Virulence characterization of Shiga-toxigenic Escherichia coli isolates from wholesale produce

The 13 Shiga-toxigenic Escherichia coli (STEC) strains isolated from wholesale spinach and lettuce consisted mostly of serotypes that have not been implicated in illness. Among these strains, however, were two O113:H21 that carried virulence genes common to this pathogenic serotype (stx(2), ehxA, saa, and subAB), suggesting that their presence in ready-to-eat produce may be of health concern.     

Serotypes among Neisseria meningitidis serogroups B and C strains isolated in Canada

Antisera made to prototype serogroup B strains of Neisseria meningitidis were used to serotype, by agar gel double diffusion, 262 meningococcal serogroups B and C strains isolated in Canada. The strains included 93 from patients and 169 from carriers. Serotype 2 was associated with 39 of 75 (52%) of group B strains and 14 of 18 (77.8%) of group C strains isolated from patients. The group B strains were mainly (87.2%) serotype 2b, while the majority (92.2%) of group C strains was serotype 2a. Other serotypes (including a new provisional serotype) represented 25.3 and 5.5% of groups B and C strains, respectively. The new serotype accounted for 13% of the group B strains. Approximately 23% of the strains isolated from patients were nontypable. The distribution of serotype 2, nontype 2 (other serotypes), and nontypable strains isolated from carriers was 2.1, 36.6, and 61.3%, respectively, for group B meningococci and 22.2, 29.6, and 48.25, respectively, for group C meningococci. Serotype 11 was the most prominent of the strains isolated from carriers. Approximately 7% of all the strains were multiple serotypes. Serotype 2 is an important virulence marker associated with meningococcal groups B and C disease in Canada, with serotypes 2a and 2b being markedly associated with groups C and B meningococcal disease, respectively.     

合肥地区动物源性大肠埃希菌的血清型分布与耐药性分析

目的了解安徽省合肥地区动物源性大肠埃希菌的血清型分布和耐药状况,以期筛选出菌苗株和指导临床合理用药。方法对46份疑似大肠埃希菌病病料进行细菌分离培养、生化编码鉴定和致病性测定。采用玻片凝集试验对分离到的46株致病性大肠埃希菌进行血清型鉴定。同时分别采用K-B纸片琼脂扩散法和双纸片增效法检测致病性大肠埃希菌的耐药性和ESBLs阳性菌株。结果46株致病性大肠埃希菌中,除7株细菌未能定型外,其余39株细菌分布于10个血清型,O127：K63血清型为优势血清型,占定型菌株的33．33％。46株致病性大肠埃希菌对21种抗菌药物均呈现不同程度的耐药性,15个ESBLs阳性菌株表现为多重耐药,对各种抗菌药物的耐药率均高于ESBLs阴性菌株。结论O127：K63血清型为优势血清型,可作为菌苗株。合肥地区动物源性大肠埃希菌耐药性较为严重,尤其是产ESBLs大肠埃希菌多重耐药更为突出。     

抗大肠埃希氏菌K88ab,K88ac和K88ad特异单克隆抗体

A panel of twelve hybridoma cell lines, secreting specific antibodies to K88 adhesin antigens of enterotoxigenic Escherichia coli (ETEC) were established from eight separate fusions between mouse myeloma cell line Sp 2/0-Ag-14 and spleen cells from mice immunized with purified K88 antigens. Among the 12 monoclonal antibodies (MCA), K-A, K-35, K-11, and K-15 were K88a specific and reacted with all K88 adhesin bearing Escherichia coli strains tested, whatever K88ab, K88ac or K88ad they might be, as shown either in enzyme-linked immunosorbent assay (ELISA) or in direct agglutination test, whereas K32, K-4, and K-3 were specific for G88ab, K88ac, and K88ad respectively. The antigen patterns of 33 K88 bearing Escherichia coli strains covering 3 serotypes of K88ab, K88ac, and K88ad were analyzed by the use of these MCAs. The preliminary results showed that all Escherichia strains with the same serotype of K88 antigen shared at least one common type-specific antigenic determinant, that K88ad and K88ac strains enjoyed one common antigenic determinant that did not exist on K88ab strains, and that there were a few K88 antigenic determinants that appeared only on limited Escherichia coli strains of the same K88 serotype.     

