REVIEW
Correlation between antimicrobial resistance and virulence in Klebsiella pneumoniae
C.Hennequin 1,2,6&F.Robin 1,3,4,5,6
Received:13November 2015/Accepted:14December 2015/Published online:30December 2015#Springer-Verlag Berlin Heidelberg 2015
Abstract Klebsiella pneumoniae is responsible for a wide range of infections,including urinary tract infections,pneu-monia,bacteremia,and liver abscesses.In addition to suscep-tible clinical isolates involved in nosocomial infections,multidrug-resistant (MDR)and hypervirulent (hvKP)strains have evolved separately in distinct clonal groups.The rapid geographic spread of these isolates is of particular concern.However,we still know little about the virulence of K.pneumoniae except for hvKP ,whose secrets are beginning to be revealed.The treatment of K.pneumoniae infections is threatened by the emergence of antimicrobial resistance.The dissemination of resistance is associated with genetic mobile elements,such as plasmids that may also carry virulence de-terminants.A proficient pathogen should be virulent,resistant to antibiotics,and epidemic.However,the interplay between resistance and virulence is poorly understood.Here,we re-view current knowledge on the topic.
Introduction
Klebsiella pneumoniae ssp.pneumoniae is the causative agent of a variety of diseases,including urinary tract and soft tissue infections,bacteremia,and pneumonia.In developed coun-tries,K.pneumoniae has traditionally been considered as an opportunistic pathogen responsible for nosocomial infections [1].However,over the past three decades,a distinctive syn-drome of community-acquired invasive infections,primarily in the form of pyogenic liver abscesses,has emerged [2].This invasive syndrome has been reported mostly in Asia,but an increasing number of cases have appeared worldwide [3].These infections are caused by hypervirulent (hvKP)isolates,mainly of serotypes K1and K2.Serotype K1strains belong to particular clones such as clonal complex 23(CC23),compris-ing ST23and ST57[4].Serotype K2strains belong to several sequence types (STs),some of which are linked to hypervirulence,such as ST86,ST375,and ST380.Isolates belonging to ST57,ST65,and ST375have been involved in invasive infections [5].
In parallel,K.pneumoniae clinical isolates have acquired increasingly high levels of antimicrobial drug resistance.For example,the rates of multidrug-resistant (MDR),extensively drug-resistant (XDR),and pandrug-resistant (PDR)isolates of K.pneumoniae were 61.4%,22%,and 1.8%,respectively,for a period of 14months between 2010and 2011in hospitals in Beijing,China [6].This makes them difficult to eradicate and has led to their rapid spread in hospitals.Klebsiella pneumoniae belongs to the ESKAPE group (Enterococcus faecium ,Staphylococcus aureus ,Klebsiella pneumoniae ,Acinetobacter baumannii ,Pseudomonas aeruginosa ,and Enterobacter species),which causes most nosocomial infec-tions in US hospitals [7].Klebsiella pneumoniae B escapes ^antibiotic treatment by becoming resistant.Most MDR K.pneumoniae isolates,which produce carbapenemases
* F.Robin
frobin@chu-clermontferrand.fr
1
Laboratoire de Bactériologie,CHU Clermont-Ferrand,58,rue Montalembert,63003Clermont-Ferrand,France
2
Clermont-Université,UMR CNRS 6023,Laboratoire
Microorganismes:Génome Environnement (LMGE),Universitéd ’Auvergne,Clermont-Ferrand,France
3
Clermont-Université,Universitéd ’Auvergne,M2iSH,Clermont-Ferrand,France
4INSERM,U1071,63001Clermont-Ferrand,France 5INRA USC2018,63001Clermont-Ferrand,France
6
Laboratoire associéRésistance des Entérobactéries
BLSE/Céphalosporinases,Centre National de Référence Résistance aux Antibiotiques,Clermont-Ferrand,France
Eur J Clin Microbiol Infect Dis (2016)35:333–341DOI 10.1007/s10096-015-2559-7
(KPC)and/or extended-spectrumβ-lactamases(ESBLs)in combination with quinolone and aminoglycoside resistance, belong to particular clones(e.g.,CC258comprising ST258, ST11,ST512,ST340,…,CC15,CC14)[5].MDR and hyper-virulent populations of this species were,for a long time,non-overlapping[5],but a few cases of MDR hvKP have recently been reported[8].ESBL-producing organisms were first de-tected in Europe,almost all in France[9].The prevalence of ESBLs in Klebsiella ranged from as low as3%in Sweden to as high as34%in Portugal in intensive care units(ICUs).In North America,6.1%of K.pneumoniae isolates from ICUs were resistant to third-generation cephalosporins.ESBLs were found in30–60%of Klebsiella from ICUs in Brazil, Colombia,and Venezuela.36.1%of K.pneumoniae isolates collected in a single South African hospital were ESBL pro-ducers.The proportion of these isolates in Australian hospitals is about5%.Rates of ESBL production by K.pneumoniae are as low as5%in Japan,compared with20–50%elsewhere in Asia[9].KPC-positive isolates have also spread worldwide. In some countries,such as Israel,Greece,and Colombia,cases are endemic,while in others,such as Australia,New Zealand, and Canada,they are only imported[10].
The main virulence factors of K.pneumoniae are capsule, fimbriae,lipopolysaccharides(LPS),and siderophores (enterobactin,aerobactin,salmochelin,yersiniabactin),and ef-flux[1].Some factors,such as fimbriae,capsule, enterobactins,and biofilm formation,are found in almost all isolates and seem to be at the origin of classical pathogenesis.
A number of putative virulence factors have been associated with hvKP,while CC258is almost entirely devoid of viru-lence genes[5].hvKP strains are characterized by the pres-ence of RmpA(a regulator of the mucoid phenotype)and aerobactin,which are both encoded by a large virulence plas-mid[11].Additional iron acquisition systems such as yersiniabactin,which is encoded by an integrative and conjugative element(ICE)ICEKp1[12],and a region associ-ated with allantoin metabolism[13]have also been associated with specific hvKP strains.Generally,the acquisition of any one of the siderophore clusters by K.pneumoniae isolates increases the risk of severe infection in humans[14].
Bacteria can acquire antimicrobial resistance by DNA mu-tation or by horizontal gene transfer[15].However,acquisi-tion of antibiotic resistance can carry a fitness cost,which reduces the competitive ability of the bacteria in the absence of antibiotics.Deletion or mutations in chromosomic genes involved in antimicrobial resistance(e.g.,porins)lead to a fitness cost.This cost is lower for plasmid acquisition[16]. Plasmids play a central role in the dissemination and acquisi-tion of resistant determinants and virulent genes in K.pneumoniae.They are responsible for so many particular properties of bacteria that certain authors consider them as independent organisms[17].Klebsiella pneumoniae is partic-ularly permeable to plasmids.The strains usually harbor more than one,including small high-copy-number plasmids and low-copy-number plasmids that are usually large[17]. Owing to the diversity of the acquisition of antimicrobial re-sistance and of the virulence factors and phylogenetic back-ground of the strains,the relation between resistance and vir-ulence is a complex issue.Like Escherichia coli, K.pneumoniae has a high degree of genomic plasticity,with gene loss or gain of genomic segments by lateral gene transfer [15,17].
The main aim of this review is to discuss the association between antimicrobial resistance and virulence in K.pneumoniae strains.
Resistance toβ-lactams
β-Lactam antibiotics are a large class of antibiotics,in-cluding penicillins,cephalosporins,monobactams,carba-penems,andβ-lactamase inhibitors.They have aβ-lactam ring in their molecular structure and are the most widely used antibiotics.The first mechanism in resistance toβ-lactams in K.pneumoniae is the production ofβ-lactamase,followed by alterations in permeability and ex-trusion by efflux pumps.
Association betweenβ-lactamase expression
and virulence
Extended-spectrumβ-lactamases
Klebsiella pneumoniae is naturally resistant to ampicillin and carbenicillin by the production of SHV-1β-lactamase encoded on the chromosome.In the early1980s,the first ESBL able to hydrolyze oxyimino-cephalosporins was iden-tified[18].Since then,third-generation cephalosporin resis-tance has been mainly due to the production of ESBLs. These enzymes are mostly plasmid-mediated and confer resis-tance against penicillins,first-,second-,and third-generation cephalosporins,and aztreonam by hydrolyzing these antibi-otics.They are inhibited byβ-lactamase inhibitors and remain inactive against cefoxitin and carbapenems.Their plasmids frequently encode other resistance mechanisms involving re-sistance to fluoroquinolones,cotrimoxazole,and aminoglyco-sides.In the early1980s,and for more than two decades, genetic variants of the classic SHV-1and TEM-2were pre-dominantly ESBLs.At the beginning of the1990s,a new ESBL family,named the CTX-M group,emerged.CTX-M enzymes are now the dominant ESBL type,with CTX-M-15 being the main enzyme currently observed in K.pneumoniae [19].
