Monoclonal antibodies to avian Escherichia coli Iss

Escherichia coli infections are a major problem for the poultry industry in the United States. Yet, the virulence mechanisms operative in avian E. coli are poorly understood. In the present studies, monoclonal antibodies (MAbs) have been generated that may facilitate study of the pathogenesis of avian colibacillosis. These MAbs are directed against the Iss protein because results from our laboratory have shown that the possession of iss DNA sequences is strongly correlated with the E. coli implicated in avian colibacillosis. As part of an overall effort to explore the role of iss/Iss in colibacillosis pathogenesis, Iss protein has been purified, MAbs to Iss have been generated, and the MAbs are being evaluated. B cells from mice immunized with an Iss fusion to glutathione-S-transferase produced antibodies specifically against Iss, and these cells were used to generate the MAbs. These anti-Iss MAbs, when used in western blotting assays, can be used to distinguish iss-positive and -negative E. coli isolates, suggesting that they may be useful as reagents in the detection and study of virulent avian E. coli.     

Iss from a virulent avian Escherichia coli

No single characteristic of virulent avian Escherichia coli has been identified that can be exploited in colibacillosis detection protocols. Research in our lab suggests a strong association between the presence of an iss DNA sequence with an isolate's disease-causing ability. The study presented here focuses on the techniques used in the expression, purification, and characterization of avian E. coli Iss protein. In brief, iss was cloned into an expression vector, the construct was transformed into a protease-deficient E. coli, and expression was induced. The protein was expressed as a glutathione-S-transferase (GST) fusion and purified by affinity chromatography. The GST portion was cleaved from Iss, Iss was harvested by affinity chromatography, and the identity of Iss was confirmed by N-terminal sequencing. Currently, purified Iss is being used to prepare hybridomas for production of monoclonal antibodies with the goal of evaluating anti-Iss as a reagent for the detection of virulent avian E. coli.     

Immune response to recombinant Escherichia coli Iss protein in poultry

Colibacillosis accounts for significant losses to the poultry industry, and control efforts are hampered by limited understanding of the mechanisms used by avian pathogenic Escherichia coli (APEC) to cause disease. We have found that the presence of the increased serum survival gene (iss) is strongly associated with APEC but not with commensal E. coli, making iss, and the protein it encodes (Iss), candidate targets of colibacillosis control procedures. To assess the potential of Iss to elicit a protective response in chickens against APEC challenge, Iss fusion proteins were produced and administered subcutaneously to four groups of 2-wk-old specific-pathogen-free leghorn chickens. At 4 wk postimmunization, birds were challenged with APEC from serogroups 02 and 078 via intramuscular injection. At 2 wk postchallenge, birds were necropsied, and lesions consistent with colibacillosis were scored. Also, sera were collected from the birds pre- and postimmunization, and antibody titers to Iss were determined. Immunized birds produced a humoral response to Iss, and they had significantly lower lesion scores than the unimmunized control birds following challenge with both APEC strains. Birds that received the smallest amount of immunogen had the lowest lesion scores. Although further study will be needed to confirm the value of Iss as an immunoprotective antigen, these preliminary data suggest that Iss may have the potential to elicit significant protection in birds against heterologous E. coli challenge.     