Evaluation of a multiplex PCR for detection of serotypes K1, K2 and K5 in Klebsiella sp. and comparison of isolates within these serotypes

A multiplex PCR using targets within the serotype-specific region of the capsular polysaccharide synthesis gene cluster of serotypes K1, K2 and K5 was evaluated using the 77 reference serotype strains of Klebsiella, and a panel of clinical isolates subjected previously to conventional serotyping. The PCR was highly specific for these serotypes, which are those most associated with virulence in humans and horses. PCR confirmed that isolates of the K5 serotype had cross-reacted with antiserum for other serotypes, particularly for K7. K5 isolates received by our laboratory were almost exclusively from thoroughbred horses, and were submitted for screening prior to breeding programmes. Most, including a reference strain isolated in 1955, belonged to a cluster of genetically similar isolates of sequence type (ST) 60. K1 isolates, all from humans, belonged to a previously identified cluster of ST 23.     

Serotypes, virulence genes, and PFGE patterns of enteropathogenic Escherichia coli isolated from Cuban pigs with diarrhea.

Thirty-six enteropathogenic Escherichia coli strains isolated from Cuban pigs with diarrhea were serotyped and screened by PCR for the presence of virulence genes. The 36 isolates belonged to 11 O serogroups and 14 O:H serotypes, with 53% of the isolates belonging to only two serotypes: O141:H- (13 isolates) and O157:H19 (6 isolates). Genes coding for STb, STa, VT2e, and LT toxins were identified in 69, 61, 53, and 6% of the isolates, respectively. The most prevalent fimbrial adhesin was F18, detected in 22 (61%) isolates. The gene encoding F6 (P987) colonization factor was identified in three (8%) isolates. None of the 36 isolates assayed contained genes encoding F4 (K88), F5 (K99), or F41. The seropathotype O141:H-:STa/STb/VT2e/F18 (13 isolates) was the most frequently detected, followed by O157:H19:VT2e/F18 (5 isolates). A genetic diversity study, carried out by pulsed-field gel electrophoresis (PFGE) of 24 representative isolates, revealed 21 distinct restriction patterns clustered in 18 groups (I-XVIII). Isolates of the same serotype were placed together in a dendrogram, but isolates of serotype O157:H19 showed a high degree of polymorphism. The results of this study demonstrate the presence in Cuba of different clusters among one of the most prevalent serotypes isolated from pigs with diarrhea. Further experiments are needed to determine whether some of these clusters have appeared recently; if so, their evolution, as well as their possible association with pathogenicity in farms should be studied.     

Evaluation of Virulence Test Procedures for Yersinia enterocolitica Recovered from Foods

Recently a heat stable toxin (ST) and the vw factors of the plague bacteria were identified in Yersinia enterocolitica recovered from human infections. The vw factors were reported to be associated with a plasmid of 42-46 M Daltons in size and were essential for the expression of virulence. With this knowledge virulence tests were developed which allowed us to assess the virulence potential of food-borne Y. enterocolitica regardless of biotypes or serotypes. The tests evaluated were: (1) rapid presumptive test for the virulence plasmid; (2) gel electrophoretic confirmation of the virulence plasmid; (3) Laird's qualitative oral feeding test with thirst stressed mice; (4) quantitative LD₅₀ determination by i.p. injection of the mouse lethal ( i.e. serotype 0:8) strains in saline; (5) quantitative LD₅₀ determination of mouse non-lethal ( i.e. serotype 0:3) strains by i.p. injection of these strains suspended in 1 ml of 10% iron dextran saline solution for virulence enhancement. These tests were evaluated with the serotypes 0:3 and 0:8 strains associated with human infections with and without the virulence plasmid with reproducible results. Then the virulence tests procedures were applied to 79 food isolates. The virulence plasmid was detected only in the Nilehn biotype 2, 3 and 4 strains, but it was absent in Nilehn biotype 1 or the atypical strains that ferment rhamnose. The virulence of food and clinical isolates of Y. enterocolitica can be assayed fairly accurately with the above tests.     