Correlation with adhesins
Klebsiella pneumoniae is present in the gastrointestinal tract of patients.Interaction with cells is,therefore,a crucial step in its colonization process.Colonization occurs by intestine cell adhesion via bacterial surface adhesins.Several studies of this species were made in our laboratory in the1990s and2000s to investigate the link between cell adhesion capabilities and resistance toβ-lactams by ESBL production.At this time, SHV-and TEM-type ESBLs were predominant.They were encoded by large conjugative plasmids called R-plasmids [17].It was firstly reported that some strains adhered to the microvilli of the Caco-2cell lines though a non-fimbrial pro-tein CF29K[20,21].The gene encoding this protein was located on an R-plasmid together with genes encoding the ESBL TEM-5,aerobactin,and its ferric aerobactin receptor. The prevalence of the cf29k gene was low among the ESBL-positive strains tested(3.5%).At the same time,Vernet et al. showed that only3.7%and7%of190ESBL-positive K.pneumoniae strains produced aerobactin and mucoid phe-notypes,respectively,unrelated to the type ofβ-lactamase, and that only2%had both factors[22].The presence of the cf29k gene was not looked for in these strains.Thereafter,this adhesin was no longer found in nosocomial strains. Interestingly,Brisse et al.recently showed that CF29K was particularly prevalent in the CC23clone(80%)and suggested that this protein could be either directly implicated in the path-ogenesis of pyogenic liver abscess or linked to another viru-lence factor on the same plasmid[23].A recent publication from South Korea also reported the presence of cf29k in only one CTX-M-14-producing,ST11isolate and in14non-ESBL-producing strains[24],suggesting an Asian diffusion for this gene.However,Struve et al.found no cf29k genes in the CC23strains that they studied[25].Another study showed the role of a28-kDa fimbrial protein in the adhesion of ESBL-producing K.pneumoniae to Caco-2cells[26].The gene encoding this protein,called KPF-28,was also located on the R-plasmid in SHV-4-producing strains.However,it has not been found subsequently.Sahly et al.also observed an increase in the adhesion and/or invasion process upon acqui-sition of ESBL(SHV-12)-encoding plasmids in parallel with an upregulation of type3fimbriae expression[27],suggesting that an element of the plasmid acted as a transcriptional regu-lator.In another study,it was shown that some isolates of K.pneumoniae producing SHV-4β-lactamase were associat-ed with a localized pattern of adhesion[28].Since45%of such localized-adhesion isolates in the same study were in-volved in severe infections[28],this phenotype could also be considered as a marker of pathogenicity.
Comments Adhesins or potential transcriptional regulators are carried upon ESBL acquisition.Interestingly,three virulence factors associated with ESBL-producing K.pneumoniae strains(adhesin CF29K,aerobactin,and mucoid phenotype,now associated with RmpA)have been found in hvKP strains and correlated with a syndrome of community-acquired inva-sive infections[25].Unfortunately,although these three viru-lence factors were found associated on a single plasmid,none of the studies clearly identified them in ESBL-producing iso-lates.Sequencing of these plasmids would bring us new in-sights into the virulence of these strains.
Correlation with capsule production
Another work showed that the frequency of serum-resistant isolates was higher among ESBL-producing strains(SHV-and TEM-types)than among non-ESBL-producing strains [29].The property of serum resistance depends on the capsule synthesis that protects the bacteria from phagocytosis.These strains are,therefore,more likely to be responsible for blood infections.However,curing ESBL-coding plasmid did not influence the serum resistance of the bacteria.On the contrary, Shin and Ko related that non-ESBL-producing K.pneumoniae showed higher serum resistance than CTX-M-producing isolates[24].Indeed,the hypermucoviscosity phenotype was more frequently identified in non-ESBL-producing isolates in this study.However,plasmids with the bla CTX-M-15gene confer higher serum resistance to transconjugants than that observed in their original host. Although there was no difference in serum resistance between CTX-M-producing and non-ESBL-producing ST11 K.pneumoniae isolates,acquisition of the plasmid by conju-gation increased the ability of the strain to survive against serum.The authors suggested that the traT gene of the plas-mid may have contributed to serum resistance in this case.In another study,we showed that clinical isolates that produced ESBL TEM-47or TEM-68was responsible for infections in newborns,despite the presence of few genes of virulence[30]. However,these strains over-expressed their capsule,which is a major virulence factor in K.pneumoniae because of its role in protecting the bacteria from the immune system. Comments These results strengthen the hypothesis that hypermucoviscosity is more associated with SHV-and TEM-type than with CTX-M-type ESBLs.However, hypermucoviscosity is sometimes related,whereas the strains were negative for RmpA or RmpA2[31].There might,there-fore,exist an unknown regulator of mucoid phenotype.RmpA is usually located on plasmids of virulence,but it has also been found in genomic islands associated with an ICE[25],which could explain why the curing of plasmids did not modify serum resistance.
These studies suggest that acquisition of ESBL-encoding plasmids increase the virulence potential of the strains be-cause,in these few cases,the gene encoding ESBLs were located on plasmids also encoding virulence factors. However,their prevalence was low among the ESBL-producing strains,and the former plasmids that also carried
these virulence genes seemed to have disappeared,at least in their original form.In these studies,acquisition of ESBL-encoding plasmids did not evidence biological cost. However,a fitness cost could have existed and would explain why the plasmids did not disseminate and persist.This was illustrated by another study from our laboratory in which we showed that the presence of ESBL-encoding plasmids altered the basal adhesion capacity of the strains,since cured strains adhered more than parental strains[32].
Correlation with non-virulence
At the end of the2000s,a clinical isolate of K.pneumoniae producing a CTX-M-15was responsible for an outbreak at the teaching hospital of Clermont-Ferrand(France)[33].The pa-tients were infected though an endoscope in which the bacteria were in a biofilm state.The strain harbored few virulence genes,despite belonging to the virulent capsular serotype K2 and,accordingly,there was no evidence of virulence in affect-ed patients.This strain has a great ability to transfer its plasmid to other bacteria and to survive in a hospital environment. There is growing evidence that capsular type is not the pre-dominant marker of virulence.For example,Wand et al. showed that K.pneumoniae strains from the Murray collec-tion(Enterobacteriaceae isolated between1917and1949) were mainly of the K1serotype but presented little or no virulence[34].Another outbreak with a strain of K.pneumoniae producing the CTX-M-15ESBL occurred at the Uppsala University Hospital(Sweden)during2005–2007 [35].A total of248patients were either infected or colonized by the isolate.The CTX-M-15-encoding plasmid carried no virulent genes but was perfectly adapted to its host and,there-fore,created no fitness cost.In contrast,fitness cost was ob-served for an E.coli recipient of this plasmid,in which it was unstable.The authors concluded that this plasmid ended in a strain particularly prone to dissemination and that it could have genetic elements to compensate for the fitness cost of the plasmid.
Comments These two examples show that the plasmids producing CTX-M-15ended in strains with different genetic backgrounds and,therefore,of variable virulence.Both had a propensity to disseminate and to cause outbreaks.These strains were adapted to the plasmids and did not seem to present fitness cost.Plasmids encoding CTX-M-15were re-cently found in CC23strains[36],which is worrying because this clone associates hypervirulent traits and multidrug resis-tance.To date,ESBL K.pneumoniae isolates have been less hypermucoviscous and less virulent than non-ESBL K.pneumoniae isolates,mostly because of concurrently lower carriage and higher mutation rates of the rmpA and rmpA2 genes[37].Plasmid-encoded cephalosporinases
This group ofβ-lactamases was formally identified at the end of the1980s[38].They hydrolyze all penicillins and first-, second-,and third-generation cephalosporins,but remain in-active against cefepime and carbapenems and are resistant to clavulanic acid and tazobactam.Various enzymes have been classified among this group(CMY,DHA,ACC,EBC,FOX), but DHA enzymes are the main cephalosporinases observed in K.pneumoniae.Since the1980s,plasmid-mediated DHA-type cephalosporinases have been reported in clinical strains of K.pneumoniae.bla DHA expression is regulated by the tran-scriptional regulator AmpR[39].DHA-producing strains have been frequently observed in nosocomial spreads[40].AmpR was also shown to be involved in the upregulation of capsule synthesis and resistance to killing by serum,in the modulation of biofilm formation and type3expression,and in adhesion to HT-29intestinal cells and the murine gastrointestinal tract [41].We showed that the virulence of clinical strains could be increased by the plasmid acquisition of transcriptional fac-tors under antibiotic pressure.