Recombinant Iss as a potential vaccine for avian colibacillosis

Avian pathogenic Escherichia coli (APEC) cause colibacillosis, a disease which is responsible for significant losses in poultry. Control of colibacillosis is problematic due to the restricted availability of relevant antimicrobial agents and to the frequent failure of vaccines to protect against the diverse range of APEC serogroups causing disease in birds. Previously, we reported that the increased serum survival gene (iss) is strongly associated with APEC strains, but not with fecal commensal E. coli in birds, making iss and the outer membrane protein it encodes (Iss) candidate targets for colibacillosis control procedures. Preliminary studies in birds showed that their immunization with Iss fusion proteins protected against challenge with two of the more-commonly occurring APEC serogroups (O2 and O78). Here, the potential of an Iss-based vaccine was further examined by assessing its effectiveness against an additional and widely occurring APEC serogroup (O1) and its ability to evoke both a serum and mucosal antibody response in immunized birds. In addition, tissues of selected birds were subjected to histopathologic examination in an effort to better characterize the protective response afforded by immunization with this vaccine. Iss fusion proteins were administered intramuscularly to four groups of 2-wk-old broiler chickens. At 2 wk postimmunization, chickens were challenged with APEC strains of the O1, O2, or O78 serogroups. One week after challenge, chickens were euthanatized, necropsied, any lesions consistent with colibacillosis were scored, and tissues from these birds were taken aseptically. Sera were collected pre-immunization, postimmunization, and post-challenge, and antibody titers to Iss were determined by enzyme-linked immunosorbent assay (ELISA). Also, air sac washings were collected to determine the mucosal antibody response to Iss by ELISA. During the observation period following challenge, 3/12 nonimmunized chickens, 1/12 chickens immunized with 10 microg of GST-Iss, and 1/12 chickens immunized with 50 microg of GST-Iss died when challenged with the O78 strain. No other deaths occurred. Immunized chickens produced a serum and mucosal antibody response to Iss and had significantly lower lesion scores than nonimmunized chickens following challenge, regardless of the challenge strain. This study expands on our previous report of the value of Iss as an immunoprotective antigen and demonstrates that immunization with Iss can provide significant protection of chickens against challenge with three different E. coli strains.     

Cloning and sequencing of the iss gene from a virulent avian Escherichia coli

Control of colibacillosis is important to the poultry industry. We have found that the presence of a gene for increased serum survival, iss, is strongly correlated with Escherichia coli isolated from birds with colibacillosis. Therefore, the iss gene and its protein product, Iss, are potential targets for detection and control of avian colibacillosis. The iss gene was amplified from a virulent avian E. coli isolate and sequenced. The sequences of the gene and the predicted protein product were compared with those of iss from a human E. coli isolate and lambda bor. The iss gene from the avian E. coli isolate has 96.8% identity with the iss gene from the human E. coli isolate and 89.4% identity with lambda bor. The Iss protein from the avian isolate has 87% identity with Iss from the human isolate and 90% identity with Bor. The low identity between the two Iss proteins is because of a frame-shift in their respective coding sequences. In sum, iss from this avian E. coli isolate is very similar to iss from a human E. coli isolate, but because of a frameshift mutation in the coding sequence of iss from the human E. coli isolate, Iss proteins from avian and human E. coli isolates have only 87% identity. The strong association of iss with E. coli isolated from birds with colibacillosis, suggests that this sequence be studied for its value as a marker or target to be used in colibacillosis control.     

Avian pathogenic Escherichia coli (APEC)

Infections with avian pathogenic Escherichia coli (APEC) cause colibacillosis, an acute and mostly systemic disease resulting in significant economic losses in poultry industry worldwide. Avian colibacillosis is a complex syndrome characterized by multiple organ lesions with airsacculitis and associated pericarditis, perihepatitis and peritonitis being most typical. Environmental factors as well as the constitution of poultry or initial viral infections influence the outcome of APEC-infections. However, several challenge experiments in chickens proofed the role of virulent APEC strains as the single aetiological agent. Currently serotypes O1:K1, O2:K1 and O78:K80 are recognized as the most prevalent, however the number of published serotypes is increasing. In addition, single APEC isolates vary profoundly in virulence, and knowledge about the molecular basis of this variability is still scarce. Known virulence factors of APEC are adhesins (F1- and P-fimbriae), iron acquisition systems (aerobactin and yersiniabactin), hemolysins (hemolysinE and temperaturesensitive hemagglutinin), resistance to the bactericidal effects of serum and phagocytosis (outer membrane protein, iss protein, lipopolysaccharide, K/1)-capsule and colilcin production) as well as toxins and cytotoxins (heat stable toxin, cyto-/verotoxin and flagella toxin). Esperimental studies have shown that the respiratory tract, principally the gas-exchange region of the lung and the interstitium of the air sacs are the most important sites of entry for avian pathogenic E. coli. APEC strains adhere to the epithelial cells of air sacs presumably through F1-fimbriae. After colonization and multiplication the bacteria enter the bloodstream, and the temperature-sensitive hemagglutinin (tsh) seems to be important int his step. After invading the bloodstream APEC cause a septicemia resulting in massive lesins in multiple internal organs and in sudden death of the birds. The ability of the bacteria to acquire iron and the resistance to the bactericidal effects of serum, predominantly conferred by the increased serum survival (iss)--protein, enables APEC to multiply quickly in their hosts. Iss is regarded a specific genetic marker for avian pathogenic E. colistrains. A critical review of the literature published so far on APEC reveals, that these pathotypes are not defined appropriately. This findings urge investigations on the population structure of APEC, enabling the establishment of appropriate diagnostic tools and avoiding the obsolete use of serotyping for APEC diagnosis. So far more than 20 APEC strains have been investigated in animal experiments, explaining contrary published results. Thus, the lack of knowledge in pathogenicity and in immunity of APEC infections urges further experimental studies. As APEC share not only identical serotypes with human pathogens but also specific virulence factors, their zoonotic potential is under consideration.     