Identification of Escherichia coli Recovered from Milk of Sows with Coliform Mastitis by Random Amplified Polymorphic DNA (RAPD) Using Standardized Reagents

A standardized-reagents commercial kit for random amplified polymorphic DNA (RAPD) analysis was used for typing 58 Escherichia coli strains that were recovered from the milk of sows, having coliform mastitis, within a single swineherd in Sweden. Previously, the 58 E. coli strains were characterized serologically and profiled biochemically. They were also evaluated for their serum resistance and their ability to adhere to fibronectin and bovine fetal fibroblasts. The RAPD analysis was fast, easily performed, and required only a nanogram of DNA. The indistinguishable banding patterns obtained with repeated analyses of 2 isolates from each strain demonstrated that RAPD analysis using standardized beads is a technique that provides reproducible results for typing E. coli strains that cause mastitis in sows. The results of the RAPD analyses demonstrated that E. coli sow mastitis strains are highly variable in serotype, biochemical profiles, virulence factors, and RAPD type, and that all 58 strains can be differentiated by means of the RAPD technique. The strains grouped into 24 RAPD types by combining the results of 2 primers, and into 38 groups by combining the results of serotype and RAPD type. No relationship between serotypes, virulence factors and RAPD types was found.     

Verotoxin-producing Escherichia coli in Spain: prevalence,serotypes, and virulence genes of O157:H7 and non-O157 VTEC in ruminants,raw beef products,and humans

In Spain, as in many other countries, verotoxin-producing Escherichia coli (VTEC) strains have been frequently isolated from cattle, sheep, and foods. VTEC strains have caused seven outbreaks in Spain (six caused by E. coli O157:H7 and one by E. coli O111:H- [nonmotile]) in recent years. An analysis of the serotypes indicated serological diversity. Among the strains isolated from humans, serotypes O26:H11, O111:H-, and O157:H7 were found to be more prevalent. The most frequently detected serotypes in cattle were O20:H19, O22:H8, O26:H11, O77:H41, O105:H18, O113:H21, O157:H7, O171:H2, and OUT (O untypeable):H19. Different VTEC serotypes (e.g., O5:H-, O6:H10, O91:H-, O117:H-, O128:H-, O128:H2, O146:H8, O146:H21, O156:H-, and OUT:H21) were found more frequently in sheep. These observations suggest a host serotype specificity for some VTEC. Numerous bovine and ovine VTEC serotypes detected in Spain were associated with human illnesses, confirming that ruminants are important reservoirs of pathogenic VTEC. VTEC can produce one or two toxins (VT1 and VT2) that cause human illnesses. These toxins are different proteins encoded by different genes. Another virulence factor expressed by VTEC is the protein intimin that is responsible for intimate attachment of VTEC and effacing lesions in the intestinal mucosa. This virulence factor is encoded by the chromosomal gene eae. The eae gene was found at a much less frequency in bovine (17%) and ovine (5%) than in human (45%) non-O157 VTEC strains. This may support the evidence that the eae gene contributes significantly to the virulence of human VTEC strains and that many animal non-O157 VTEC strains are less pathogenic to humans.     

Virulence of Yersinia pseudotuberculosis isolated from pork and from the throats of swine

Yersinia pseudotuberculosis was isolated from retail pork and from healthy swine throats. These wild-type strains and their representative cured isogenic strains were tested for the presence of plasmids and several virulence factors, and these characteristics were compared with those of virulent strains from humans. Two pork isolates (serotype IVB) and four swine isolates (serotypes IIB, IIC, III, and IVB) harbored a 42- to 48-megadalton plasmid which had similar fragmentation patterns resulting from digestion with restriction endonuclease. These six strains were lethal for mice via oral challenge and were positive in autoagglutination and calcium dependency tests. They also invaded HeLa cells and induced cytotoxicity. Histopathological examination and indirect fluorescent-antibody staining provided definite evidence of the pathogenicity of these strains when tissue sections from orally infected mice were used. The virulence factors of wild-type pork and swine isolates with the 42- to 48-megadalton plasmid were identical to those of two human isolates (serotypes IVB and VB). Hence, these pork and swine isolates should be considered potentially pathogenic for humans. The finding suggests that retail pork and swine may play an important role in the epidemiology of human infections caused by Y. pseudotuberculosis.     

Virulence of Yersinia pseudotuberculosis isolated from pork and from the throats of swine.

Yersinia pseudotuberculosis was isolated from retail pork and from healthy swine throats. These wild-type strains and their representative cured isogenic strains were tested for the presence of plasmids and several virulence factors, and these characteristics were compared with those of virulent strains from humans. Two pork isolates (serotype IVB) and four swine isolates (serotypes IIB, IIC, III, and IVB) harbored a 42- to 48-megadalton plasmid which had similar fragmentation patterns resulting from digestion with restriction endonuclease. These six strains were lethal for mice via oral challenge and were positive in autoagglutination and calcium dependency tests. They also invaded HeLa cells and induced cytotoxicity. Histopathological examination and indirect fluorescent-antibody staining provided definite evidence of the pathogenicity of these strains when tissue sections from orally infected mice were used. The virulence factors of wild-type pork and swine isolates with the 42- to 48-megadalton plasmid were identical to those of two human isolates (serotypes IVB and VB). Hence, these pork and swine isolates should be considered potentially pathogenic for humans. The finding suggests that retail pork and swine may play an important role in the epidemiology of human infections caused by Y. pseudotuberculosis.     