Carbapenemases
Carbapenemases have been observed since the beginning of the1980s.While they all hydrolyze penicillins and carba-penems,their activity against cephalosporins differs accord-ing to their family.Metallo-carbapenemases(VIM,IMP, NDM)hydrolyze all cephalosporins but remain inactive against aztreonam.Carbapenemases belonging to class A of the Ambler classification hydrolyze all cephalosporins and aztreonam.Finally,OXA-type carbapenemases are only active against first-generation cephalosporins,cefotaxime, ceftriaxone,and cefepime,and remain inactive against cef-tazidime and aztreonam.KPC,whose gene was plasmid-encoded,was first isolated in1996[42].Since then,these strains have disseminated worldwide owing to the rapid spread of broad host-range conjugative plasmids.Eleven KPC variants(KPC-2to KPC-12)have been identified that differ by a few amino acids changes,and of which KPC-2is the most frequent[43].Additional carbapenemases have now been found.bla NMD(New Delhi metallo-beta-lactamase-1)was isolated from a patient who acquired the bacterium in New Delhi,India[44]and has rapidly spread. bla O X A48(OXA-48)was first identified from a K.pneumoniae isolate in Turkey[45])and has gradually disseminated in Europe[46].The explanation for the epide-miological success of KPC remains unclear.No specific vir-ulence factor has been associated with KPC-producing strains[47].In2012,in the north-west of Italy(the second place in Europe after Greece in terms of resistance),of all the isolates of K.pneumoniae,17.5%were KPC-producing strains.KPC was more frequently isolated in tertiary care
referral hospitals and from urine samples(50%),and31% of KPC were identified in patients admitted to medical wards,followed by ICUs(15%),surgical wards(13%), and emergency departments(14%).Risk factors for KPC colonization were identified,such as duration of hospitaliza-tion,number of antimicrobials administered,number of co-morbidities,and number of invasive catheters[48].KPC enzymes are frequently associated with resistance to other antibiotics,such as fluoroquinolones,aminoglycosides,and cotrimoxazoles.All these factors could explain why a20% absolute increase in hospital mortality for patients with KPC was observed[49].In these conditions,it is difficult to link the epidemiological success of these strains to their own virulence or to the best environmental conditions for their development that were inadvertently created.De Rosa et al. concluded their review on a particularly interesting but not easily applicable strategy for limiting the expansion of these isolates,even though it seems currently difficult to apply. Klebsiella pneumoniae is very well adapted to the gastroin-testinal tract,especially when the protective bacteria are eliminated by a broad-spectrum antimicrobial treatment. The authors suggested,therefore,that,to limit the multipli-cation of KPC,there is a great need to restore the integrity of the gut by improving the specificity of diagnosis and limiting the duration of treatments[48].
There have been few studies on the fitness cost of carbapenemase production in K.pneumoniae.Beyrouthy et al.studied the virulence of six OXA-48-producing K.pneumoniae strains and showed that they had no par-ticular trait of virulence[50].Fuursted et https://www.doczj.com/doc/ee10369475.html,pared the virulence of five strains of K.pneumoniae,including one carrying NDM-1[51].In a murine sepsis model,they showed that the NDM-1-producing strain was the most virulent strain and possessed several virulence factors(cap-sular serotype K2,strong biofilm production,and resis-tance to killing in human and murine serum).However, to draw any definite conclusions,this study needs to be extended to other NDM-1-producing strains.In contrast, Lavigne et al.showed that the presence of bla KPC-2gene decreased the virulence of strains in a Caenorhabditis elegans model[43].This last report had the advantage of studying the effect of the gene alone on bacterial virulence, whereas that of Beyrouthy et al.studied the virulence of the entire bacterium,including the plasmids.Another re-port showed a decreased virulence for KPC isolates com-pared to non-producing carbapenemase in a Galleria mellonella model.Opposite findings were observed in pa-tients[49].However,we saw above that the virulence of these strains could be linked to factors external to the strains,such as the antibiotic treatment and the number of comorbidities.In the light of these observations,the success of these strains does not seem to be linked to their particular virulence.Association between outer membrane proteins
and virulence
The outer membrane of Gram-negative bacteria serves as a channel to regulate the influx and efflux of substances such as ions,nutrients,and antibiotics[52].As they are surface-exposed,they could be highly immunogenic and used for vaccination trials[53].High-level carbapenem resistance may develop via the loss of the OmpK36porin coupled with the expression of variousβ-lactamases[54].The two major porins,OmpK35and OmpK36,are often missing in MDR strains(especially the ESBL-positive strains).The loss of OmpK36resulted in decreased resistance to the neutrophil phagocytosis and increased mice DL50compared to parental strains[52,54],thereby impairing the virulence potential of the strain.This result was confirmed in another study,where it was shown that the loss of non-specific trimeric porins such as OmpK35and OmpK36was associated with a decrease in virulence in a C.elegans model[55].Klebsiella pneumoniae outer membrane proteins(OMPs)contribute to phagocytosis resistance.OmpA thwarts the innate system,but March et al. showed that OmpK36also contributes to phagocytosis resis-tance by Klebsiella.In contrast to OmpA,OmpK36does not play any role in resistance to antimicrobial peptides and,un-like OmpK36,OmpA does not play any role in resistance to antibiotics[56].OmpA is a multifunctional OMP,one of the major proteins in the outer membrane of many members of the Enterobacteriaceae.
We also showed that isolates resistant to cefoxitin,chlor-amphenicol,and quinolones had a significantly lower adhe-sion index to Int-407cells compared to a median adhesion index made with all the tested isolates[57].The isolates resis-tant to cefoxitin,chloramphenicol,and tetracycline showed a greater ability to mutate.Resistance to cefoxitin could be due to impermeability resulting from the loss or modification of porins or,more generally,bacterial surface components fol-lowing mutations.These proteins could also be involved in the adhesion process.To survive the presence of antibiotics, bacteria sacrifice one or several proteins,thereby losing cer-tain abilities.These results illustrate the biological cost of acquiring antibiotic resistance via mutations.
Association between efflux pumps and virulence
The K.pneumoniae genome harbors several operons encoding efflux systems.These pumps export not only antibiotics but also other substances,such as dyes and detergents[58]. Klebsiella pneumoniae encodes an AcrAB multidrug efflux system,which is involved in resistance to quinolones (nalidixic acid,ciprofloxacin)and other antibiotics(cefoxitin, chloramphenicol,erythromycin,tigecycline).This system is also involved in virulence by mediating resistance against antimicrobial peptides present in the lung[59].The
overexpression of this efflux pump was correlated with an increase in virulence in a C.elegans model[55].Another efflux system,OqxAB,involved in resistance to nalidixic ac-id,ciprofloxacin,chloramphenicol,and cefoxitin,was recent-ly described[60].It belongs to the rarA-oqxABR locus,with RarA acting as a transcriptional regulator of oqxAB and OqxR acting as a transcriptional repressor of oqxAB and rarA[61].A mutation in this latter repressor was responsible for the multi-drug resistance and the increased virulence of the strain ob-served in a C.elegans model.KexD,a resistance–nodulation–cell division(RND)-type efflux pump from K.pneumoniae, was shown to contribute to multidrug resistance,but its role in virulence has not been studied[62].Other pumps such as EefABC,although involved in colonization of the murine di-gestive tract,were not linked to any antimicrobial drug resis-tance phenotype[63].The multidrug and toxic compound extrusion(MATE)KetM was also not significantly correlated with resistance to antibiotics[64].
Resistance to other antibiotics
Resistance to fluoroquinolones
Fluoroquinolone resistance has been associated with muta-tions in the quinolone resistance-determining regions (QRDRs)of the gyrA(gyrase)and parC(topoisomerase IV) genes,plasmid-mediated resistance to quinolones,altered per-meability(porin loss),and also with a lower uptake of quino-lones because of efflux overexpression[65].The prevalence of qnr genes was3.9%in K.pneumoniae strains isolated from patients’blood in Taiwan[66].Tóth et al.suggested that there was a strong link between fluoroquinolone resistance and fit-ness[67].However,this phenomenon seems related to an enhancing activity of efflux rather than to amino acid substi-tutions in the quinolone resistance regions.
Resistance to colistin
Colistin acts by interacting with lipid A,leading to outer mem-brane disruption[68].Colistin resistance in K.pneumoniae is mainly related to LPS modification following the addition of 4-amino-4-deoxy-L-arabinose to lipid A.This modification is associated with the pbgPE operon,which is regulated by PmrAB and PhoPQ.Insertional activation of the PhoQ/PhoP MgrB regulator has also been proposed as a determinant of colistin resistance[69].Colistin resistance was related to mu-tations in three different genes,mgrB,phoQ,and ccrAB,a two-component regulatory system;ccrAB,however,is present in only some strains[70].Choi and Ko showed that resistance to colistin in hypervirulent K.pneumoniae ST23strains may result in an in vitro fitness defect and in defects in hypermucoviscous,CPS production,and serum resistance [71].Recently,Liu et al.characterized the plasmid-encoded phosphoethanolamine transferase MCR-1,which confers re-sistance to colistin.This enzyme is rare in K.pneumoniae strains(0.7%)and its role in virulence remains unknown[72].