Comparative genomics of multiple plasmids from APEC associated with clonal outbreaks demonstrates major similarities and identifies several potential vaccine-targets

Avian pathogenic Escherichia coli (APEC) is associated with several types of extraintestinal infections, collectively known as colibacillosis. A heterogeneous population structure has hindered development of vaccines protective against all APEC. Recently, however, the existence of different APEC subpathotypes have been suggested, which are defined by specific disease syndromes and associated virulence genes. A collection of 14 APEC isolates representing clonal outbreaks of salpingitis accompanied by peritonitis and sepsis were characterized in the present study. All the strains carried large plasmids and the aim of the study was to investigate the similarity of these by sequencing, annotating and comparative analysis to identify potential vaccine targets. In addition, a comparison with gene content of human extraintestinal E. coli (ExPEC) subtypes was conducted. Results obtained demonstrated highly similar plasmid contents of the 14 APEC strains, despite the diversity of their chromosomal background. All 14 APEC carried the colicin V operon and numerous virulence genes. These included iss, traT, hlyF, eitABC, ompT, iroBCDEN, sitABCD, iutA and lucABCD. Several of these are shared with human ExPEC, implicating a possible zoonotic potential. Despite a diverse chromosomal background, it was concluded that the plasmid content of virulence genes are highly similar for the investigated APEC subpathotype. Based on their frequency, protein uniformity and subcellular localization iroN, iutA, iss, traT, ompT and etsC are suggested as vaccine-candidates. Experimental studies are, however, necessary to determine the protective potential of the candidates against the APEC subpathotype characterized by salpingitis, peritonitis and possibly septicaemia.     

Characterization of a series of transconjugant mutants of an avian pathogenic Escherichia coli isolate for resistance to serum complement

Colibacillosis, caused by avian pathogenic Escherichia coli (APEC) is a major problem for the poultry industry resulting in significant losses annually. Previous work in our lab and by others has shown that the increased serum survival gene (iss) is a common trait associated with the virulence of APEC. This gene was first described for its contributions to E. coli serum resistance. However, recently published research has called the contribution of iss to this trait into question. In the present study, the level of serum resistance conferred on an E. coli isolate by iss is examined. Additionally, the contribution of lambda bor gene to E. coli serum resistance is studied, as iss is thought to be derived from bor and bor occurs commonly among E. coli. To better understand the iss and bor contributions to serum resistance, a series of iss and bor mutants was generated. An iss deletion (iss-) mutant showed a significant drop in its resistance to serum. Similarly, a bor mutant showed a drop in serum resistance but not as drastic as that observed with the iss mutant, suggesting that iss contributes more to serum resistance than bor in this E. coli strain. Also, when iss was reintroduced into the iss- mutant the wild-type level of serum resistance was restored, confirming that the deletion of iss was responsible for the change in resistance seen in the mutant.     