Escherichia coli serotyping and disease in man and animals.

Serotyping of Escherichia coli is useful, but complex, with 173 O antigens, 80 K antigens, and 56 H antigens, which can all be subdivided into partial antigens. The O, K, and H antigens can be found in nature in many of the possible combinations. The final number of E. coli serotypes is very high, 50,000-100,000 or more. The number of frequent pathogenic serotypes is, however, limited. Two main groups of such frequent serotypes are (i) serotypes from diarrhoeal disease and (ii) serotypes from extraintestinal disease. Serotypes from diarrhoeal diseases are mostly species specific, and could at present be used as epidemiological markers for bacterial clones equipped with special virulence markers, such as toxins and adhesins. Their O-antigen lipopolysaccharides may be regarded as virulence factors. These strains are not inhabitants of the normal intestine. Serotypes from extraintestinal diseases constitute a different set of clones, which are good colonizers of the intestinal tract, that under certain conditions succeed in invading host tissues. They are characterized by virulence factors different from those found in strains from diarrhoeal disease. Thus, the two groups of pathogenic E. coli are both composed of a limited number of clones for which the O:K:H serotypes are excellent, although not faultless, markers.     

Escherichia Coli and Virus isolated from “Sticky Kits”

A total of 121 Escherichia coli strains isolated from 3-week-old mink kits were serotyped and examined for virulence factors. 56 strains were isolated from healthy kits while 65 were from “sticky kits”. Among these, 34 different serotypes were detected. No difference in serotypes or the presence of virulence factors could be detected between healthy and diseased kits. By electron microscopy of faecal samples corona-, rota-, and calicivirus were demon-strated among healthy as well as diseased kits.     

一种新的人兽共患传染病——狐阴道加德纳氏菌病的研究 V.狐阴道加德纳氏菌菌株的血清分型研究

将我国6个省(区)、13个主要狐场分离的145株狐阴道加德纳氏菌,进行抗原性、免疫原性测定,从每场分离菌中选出1～3个优良株进行血清型研究。凝集素交叉吸收试验证实,选出的26株菌可划分为3个血清型,以此3个血清型代表株制备因子血清,余下119株菌中,108株在所划分的3个血清型内,11株未能定型。在3个血清型中,Ⅰ型菌株数占定型菌数的79.1％,因而确定,Ⅰ型菌是国内狐场狐阴道加德纳氏菌主要流行型。试验还明确了从貉分离的5株、水貂分离的4株、犬分离的2株阴道加德纳氏菌也属于血清Ⅰ型。将3个血清型代表菌株制成超声抗原,经免疫琼脂扩散试验证实,各型抗原与同型或异型免疫血清均可形成一条明显的融合沉淀线,表明各型菌间有共同抗原成分。同型菌免疫琼脂扩散试验表明,各株菌形成的沉淀线完全融合,从而证实了血清分型的可靠性。     

胸膜肺炎放线杆菌萘啶酸抗性菌株的选育和信号标签突变株的构建

胸膜肺炎放线杆菌(APP)是重要的猪呼吸道病原菌,给世界养猪业造成严重的经济损失.信号标签突变(STM)技术是在宿主动物体内鉴定病原菌毒力因子的高通量方法.通过体外传代选育出APP血清1型和3型萘啶酸抗性菌株,再以萘啶酸抗性菌株为受体菌,以携带mini-Tn10的标签质粒(pLOF/TAG1-48)的E.coli CC118 λ pir或S17-1λpir为供体菌,在或不在E.coli DH5α(pRK2073)的辅助下,进行三亲本或两亲本接合,通过抗性筛选、PCR和Southern杂交鉴定转座突变株.结果表明:体外萘啶酸加压传代很容易选育出萘啶酸抗性APP菌株,该抗性的产生与DNA促旋酶A亚基基因gyrA的突变有关.在APP与E. coli接合实验中,两亲本接合比三亲本接合操作更简单,效率也较高;APP不同菌株在接合和转座效率上存在很大差异,血清1型菌株高于血清3型菌株,3型标准菌株高于地方分离株JL03-R.本研究为APP STM突变体库的构建与毒力基因的鉴定奠定了基础.     