The contribution of whole-genome sequencing Whole-genome sequencing allows an in-depth characteriza-tion of bacterial strains and will serve as a powerful tool to study and compare strains in nosocomial infections and out-breaks.This method is still in its infancy but will greatly contribute to the better understanding of the virulence and epidemiology of K.pneumoniae strains.For this purpose, Brisse et al.have developed a freely accessible BIGSdb-Kp database.A few studies have already used high-throughput sequencing,in particular to obtain the genomes of hyperviru-lent(CC23)and MDR strains(CC258),which are endemic. Bialek-Davenet et al.showed that CC258was entirely devoid of virulence genes and that MDR and hypervirulent strains are largely non-overlapping[5].Struve et al.suggested that CC23 isolates are highly effective colonizers of the human intestinal tract.Homologs of a large virulence plasmid encoding two siderophores,aerobactin and salmochelin,and RmpA were detected in all hvKP.They possessed additional siderophores, such as yersiniabactin,colibactin,and microcin E492associ-ated with an ICE.These strains,which have remarkable ge-nome plasticity,seem to be characterized by recombination events with gain/loss of genomic segments[25,73].
Conclusion
In Klebsiella pneumoniae,the most common mechanism of resistance consists of enzyme synthesis and,more particularly, that ofβ-lactamases:extended-spectrumβ-lactamases (ESBLs),cephalosporinases,and carbapenemases.Currently, the large class of antibiotics calledβ-lactams is the most fre-quently used in human therapeutic situations.The genes encoding these enzymes are carried by plasmids that also car-ry other genes,including genes of virulence factors.Thus,it is difficult to appreciate the real effect of the mechanism of re-sistance on the fitness cost for the bacteria.In the1980s and 1990s,ESBL(SHV-and TEM-types)encoding genes were observed on plasmids of virulence.Consequently,acquisition of these plasmids by the bacterium increased its virulence potential.Currently,the epidemiology has shifted to a major-ity of CTX-M-type ESBLs and the spread of K.pneumoniae carbapenemases(KPC).The plasmids that carried these en-zymes do not seem to have fitness cost,but the strains are less virulent.At present,clonal complexes of hypervirulent (hvKP)and multidrug-resistant(MDR)strains are non-over-lapping.Let us hope that MDR plasmids will not be able to
maintain stability in hvKP,because this would lead to the emergence of a B super^bug.Much work remains to be done to fully understand the relationship between virulence and resistance in K.pneumoniae.In particular,little is known about the influence of the acquisition of several antibiotic mechanisms by the bacteria on fitness cost.
Compliance with ethical standards
Conflict of interest The authors declare that they have no conflict of interest.
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among between为近义词，皆可表示“在……之间”，但用法大不相同，现归纳比较如下： 一、among一般用于三者或三者以上的“在……中间”，其宾语通常是一个表示笼统数量或具有复数意义的名词或代词。 His house is hidden among the trees． 他的房子隐藏在树林之中。 She sat among the children． 她坐在孩子们中间。 二、between一般指两者之间，其宾语往往是一个具体数目的人(物)，或者是由and连接的两个具体的人(物)。 There was a fight between the two boys． 这两个男孩间发生了一场格斗。 I am sitting between my parents． 我正坐在我父母中间。 三、把两者以上的为数不多的人或事物单独地看待，用and连接时，要用between；把两者以上的人或事物看成一群、一堆或一组而不是个体时，要用among。 Switzerland lies between France,Italy,Austria and Germany． 瑞士位于法国、意大利、奥地利和德国之间。 The old man’s cottage lies among the trees． 老人的小木屋在树林中。 四、between也可用于三者以上的事物之间，强调一物与数物之间的关系。 The small village lies between the three mountains．
小村庄位于三座大山之间。 I saw something lying between the wheels of the train． 我看见火车轮子之间有什么东西。 五、涉及人或事物之间的区别以及人或事物之间的关系时，一般要用between。 We must find out the difference between the three companies. 我们必须查清这三家公司之间的区别。 The relations between various countries are very important. 各国之间的关系是很重要的。 六、表示“由于……合作的结果”时，要用between。 Between them they landed the fish． 他们协力把鱼拖上了岸。 Between the five companies the project was soon completed． 在五家公司的齐心协力下，这项工程不久就完成了。 七、当and连接三者或三者以上的人(物)而仍然强调两者的并列时，常用between。 The hospital lies between a river and hills． 医院坐落在一条河与群山之间。 The park lies between a road and the woods． 公园位于一条马路与树林之间。 八、在divide,share等表示“分享”之类的动词之后。若接一个表示三者或三者以上的复数名词时，用among或between均可。 The father divided his money among/between his three sons。