Evidence for two evolutionary lineages of highly pathogenic Yersinia species.

Sensitivity to Yersinia pestis bacteriocin pesticin correlates with the existence of two groups of human pathogenic yersiniae, mouse lethal and mouse nonlethal. The presence of the outer membrane pesticin receptor (FyuA) in mouse-lethal yersiniae is a prerequisite for pesticin sensitivity. Genes that code for FyuA (fyuA) were identified and sequenced from pesticin-sensitive bacteria, including Y. enterocolitica biotype 1B (serotypes O8; O13, O20, and O21), Y. pseudotuberculosis serotype O1, Y. pestis, two known pesticin-sensitive Escherichia coli isolates (E. coli Phi and E. coli CA42), and two newly discovered pesticin-sensitive isolates, E. coli K49 and K235. A 2,318-bp fyuA sequence was shown to be highly conserved in all pesticin-sensitive bacteria, including E. coli strains (DNA sequence homology was 98.5 to 99.9%). The same degree of DNA homology (97.8 to 100%) was established for the sequenced 276-bp fragment of the irp2 gene that encodes high-molecular-weight protein 2, which is also thought to be involved in the expression of virulence by Yersinia species. Highly conserved irp2 was also found in all pesticin-sensitive E. coli strains. On the basis of the fyuA and irp2 sequence homologies, two evolutionary groups of highly pathogenic Yersinia species can be established. One group includes Y. enterocolitica biotype 1B strains, while the second includes Y. pestis, Y. pseudotuberculosis serotype O1, and irp2-positive Y. pseudotuberculosis serotype O3 strains. E. coli Phi, CA42, K49, and K235 belong to the second group. The possible proximity of these two iron-regulated genes (fyuA and irp2), as well as their high levels of sequence conservation and similar G+C contents (56.2 and 59.8 mol%), leads to the assumption that these two genes may represent part of an unstable pathogenicity island that has been acquired by pesticin-sensitive bacteria as a result of a horizontal transfer.     

Production of cytolethal distending toxin and other virulence characteristics of Escherichia coli strains of serogroup O86

Genetic and phenotypic virulence markers of different categories of diarrhoeagenic Escherichia coli were investigated in 106 strains of enteropathogenic E. coli (EPEC) serogroup O86. The most frequent serotype found was O86:H34 (86%). Strains of this serotype and the non motile ones behaved as EPEC i.e., carried eae, bfpA and EAF DNA sequences and presented localised adherence to HeLa cells. Serotypes O86:H2, O86:H6, O86:H10, O86:H18, O86:H27 and O86:H non determined, belonged to other categories. The majority of the strains of serotype O86:H34 and non motile strains produced cytolethal-distending toxin (CDT). The ribotyping analysis showed a correlation among ribotypes, virulence markers and serotypes, thus suggesting that CDT production might be a property associated with a universal clone represented by the O86:H34 serotype.     

Phenotypic and genotypic analysis of pathogenic Escherichia coli virulence genes recovered from Riyadh,Saudi Arabia

The current study was carried out to evaluate the phenotypic and genotypic characterization of avian pathogenic Escherichia coli recovered from Riyadh, Saudi Arabia. During the period of 10th February–30th May 2015, 70 E. coli strains were isolated from chicken farms located in Riyadh, Saudi Arabia. All strains were tested phenotypically by standard microbiological techniques, serotyped and the virulence genes of such strains were detected by polymerase chain reaction (PCR). Most of the recovered strains from chickens belonged to serotype O111:K58 25 strains (35.7%), followed by serotype O157:H7 13 strains (18.57%), followed by serotype O114:K90 10 strains (14.29%), then serotype O126:K71 9 strains (12.9%), serotype O78:K80 8 strains (11.43%) and in lower percentage serotype O114:K90 and O119:K69 5 strains (7.14%). The virulence genotyping of E. coli isolates recovered from broilers revealed the presence of the uidA gene in all the field isolates (6 serovars) examined in an incidence of 100%, as well as the cvaC gene was also present in all field isolates (6 serovars), while the iutA gene and the iss gene were detected in 5 out of 6 field serovars in an incidence of 81.43% and 64.29%, respectively. Phenotypical examination of the other virulence factors revealed that 65 isolates were hemolytic (92.9%), as well as 15 isolates (21.42%) were positive for enterotoxin production. Meanwhile, 21 isolates (30%) were positive for verotoxin production, 58 isolates (82.86%) for the invasiveness and 31 isolates (44.29%) for Congo red binding activities of the examined serotypes.     

Pandemic strains of O3:K6 Vibrio parahaemolyticus in the aquatic environment of Bangladesh

A total of 1500 environmental strains of Vibrio parahaemolyticus, isolated from the aquatic environment of Bangladesh, were screened for the presence of a major V. parahaemolyticus virulence factor, the thermostable direct haemolysin (tdh) gene, by the colony blot hybridization method using a digoxigenin-labeled tdh gene probe. Of 1500 strains, 5 carried the tdh sequence, which was further confirmed by PCR using primers specific for the tdh gene. Examination by PCR confirmed that the 5 strains were V. parahaemolyticus and lacked the thermostable direct haemolysin-related haemolysin (trh) gene, the alternative major virulence gene known to be absent in pandemic strains. All 5 strains gave positive Kanagawa phenomenon reaction with characteristic beta-haemolysis on Wagatsuma agar medium. Southern blot analysis of the HindIII-digested chromosomal DNA demonstrated, in all 5 strains, the presence of 2 tdh genes common to strains positive for Kanagawa phenomenon. However, the 5 strains were found to belong to 3 different serotypes (O3:K29, O4:K37, and O3:K6). The 2 with pandemic serotype O3:K6 gave positive results in group-specific PCR and ORF8 PCR assays, characteristics unique to the pandemic clone. Clonal variations among the 5 isolates were analyzed by comparing RAPD and ribotyping patterns. Results showed different patterns for the 3 serotypes, but the pattern was identical among the O3:K6 strains. This is the first report on the isolation of pandemic O3:K6 strains of V. parahaemolyticus from the aquatic environment of Bangladesh.     

山东省某地区奶牛源大肠埃希菌的血清型、耐药特性及分子特性

【目的】旨在对从山东省某地区4个健康奶牛养殖场分离到的大肠埃希菌进行优势血清型、耐药特性、Ⅰ类整合子基因盒携带情况以及系统进化群分析。【方法】采集194份来自山东省某地区4个规模化奶牛场奶牛新鲜粪便样品,进行大肠埃希菌分离和鉴定,利用常用大肠埃希菌诊断血清进行血清型鉴定;利用10%的绵羊血平板检测溶血性;利用K-B法检测对14种常规抗菌药物的敏感性;利用聚合酶链式反应(PCR)检测革兰阴性菌常见的6大类24种耐药基因、Ⅰ类整合子基因盒结构并对目的条带测序分析;利用细菌多位点序列分型(Multilocus sequence typing,MLST)技术分析大肠埃希菌的ST型并使用eBURST v3软件分析菌株之间的克隆关系。【结果】从194份新鲜粪便样品中分离到171株大肠埃希菌,其中主要为致病性(19.9%)和侵袭性大肠埃希菌(17.0%),优势血清型分别为O128:K67(12/171)和O143:K7(12/171)。另外,具有溶血性的大肠埃希菌阳性率为9.4%(16/171);药敏试验结果显示多重耐药菌株的比率为22.2%,其中对氨苄西林耐药率最高为33.9%,四环素次之,为24.0%;PCR检测耐药基因和整合子结果显示,59.1%的菌株携带β-内酰胺类耐药基因blaTEM,59.1%的菌株携带氨基糖苷类耐药基因ant(2′),未检测到四环素耐药基因tetA和tetB;Ⅰ类整合子的阳性率为4.1%(7/171),dfrA12-aadA2-sul1为优势基因盒结构(4/171);MLST将大肠埃希菌分为8种ST型,其中,ST155(10/171)和ST58(45/171)形成一个克隆复合物且没有发现新的ST型。【结论】本研究证实,从该地区规模化健康奶牛场新鲜粪便中分离到的大肠埃希菌优势血清型为O128:K67和O143:K7;少部分大肠埃希菌具有溶血性;仅对氨苄西林、四环素等具有较高的耐药率;优势基因盒结构为dfrA12-aadA2-sul1;MLST分型显示不同奶牛场分离出亲缘关系较近的菌株,其分布具有多态性,血清型与ST型之间无相关性。本研究表明源自表观健康的奶牛的大肠埃希菌存在多重耐药现象,具有食品公共卫生安全隐患,该研究对于提升规模化奶牛场奶制品的安全生产与质量评估具有一定的理论指导意义。