四年级一件难忘的事400字作文【五篇】 四年级一件难忘的事400字精选作文篇一 在童年里有许许多多的事情就像在海滩上捡贝壳，有绚烂的笑，是开心的往事；有黯淡的，像是勾起一段伤心的往事，都使我非常难忘。 早晨，风和日丽，我去帮妈妈和爸爸买早餐，等我去到早餐店，我发现早餐店旁有一大群人，我带着好奇心走去，让我大吃一惊，我发现那有一位老奶奶绊倒在地上。 我心想：为什么一位老奶奶搬绊倒在地上，但每一个人都无动于衷，还围观老奶奶，真是不尊重老人！我气得暴跳如雷，我就问我旁边的大人：“你们为什么不扶老奶奶”那人就说：“因为最近在新闻里，有很多老人成心绊倒在地上让路人扶起她。”谁知老人就说：扶起他的人是推他的人。老人还说：赔我钱。我听完就有一点不敢拉老奶奶了。 这时，一位青年人走过来，马上扶起老奶奶，旁边的人说，那个青年人真笨，青年人就说：“难道金钱比生活还重要吗？”青年人问奶奶要紧吗？奶奶说，不要紧，谢谢你，小伙子。青年人就说不用谢，说完就走了，我看到青年人的身影，我觉得很惭愧。 这一件事使我非常难忘，假如社会的人都献出一点爱心，那么社会就是充满爱心的。
四年级一件难忘的事400字精选作文篇二 每一个人都有自己最难忘的事，我最难忘的事就是三年级那次集体打连响儿。 记得那一次我们拿着漂亮的连响儿，排着整齐的队伍，伴随着美好的音乐，踏着坚决的步伐，熟练地打了起来。 看到这样整齐划一的队伍，我不由心生骄傲：看来我的努力还是没有白费呀！但放眼又一望，我们四周可以说是人山人海，有无数双眼睛正盯着我们，我心里不知从哪里来了一种莫名的紧张，我头上冒着冷汗，腿不由的发着抖。就在这一霎时，我做错了一个动作，登时，我感觉所有人的目光都投向了我一个人，我这颗本来就不平静的心，现在显得更加紧张了、更加不安了，我一下子都不知该干什么了，我真想有一个洞，让我钻到洞里去，我以前打的是那么灵敏，可现在却有气无力，仿佛有一个小精灵把我这活泼的精神给夺走了，经过漫长的煎熬，我好不容易才等到了结束的那一刻。 我回家把这件事告诉我妈妈，妈妈说：想做好一件事就要加倍的努力，不要认为自己会做了就可以了，妈妈的这次教导，将使我一生受益。 四年级一件难忘的事400字精选作文篇三 我家有各种各样的陶瓷器皿，比如：碗，花瓶，勺子，水杯等。它们形态各异，五彩缤纷。它们站在一起，仿佛都在展示自己曼妙的
人 教 版 四年级（上册） 数 学 基 础 知 识 专 项 训 练
人教版四年级上册数学基础知识填空题专项训练 １、由5个千万、4个万、8个十和9个一组成的数是（），读作（），取近似值到万位约是（）。 2、406000000读作（），这个数中的6在（）位上，表示（），改写成用万作单位是（）。 3、一周角=（）平角=（）直角。 4、367÷23把23看作（）来试商比较方便。 5、下午3：00时针和分针夹成的最小角是（）度。 6、在数位顺序表中，从右起第四位是（）位，这个数的计数单位是（），如果这个数位上的数字是8，8表示（）。 7、5个一百万、4个十万、2个千和4个一组成的数是（）。读作（），它有（）个计数单位。 8、在9、8中间添（）个0，这个数才是九千万零八。 9、一个数加上2的和比最小的五位数多1，这个数减2是（ 10、在数位顺序表中，从右起第四位是（）位，这个数的计数单位是（），如果这个数位上的数字是8，8表示（）。 11、5个一百万、4个十万、2个千和4个一组成的数是（）。读作（），它有（）个计数单位。 12、在9、8中间添（）个0，这个数才是九千万零八。 13、一个数加上2的和比最小的五位数多1，这个数减2是（） 14、120分米=（）米 540秒=（）分 72小时=（）天 132个月=（）年 15、计量角的单位是（）。（）是量角的工具。 16、角的大小要看两边（），（）越大，角越大。 17、线段有（）个端点。把线段的一端无限延长，就得到一条（），把线段的两端无限延长，就得到一条（），它（）端点。 18、过一个点可以画（）条直线，过两点可以画（）条直线。 19、按照从大到小的顺序排列下面各数 88000 80800 80008 80080 ________________________________________________ 20、把锐角、平角、钝角、直角、周角按下列顺序排列。 （）＞（）＞（）＞（）＞（） 21、4293÷4口，要使商是二位数，口可以填（）
2018初中英语词汇之among与between的用法区别 一般说来，among 用于三者或三者以上的在中间，其宾语通常是一个表示笼统数量或具有复数(或集合)意义的名词或代词；而between 主要指两者之间，其宾语往往是表示两者的名词或代词，或者是由and 连接的两个人或物： They hid themselves among the trees. 他们躲在树林中。 There was a fight between the two boys. 这两个孩子打过一次架。 Im usually free between Tuesday and Thursday. 我通常在星期二与星期四之间有空。 在下列情况，between 可用于三者： (1) 当两个以上的人或物用and 连接时： between A, B and C 在A、B和C 之间 (2) 涉及事物之间的区别或各国之间的关系时： the difference between the three of them 他们三者之间的区别 the relations between various countries 各国之间的关系 (3) 表示由于合作的结果时： Between them they landed the fish. 他们协力把鱼拖上了岸。
(4) 在divide, share 等表示分享之类的动词之后，若接一个表示三者或三者以上的复数名词时，用among 和between 均可：He divided his money among [between] his five sons. 他把钱分给了5 个儿子。
关于难忘的一件事四年级作文3篇 【导语】每个人都有难忘的事，我当然也不会例外。下面是小编为大家整理的关于难忘的一件事四年级作文，仅供参考，欢迎大家阅读。 关于难忘的一件事四年级作文一 六岁那年，发生了一件令我难忘的事情——在商场里我差点儿走丢了！ 那天，天气很冷，我们一家人去商场买东西。买完后，我们在一楼闲逛。走着走着，我松开了妈妈的手，跑去看一款造型独特的吸尘器，当回过头时，爸爸妈妈已经走得无影无踪了。商场里人很多，我非常害怕，心想：“妈妈也一定急坏了！怎么办？” 我在商场里转了一圈，越来越害怕、恐惧，忽然，我想起爸爸的车在路边停着，干脆就“守株待兔”吧！所以就跑去爸爸的车旁边等。过了一会儿，爸爸妈妈终于找到我了，妈妈冲过来，一把把我紧紧地搂进怀中，口里不停的说：“吓死妈妈了，妈妈腿都软了！”然后又语无伦次的夸我聪明，没瞎跑，唉！我心里就像打翻了五味瓶一样，说不出的'滋味儿！ 从那以后，我再也不敢轻易放开妈妈的手了。 关于难忘的一件事四年级作文二
一个美丽的晚霞中，那七元五角的事使我永刻不能忘怀。 我背着书包回了家，对妈妈和蔼地说：“妈咪，我回来了！哦，对了妈咪，老师吩咐我们要买一支黑笔，您可不可以给我两块半。”说完，妈咪点了点头，从黑纹色的包里拿出了两块半，我拿起了钱就往商店里奔驰去。 从商店里拿起了一支晶亮的黑笔，到收银台付了钱，就发现了里头还藏了个五元，我的贪婪之心有苏醒了。付了钱后就在半路上犹豫到底要不要把这五元给妈咪呢？在犹豫时就看见了一个小妹妹，她看见前面的叔叔掉了十元钱，就还给了那个叔叔，我就在想，连一个比我还小的妹妹都那么听话，我不能落后，就跑回了家，把五元还给了妈咪。 做人要道德，道德只是个简单的是与非的问题，实践起来却很难，把五元还给妈咪，一个人要是从小就受到这样的教育的话，就会获得道德实践的勇气和力量。 关于难忘的一件事四年级作文三 在我的脑海里发生过许多难忘的事情，但是有一件事令我记忆犹新，那是发生在去年的一件事。 那是一天下午我在我在学校上学的时候，我借了刘利娜的语文作业抄，下课了，我抄完了。就和女同学玩橡皮筋。正在这时候周老师来了，我们吓得腿都软了。 在回家得路上我心里忐忑不安，心想这样做对不对呢？
2020年四年级数学上册解决问题专项综合练习 1. 某小学学生向汶川地震灾区踊跃捐款，请根据下图填空． （1）______年级的捐资金额最多，是______元． （2）六个年级捐资金额这组数据的中位数是______元． （3）二年级捐资金额是四年级捐资金额的______%． （4）四年级捐资金额比五年级少______%． 2. 特快列车每小时行170千米,普通列车每小时行107千米 （1）特快列车40小时行多少千米？ （2）普通列车40小时行多少千米？ 3. 2010年第六次全国人口普查显示，我国人口总数大约是1339720000 人，读横线的数。 4. 如图是广本汽车销售店2013年一月至五月的销售情况统计图．
请你根据上图，完成以下的填空： （1）______月的销售量最少． （2）二月份的销售量比一月份多______台． （3）五月份的汽车销售量是三月份的______%． （4）四月份的汽车销售量比二月份增加了______%． 5. 下图显示的是某班20人在“献爱心”活动中捐图书的情况，该班级人均捐了多少册书？ 6. 小强拆了甲、乙两架纸飞机，下面是这两架飞机前5次试飞情况的统计图． （1）估一估，前5次试飞，哪一驾纸飞机飞行的距离远一些？ （2）第6次试飞，甲飞机的飞行距离是21米，乙飞机的飞行距离是10米．在图中画出表示这架纸飞机第6次试飞飞行距离的直条，并计算这架纸飞机6次飞行距离的平均数各是多少．
7. 早餐店炸油饼，油锅一次最多能炸2张饼，炸熟一张饼要4分钟（正反面各2分钟）．如果一个客人要9张饼，早餐店老板最快多少分钟可以把油饼给他？ 8. 某机床厂各车间男、女工人统计图． 看上面的统计图回答下面的问题． （1）三个车间共有工人______人，其中女工共______，男工共______人．（2）三个车间平均每车间有______人． （3）第三车间男工人数比第二车间男工人数多______ %． 9. 学校进行团体操表演，每行站20人，正好站24排．如果要站成16排，那么每行需要站多少人？ 10. 50枚棋子围成圆圈，编上号码1、2、3、4、…50，每隔一枚棋子取出一枚，要求最后留下的枚棋子的号码是42号，那么该从几号棋子开始取呢？ 11. 估一估，哪个算式的商最接近圈中的数，画上“√”。 先估算出每个圆中的四个算式的商是多少，再比较一下，看哪个算式的商最接近圈中的数。 12. 小红在计算一个数乘2.5时，忽略了2.5的小数点，得到的结果是75，正确的结果是多少？
英语地点介词的正确使用方法 地点介词主要有at ，in，on，to，above，over，below，under，beside，behind ，between。它们的用法具体如下： 1、at （1）at通常指小地方：In the afternoon，he finally arrived at home。到下午他终于到家了。 （2）at通常所指范围不太明显，表示“在……附近，旁边”：The ball is at the corner。球搁在角落里。 2、in （1）in通常指大地方：When I was young，I lived in Beijing。我小时候住在北京。 （2）在内部：There is a ball in in the box。盒子里有只球。 （3）表示“在…范围之内”（是从属关系）： Guangdong lies in the south of China。深圳在中国的南部。 3、on
（1）on主要指“在……之上”，强调和表面接触： There is a book on the table。桌上有一本书。 （2）表示毗邻，接壤（是相邻关系）： Canada lies on the north of America 加拿大在美国的北边（与美国接壤）。 4、to 主要表示“在……范围外”，强调不接壤，不相邻。 Japan is to the east of China。日本在中国的东面。 注意： （1）at 强调“点”，on 强调“面”，in 强调“在里面”，to 表示“范围外”。 （2）on the tree：表示树上本身所长着的叶子、花、果实等 in the tree：表示某物或某人在树上 on the wall：表示在墙的表面，如图画、黑板等 in the wall：表示在墙的内部中，如门窗、钉子、洞、孔 5、above
每个人的每一天都充满着快乐、伤心、害怕或者难过，并且会有一些难忘的事情。你难忘的一件事是什么呢？下面是小编和大家分享的“四年级难忘的一件事作文400字”，一起来看看吧，希望大家喜欢。 四年级难忘的一件事作文400字（一） 虽然我才上四年级，但是我的生活里也有很多难忘的事。我就给大家讲一件让我难忘的事吧。有一次爸爸让我做大米饭，给我留下了深刻的印象。 那天中午，爸爸对我说：“今天中午你来帮妈妈做大米饭吧，正好也锻炼锻炼。”我听了以后高兴地说：“好啊，小菜一碟。”说完以后我就开始动手了。我先把大米放在盆里淘干净，再倒进锅里，放在炉子上就到客厅去看我喜欢看的电视去了。在我看得正高兴的时候，忽然就闻到厨房里传出来一股烧糊的味儿来。我一看表，已经过去了半个小时了，我马上跑进厨房，把锅端下来。打开锅盖，就看见了变黑的锅底。可我没有放弃，又做了一次。 我又把米淘干净倒进锅里以后，我又去写作业去了。过了一会儿，我又闻到了那股烧糊的味道。我跑到厨房一看，锅底又黑了。这一看，我的眼泪都快掉下来了，这可怎么办啊？这时候，爸爸来了，对我说：“不用哭，做什事都要专心，再来一次。”我点了点头。第三次，我终于把大米饭做熟了，看见自己的成果，我感到很高兴。 这件事让我明白了：做什么事都要专心致志，不能三心二意。 四年级难忘的一件事作文400字（二） 今天，是我和姐姐第一次去买菜。 妈妈给了我们钱，说：＂要注意安全哦。＂我们说：＂知道了。＂ 一路上，我的心在扑通扑通地跳。来到菜市场，我都目瞪口呆了，各种各样的蔬菜放在我面前，有西兰花、芫茜、冬瓜……还有许多嘈杂声呢！ 我回忆起妈妈要我买的菜，哦！要买鱼。我走进市场左看看，右望望，嘿，找到了！我走过去，正准备对卖鱼的叔叔说话，一位阿姨便先说了一步。阿姨买完了鱼，我又准备说，谁知，又有一位阿姨要买。5分钟过去了，我还没买到，我着急了，对着叔叔大声说：＂叔叔，请给我一块鲜鱼肉！＂叔叔笑眯眯地说：＂你要多少呀？＂＂4元。＂我回答说。叔叔弄好了鱼，用袋子好了给我，我付了钱并说了一声＂谢谢＂就走了。 我又想到要买菜，对了，还有芫茜！我找到了买我所需要的地方，就跟那位婆婆要了一点。当我走出市场时，天空下起了牛毛似的雨，我跟姐姐说：＂我们快走吧！＂接着，我们骑着自行车，飞快地奔回家。 骑到半路，雨就更大了，我们到了一棵树下遮雨，但是好久都不见天空好转。为不拖延时间，我和姐姐顶着风冒着雨冲回家。
薄弱环节专项全练全测(一) 数与代数【薄弱点一】大数读、写的准确性 一、读出或写出下列各数。(8分) 30050082 读作：______________________ 3960400090 读作：______________________ 一千零四十万四千零二十写作：________________ 七千零三亿零二十万零五写作：________________ 二、求近似数。(8分) 1．省略万位后面的尾数。 16493560≈9528641≈ 2．省略亿位后面的尾数。 2709546312≈9953364778≈ 三、选择。(6分) 1．下面三个数中，一个0也不读出来的是( )。 A．50000800 B．50080 C．5008000 2．下面各数中，用9，8，7，5，0这五个数字组成的最接近8万的数是( )。 A．89750 B．80579 C．85079 3．74□7600000≈75亿，□里最大能填( )。 A．8 B．9 C．7 【薄弱点二】乘、除法计算的准确性 四、改错。(12分)
五、笔算下面各题，带※号的要验算。(24分) 240×38＝207×40＝※360×50＝ 380÷70＝ 694÷72＝※633÷21＝ 【薄弱点三】用乘、除法知识解决问题的正确性 六、解决问题。(34分) 1．一块长方形草坪的面积是120平方米，改建后，长扩大到原来的4倍，宽扩大到原来的3倍，改建 后草坪的面积是多少？(8分) 2．某花店在教师节到来之际，搞促销活动：每盆月季花16元，买3盆送1盆。 照这样计算，买4盆花，每盆比原来便宜多少钱？(8分) 3．一根木头长15米，把它平均分成5段，每锯一段需8分钟，锯完一共要花多少分钟？(8分) 4．一辆长途客车6小时行了348千米。照这样的速度，它12小时可以行多少千米？(10分) 解法1：先求客车1小时行驶多少千米，再求12小时可以行驶多少千米。 解法2： 先求12小时中有几个6小时，再求几个348千米是多少。 【薄弱点四】合理安排时间 七、小红家来了客人，她要沏茶招待客人。(8分) 找茶叶：1分钟沏茶：3分钟接水：1分钟洗杯子：2分钟洗水壶：2分钟烧水：10分钟 应该怎样安排时间才能使所用时间最短？至少要多长时间？
大年三十，踩气球 白雪皑皑的夜晚，从一间小屋里传出了阵阵欢快的笑声。那是大年三十，人们正忙着发压岁钱。只见家人们从裤兜里掏出一张比一张大的钱，塞到满怀欣喜的孩子们的手里。我们拿着钱就去买了许许多多的气球。 我们一家人开始给气球打气。我们不知疲惫地打了一个又一个。不一会儿，整整地堆满了一间大房子。气球的颜色和形状各不相同，有大的、有小的、红的、绿的、白的……在灯光的照耀下，非常漂亮。 我挑了一些漂亮点儿的气球，挂了起来，把客厅装扮得像春节联欢晚会现场一样光彩亮丽。时钟刚好走到12点时，我便急得像上锅的蚂蚁一样，立即跳向气球堆。“啪啪……”气球发出响亮的爆炸声。这清脆的爆炸声吸引了爸爸妈妈和弟弟的围观。他们被我那热乎劲儿感染了，也过来跟我一起踩气球。“噼噼啪啪……”一阵响，就像放鞭炮一样，既吓人又好玩。清脆的爆炸声给热闹的夜晚增添了一份别具一格的色彩。 不一会儿，整整一屋的气球就只踩剩一个了。我上前去一踩，没想到它竟然跑了，气得我张牙舞爪直瞪眼，恨不得一脚把它踩扁。我那滑稽的举动惹得爸妈们突地大笑起来。我不服气，气鼓鼓的又跑上去，对着气球狠狠地一踩，心想这回肯定能踩扁它。可谁知，我一个不小心，脚下一滑，便跌坐在气球上。“啪……”完了，气球在我的屁股上炸开了花。这下子，可把我的爸妈们逗得更乐了。 欢快的笑声在小屋的四周回荡，也回荡在我生命的旋律中…
捉螃蟹 记得去年冬天的一个早晨，天气晴朗，阳光明媚，我和表妹一起到河边的沙滩上捉螃蟹。 河里的水非常的少，到处都是石头。螃蟹，就藏在这些奇形怪状的石头下面。我们一边沿着岸边走，一边翻着旁边的石头。突然，表妹发现了一只螃蟹，激动万分地对我说：“哥哥，这块石头下面好像藏了一只螃蟹。”我们连忙轻轻地走过去，慢慢地蹲下去，小心翼翼地把石头搬开，快速伸过手去捉它，没想到螃蟹用两只大钳子把我的手狠狠地夹住了，痛得我脸都涨红了。我赶紧用力的把手往外一甩，才摆脱掉这该死的螃蟹。“呀！流血了！”表妹惊呼道。我低头一看，才发现是自己的手被螃蟹夹了一条大口子，直流血。我淡然地擦擦手说没事，可怎知表妹被我那流血直流的手吓得大哭起来。忍者疼痛，我迅速一抓，就把螃蟹扔进了瓶子里，表妹立即破涕为笑。 我们终于捉了一只螃蟹，心里有说不出的高兴，我那鲜红的手指和表妹那还挂着晶莹泪珠的脸，在阳光下，闪闪发光……
填空题 1、当除数是34时，试商时可以把除数看作( )，这样初商容易偏( )。 2、（）个26相加的和是468；（）比12个15多20。 334＝21），这时被除数是（）。 4、在括号里填上合适的数。 480秒=（）分540厘米=（）分米624时＝（）日 5、我们戴的红领巾上有一个（）角，两个（）角。 6、钟面上，分针转动360度，相应地时针转动（）度。 从3：00走到3：15，分针转动了（）度。 6点时，时针和分针所组成的角是（）度，是（）角， 3点时，时针和分针所组成的角是（）度，是（）角。 7、把“78÷26=3，26+3=29”合并成一个综合算式是（）。 8、在5○1÷58中，如果商的最高位在十位上，○中最小填（），还可以填（）。如果3□2÷36的商是一位数，□里的数最大可以填（），最小可以填（）。在算式□17÷53中，要使商是两位数，□最小填（）；要使商是一位数，□最大填（）。 9、在公路上有三条小路通往小明家，它们的长度分别是125米、207米、112米，其中 有一条小路与公路是垂直的，那么这条小路的长度是（）米。 10、李阳从1楼到3楼用了12秒，她从一楼到六楼需要（）秒。 11．二百零六亿八千万写作（），改写成用“万”作单位的数是（）万，用“亿”作单位这个数的近似数是（）亿。 12．2个千万、7个万、8个百和5个十组成的数是（），这个数读作（）。 13．由6、7、5、1、0组成的最大数是（），最小数是（）。14、计算除法时，错将除数36看成63，结果得到商12。请你帮他算一算，正确的商应该是（）。
15．一个边长24厘米的正方形面积是（）平方厘米。 16、把两道算式组成综合算式，再用递等式计算。 14×2=28 21×4=84 10×5=50 28-15=13 200-84=116 30＋50=80 （）（）（） 选择、判断题 1、30度的角被投影仪投到屏幕上时角就变大了。（） 2、570÷40＝14……1。（） 3、在方向板上，北和西南之间夹角是135°。（） 4、在同一平面内，两条直线不是相交就是平行。（） 5、在同一平面内，两条直线不是平行就是垂直。（） 6、4个同样大的正方体可以拼成一个较大的正方体。（） 7、在10倍的放大镜下看15度的角就变成了150度。（） 8、六位数一定比七位数小。（） 9、平角就是一条直线，周角就是一条射线。（） 10、三位数除以两位数，商不可能是三位数。（） 11、三位数除以两位数，商最多是两位数。（） 12、过一点能画无数条直线。（）13．两条直线相交成直角时，这两条直线就互相垂直。（）14．观察物体时，在同一位置看到相同的形状可能有不同的摆法。（）15、两条不相交的直线叫做平行线。（） 1、在4□7÷46的商是两位数，□中的数最小是（）。 ①7 ② 6 ③5 2、想使物体从斜面上向下滚动时尽可能地快，下面的选项中，木板与地面的夹 角是（）度最符合要求。 ①20 ②38 ③10 ④80
难忘的一件事作文400字四年级 难忘的一件事作文400字四年级 最令我难忘的一件事，就是偷吃四色小辣椒了，那四色辣椒的滋味，真是难以形容，令人难忘。在我家楼下有一个小菜园，那里是爷爷的乐园，他每天都会去那，乐呵呵地给他种的蔬菜浇水。在这个菜园里，有白菜、罗卜和辣椒。最引起我主意的就是辣椒了。爷爷种的不是一般的辣椒，是四色小辣椒。四色小辣椒是一种能变色的小辣椒，在每个季节变一种色，春天是绿色、夏天是黄色、秋天是紫色、冬天是红色，非常神奇。一天我在楼下散步，当时正是秋天的一个。我走到菜园时，看见里面的小辣椒，紫紫的，看起来很好吃。心想：着么紫的小辣椒难道还会辣吗？肯定能好吃，心中便有了一种想法，摘一个尝一尝，反正爷爷也不在。我以飞一般的速度跑了过去。用手一拽，边摘了下来。我像得了一件宝贝似的，小心翼翼地捧着四色小辣椒，向家跑去。到了家，见没人，才放了心。跑进厨房把四色小辣椒洗干净，然后把放四色小辣椒放进嘴里嚼，不嚼不要紧，一嚼可坏了。顿时，舌头、喉喽就像要喷火一样，辣的我说不出话来。我跑到水
龙头前大口大口的喝起水来。这才好点，没想到这四色小辣椒比别的辣椒辣上十倍。这件事一直刻在我的心头，甩也甩不掉呢。 四年级:戴恋佳 难忘的一件事作文400字四年级 最令我难忘的一件事，就是偷吃四色小辣椒了，那四色辣椒的滋味，真是难以形容，令人难忘。在我家楼下有一个小菜园，那里是爷爷的乐园，他每天都会去那，乐呵呵地给他种的蔬菜浇水。在这个菜园里，有白菜、罗卜和辣椒。最引起我主意的就是辣椒了。爷爷种的不是一般的辣椒，是四色小辣椒。四色小辣椒是一种能变色的小辣椒，在每个季节变一种色，春天是绿色、夏天是黄色、秋天是紫色、冬天是红色，非常神奇。一天我在楼下散步，当时正是秋天的一个。我走到菜园时，看见里面的小辣椒，紫紫的，看起来很好吃。心想：着么紫的小辣椒难道还会辣吗？肯定能好吃，心中便有了一种想法，摘一个尝一尝，反正爷爷也不在。我以飞一般的速度跑了过去。用手一拽，边摘了下来。我像得了一件宝贝似的，小心翼翼地捧着四色小辣椒，
第一单元专项练习 一、填空。 （2）从个位起，往左第五位是（）位，第（）位是亿位，万位的左面第一位是（）位，最高位是百亿位的数是（）位数。 （3）在854006007中，“8”在（）位上，表示（），“5” 在（）位上，表示（），“6” 在（）位上，表示（）。 （4）10个10亿是（），10个（）是一千万，（）个一千万是一亿。（5）四十六万八千零四十是由（）个十万，（）个万，（）个千和（）个十组成，它写作（）。 （6）十位上和千位上都是4的五位数中，最大的数是（），最小的数是（）。它们相差（）。 （7）最小的七位数是（），最大的六位数是（），它们的和是（），差是（）。 （8）一个十一位数，最高位是6，第七位是8，最低位是3，其余各位是0，这个数写作（），读作（），四舍五入到亿位写作（）。 （9）一个九位数，最高位和最低位上的数字都是6，十万位上的数字是5，其余数位上的数字都是0，这个数写作（），读作（）。（10）由五个千万，六个万，七个百和八个十组成的数写作（）。二、写出或读出下列各数。 八万三千零八写作（）；一千零一十万二千写作（）；六亿零七千写作（）； 五百六十亿零三万写作（）；814200读作（）； 108078000读作（）；200003050读作（）；603000004读作（）； 三、在○里填上“＞” “＜”或“＝”。（8分）
84亿○8400000000 639925○64亿 790000○709999 3404万○30440000 8888788○8887888 79018万○8亿 1101万○11010000 9999899○9998999 四、判断题。 （1）一千一千地数，数十次是一万。（） （2）最大六位数与最小六位数相差1。（） （3）两位数共有九十个。（） （4）8亿是8位数。（） （5）984650改写成万作单位约98万。（） 五、选择题。 （1）最高位是万位的数是（）。 A.五位数 B.六位数 C.七位数 D.十位数 （2）比10000少1的数是（）。 A.9000 B.9900 C.9009 D.9999 （3）八个千和八个十组成的数是（）。 A.800080 B.808 C.8080 D.8800 （4）把594900四舍五入到万位约是（）万。 （5）A.60 B.59 C.61 D.595 （5）李村去年工农业纯收入六百四十万零七十元，写作（）元。 A.64070 B.6407 C.6400070 D.640070 （6）四十、四万、四亿组成的数是（）。 A.4000040040 B.400040040 C.400004040 D.4000004004 （7）比最小的九位数少1的数是（）。 A.99999999 B.999999999 C.1000000001 D.9999999 六、把下列各数改写成用“万”或“亿”作单位的数。 230000=（）万 36700000=（）万 635000000=（）万
用on, across from, next to或between…and 完成下列句子。 1. 学校在书店的对面。 The school is ____ the bookshop. 2. 我坐在他旁边。 I sit ____ him. 3. 我们把这个书桌放在床和椅子之间。 Let’s put the desk ____ the bed ____ the chair. 4. 墙上有一些画。 There are some pictures ____ the wall. [Key: 1.across from 2.next to 3.between；and 4.on] 【辨析】这几个词语均用来表示位置，但用法有别。 ⑴on 作介词，常用于表示方位和地点，侧重指紧贴着某物，意为“在……上面”。 例如： 在教师的讲桌上有一些花。 There are some flowers on the teacher’s desk. There is a map of China on the wall. 墙上有一张中国地图。 注意：在美式英语中，表示“在街上”用介词on，但在英式英语中，则常用 介词in。例如： The Greens live on(in) Wall Street. 格林一家住在华尔街上。 Their house is on(in) that street. 他们家就在那条街上。 ⑵across from 相当于介词，在美式英语中表示“在……的对面；在……的对侧”， 表示此含义时，在英式英语中也可以只用across。例如： The pay phone is across from the library. 公用电话就在图书馆的对面。 Can you see the shop across from the river? 你能看到河对岸的商店吗？ ⑶next to 相当于介词，意为“紧挨着，紧靠着”。例如： There is an airport next to the park. 有个机场紧挨着那个公园。 The pay phone is next to the post office. Go along and turn left, you’ll see it. 公用电话紧挨着邮局。你向前走，往左拐就能看到它。 ⑷between…and 意思是“在……和……之间”，用于表示两者间。between是介 词，后接人称代词时，该人称代词必须用宾格形式。例如： The library is between the hotel and the supermarket. 图书馆在旅馆和超市之 间。 You sit between him and me. 你坐在我和他之间。 注意：between 后面也可以接代表两者的复数名词或代词。例如： The video arcade is between two shops. 电子游戏中心在两个商店之间。 They are twins. I can’t tell the difference between them. 他们是双胞胎，我看不出他们之间的区别。
大家都经历过让自己难忘的事，究竟是哪件事让你印象深刻至今难忘呢？下面是小编整理的“四年级难忘的一件事400字作文”，仅供参考，欢迎阅读，希望对大家有所帮助。 四年级难忘的一件事400字作文（一） 我已在世上度过了十个春秋，在我的记忆里，好多事已随着时间的流逝变得模糊啦，可是有件事像是烙在了我的脑海里，使我怎么也忘不掉。 记得那是一年夏天，天热的出奇，我和爸爸妈妈去文化宫玩，当时我穿着一件小背心和一条小短裤，鞋子走起路来还会发出“吱吱”的响声，我和爸爸妈妈坐在凳子上乘凉，我看到一群鸽子飞来，好美呀！有白的有灰的，可鸽子为什么又叫和平鸽呢？我疑惑的问爸爸，爸爸说：“传说在古代的时候，鸽子可以传递消息，因为鸽子象征着和平象征着祖国美好的未来，所以鸽子又叫和平鸽。”哦！原来是这么回事，那我就要犒劳犒劳它们啦，我买了一包玉米去喂鸽子，有一只鸽子很丑，好象是安徒生笔下的丑小“鸽”，可是，它并没有为它自己的丑而感到自卑，当我喂它们食物时它也和别的鸽子来抢食物，它的嘴啄一粒，就慢慢的吞下去，那样子真有趣，等它吃饱了，它就用翅膀盖住身子和头在那睡上了大觉！我喜欢上了那只“灰小鸽”，喜欢它那种自信，喜欢它那种乐观向上，喜欢它那种永不放弃。 每当我一看到鸽子，我就想起那只“灰小鸽”这也就成了我记忆最深的一件事，因为它使我明白了一个做人的道理，要自强-自信-自立！ 四年级难忘的一件事400字作文（二） 在我的成长历程里，有许多让我难忘的事，但它们都如过眼云烟，但今年暑假的这件事最让我刻骨铭心。 不知道从什么时候起，我的眼睛变的斜视，虽然不影响视力，但是却不美观。到医院去检查医生说：“需要做个小手术，不然长大后就不好做了。” 于是爸爸和奶奶带我到沈阳爱尔医院去做手术。在做手术之前经过一系列的检查后，准备做手术，爸爸和奶奶都很担心，妈妈也很挂念我，一个接一个的电话打过来，问我害怕吗？我还听见她在电话里哭了。其实，我很兴奋，一点也不害怕，因为我终于等到了那一刻，走进了手术室后，医生给我打上了麻醉针，我就什么也不知道了，美美的睡了一觉，当我醒来的时候，爸爸已经守护在我的床边，问长问短，我觉得很幸福。这时候妈妈又一个电话追来问我做的怎么样，爸爸说：＂做得很成功＂，妈妈心里的一块石头终于落地了。 第二天，打开纱布，我照镜子发现只是眼睛又红又肿，但是一点也不疼，在爸爸和奶奶的精心照顾下我终于出院了。 我觉得我很勇敢，我想大声对爸爸妈妈说：“我长大了，不要太为我担心了。” 四年级难忘的一件事400字作文（三） 今年暑假，社区组织我们参加“老少共读一本书《跨越时代的呼唤----焦裕禄》”活动。
------精品文档!值得拥有！------ 2012高考英语between…and…的用法 （1） I’ll phone you between lunch and three o’clock. 我将在午餐后三点钟以前给你打电话。 He felt something between laughter and anger. 他既觉得好笑，又感到气愤。 (2) 由于……和……(表示原因)。如： Between the noise outside and lack of sleep, he couldn’t concentrate. 由于外面的噪音加上睡眠不足，他无法专心。 注：between...and 不仅可连接两者，也可连接三者。如： Luxemburg lies between France, Germany and Belgium. 卢森堡位于法国、德国和比利时之间。 Between cooking, writing and running the farm he was kept very busy. 他又是做饭，又是写作，还要打理农场，忙得不可开交。 (3) 用于习语：between ourselves [you and me] 仅你我知道的秘密。如： Between ourselves, I don’t think he will live much longer. 咱们私下说说，他活不久了。 Between you and me, he’s not very reliable. 这是只有你我之间才说，他不是很可靠。 ------珍贵文档!值得收藏！------
最难忘的一件事四年级作文3篇 这件事，使我终生难忘。给了我鼓励与力量；给了我信心与希望，它已深深地印在我心里，伴我成长。下面是小编为大家整理的最难忘的一件事四年级作文，仅供参考，欢迎大家阅读。 最难忘的一件事四年级作文一今天我在家里找东西的时候，看到了一张照片，照片上一个小男孩拉着一个小购物车站在超市门口，开心地笑着。使我陷入了回忆。 哪天妈妈让我一个人去超市购物，我高兴的一蹦三尺高，因为那可是我长到现在从来没有过的事情啊！我拉着妈妈给我的小推车，走进了超市的大门。我先把我的小车车存在超市的储物台，就在超市里找了一个公用车车，推进了超市。超市里的东西真多，看得我琳琅满目。我第一个要买的东西是四盒0.5的铅，我找来找去都没找到，最和终于在文具类的下面，找到了0.5的铅。但是我又左右为难了有两种前一种是彩铅后一种不是彩铅。彩铅稍微贵一点，普通的铅不是很贵。我先来想去，最后买了彩铅，因为就贵几毛钱吗，还是一漂亮为主吧。 后来我又买了许多其他的东西，就去收银台付钱了。我先把我卖的所有东西放到收银台上，等收银的阿姨算完了
钱，统一结账。这次买的钱是40元，我给了阿姨100元，阿姨给我找了60元。等我把所有东西装进我带来的小车车，就回家了 通过这次买东西，我收获了自立。 最难忘的一件事四年级作文二在我的记忆里，我的童年有许多许多难忘的事，但其中有一件事更是让我最难忘。 这件事现在想起来，我都觉得很好笑——那就是掉牙!我换第一颗牙时，是妈妈带我去医院，医生给我打了麻药，把还没有松动的牙硬生生的给拔了下来，听医生说不拔就会影响新牙的`生长。等换第二颗牙时，我才深深地体会到了换牙的滋味。先是牙齿有点松动，吃东西时觉得有点碍事，但又好像不影响什么。于是我有事没事的就用舌头摇摇它，也不疼，心里总想它早早掉了就好了，可它偏偏好像快掉了，但又牢牢的不掉。渐渐的它越来越来松动了，我都感觉到好像就只有那么一点点肉连着它了，可它还是不掉。 有一天中午，妈妈做了我最爱吃的红烧排骨，我高兴地大吃了起来，正美美地吃着，突然，我咬到了块硬硬的东西，我以为我咬到了骨头，结果吐出来一看，是带着血的小白牙，其实这样掉牙一点儿都不疼，但是它掉得可真不是时候，那香喷喷的排骨是不能吃了，真是让我遗憾。 这件事虽然已经过去了很久，但这次掉牙的经历却让我
四年级数学（上册）期末考试试题 一、填空题（22分） 1. 四十五亿七千五百万写作( )，改写成用亿作单位的数是( )，省略亿后面的尾数的近似数是( )。 2. 两个数相除，商是56，余数是0.3，若被除数和除数同时扩大100倍，商是( )余数是( )。 3、写出下面各数的近似数.。 （1）省略万位后面的尾数1746003≈35482954≈ （2）省略亿位后面的尾数3708500000≈9964000000≈ 4、有一个七位数，减去1就变成六位数，这个七位数是（）。 5、比较大小。 527023○4969200 160000000○16亿180÷12○180÷15 150×2○15×20 6、712÷42，除数可以看成（）来试商，它的商是（）位数。 7、如图，已知∠1=60o，∠2=（），∠3=（），∠4=（） 2 1 3 8、一个数除以73商是6，且有余数，余数最大是（）。 4 9、甲数是乙数的3倍，甲数除以乙数的商是（）；如果甲数缩小5倍，要使商不变，乙数应当（）。 二、想一想，下面的说法对不对？对的打“√”，错的打“×”。（6分） 1、长方形是特殊的平行四边形。……………………（） 2、两个锐角的和一定比直角大。……………………（） 3、一个三位数除以两位数，商可能是一位数，也可能是两位数。…（） 4、估算3198÷39的结果约是100。………………………………（）。 5、638÷27=22…44 ……………………………………（）。 6、直线的长度是射线的两倍。…………………………………（）。 三、选择题(8分) 1. 下面各数中，一个零也不读出来的是( ) A 202200 B 200220 C 202020 2. 在一个整数的末尾添上两个“0”，原来的数就( ) A 扩大2倍 B 缩小2倍 C 扩大100倍 D 缩小100倍 3. 在有余数的除法中，除数和余数比较( ) A 除数比余数大 B 余数比除数小 C 除数和余数一样大 4、下列图形中，具有稳定性的是（）。 A、长方形 B、平行四边形 C、三角形 5、一个数除以13，商是103，余数是11，这个数是（）。 A、1339 B、1328 C、1350 6、382770000000≈（）亿 A、38277 B、3828 C、383 7、一个数，亿位和千位上都是7，其余各位上都是0，这个数写作（） A、700070000 B、700007000 C、700700000 8、9÷3=（9×3）÷（3×3）成立的依据是（）。 A、商不变的性质 B、乘除法的关系 C、小数的性质 四、计算我能行。（30分） 1、直接写出得数。（12分） 7×10= 360×2= 480÷60= 78÷13=