Complement resistance-related traits among Escherichia coli isolates from apparently healthy birds and birds with colibacillosis

In this study, 294 Escherichia coli isolates from birds with colibacillosis were collected from disease outbreaks throughout the United States and were compared with 75 fecal E. coli isolates of apparently healthy chickens by their possession of several purported virulence genes, resistance to rough-lipopolysaccharide-specific bacteriophages (rLPSr), and elaboration of capsule. Traits were selected for study on the basis of their association with complement resistance. The genes targeted in this study included those encoding colicin V (cvaC) and the outer membrane proteins TraT (traT), OmpA (ompA), and Iss (iss). No significant differences were found between the two groups of isolates in the occurrence of cvaC-, traT-, or ompA-homologous sequences or in rLPSr. Only a few isolates were encapsulated, and the isolates of healthy birds were significantly more likely to be encapsulated than were the isolates of sick birds. However, iss, whether detected through hybridization or amplification, was found in more of the disease-associated isolates than in those of healthy birds. This difference was highly significant. Further, iss sequences were widely distributed among isolates of different serotypes from various avian host species and sites within these hosts. Such results suggest that possession of the iss sequence by an avian E. coli isolate may be a good indicator of that isolate's potential to cause disease. This association warrants further study because iss and the protein it encodes may be useful targets of future colibacillosis control efforts.     

Virulence factors of avian pathogenic Escherichia coli strains isolated from chickens with colisepticemia in Japan

The virulence factors of avian pathogenic Escherichia coli (APEC) isolated in Japan were investigated. Serogroups O, serotypes K1 and K5, and genes cva C, iss, iutA, papA, tsh, and usp, which have been thought to be related to virulence, were examined for their association with E. coli strains isolated from diseased and healthy chickens. The frequently recognized serogroups O1, O2, and O78 were found in 56 of 125 (44.8%) strains of diseased chickens (APEC) versus 13 of 100 (13.0%) strains of healthy chickens (commensal E. coli), a significant difference at risk ratio < 0.01. Although iss, iutA, and tsh were widely distributed in the APEC irrespective of O serogroup, papA, usp, and the K1 serotype were detected in serogroup O2 of APEC. The kfiD gene related to the K5 capsule and VT, LT, and ST genes related to exotoxins were not detected in any strains examined.     

禽病原性大肠杆菌部分毒力相关基因的克隆及序列分析

克隆并分析禽病原性大肠杆菌(avian pathogenic Escherichia coli,APEC)部分毒力基因,探寻APEC毒力因子的变异和进化发生关系。以APEC中国分离株为模板,PCR扩增其部分毒力基因(fimC,kpsM,csgA,papC,felA,cvaC,iss),并测定了这些毒力基因扩增片段的核苷酸序列,与GenBank中的同一基因进行序列比较。PCR扩增产物经克隆、酶切鉴定,均与预期结果一致,序列分析结果表明上述毒力因子在APEC中的保守性非常高,均能达到99%以上。与其他来源大肠杆菌的同一基因的同源性也非常高,但不同基因间有所差别。APEC分离株的受试部分毒力因子的变异程度非常小,保守性很高,一些毒力因子与人源致肠外感染大肠杆菌的毒力因子同源性也很高,说明APEC与人源致肠外感染大肠杆菌的亲缘关系很近。     

Characterization of extraintestinal Escherichia coli isolated from a peacock (Pavo cristatus) with colisepticemia

Extraintestinal infections by avian pathogenic strains of Escherichia coli (APEC) are commonly reported in poultry, but there is little information on infections by APEC in other bird species. Here we report on the characterization of extraintestinal E. coli isolated from a domesticated peacock, from the south of Brazil, that died of colisepticemia. Necropsy examination revealed congested liver, hypertrophied kidneys, peritonitis, severe typhlitis suggestive of coligranuloma, pneumonia, and airsacculitis--typical signs of colisepticemia. The isolates from lungs, kidney, heart, intestine, liver, and bone marrow all harbored the same virulence-associated factors (iucD, colV, iss, mat, fimC, ompA, traT crl, csgA vgrG, and hcp), yielded the same band pattern in amplified ribosomal DNA restriction analysis, and were allocated to the Escherichia coli Reference Collection group B1. The isolates were resistant to bacitracin, trimethoprim, and tetracycline, but displayed slight differences in their resistance to other antimicrobials. The isolates also differed in their virulence in 1-day-old chickens, but none displayed high virulence in vivo. We conclude that the peacock died of colisepticemia after it was infected with an extraintestinal E. coli strain of low virulence that nevertheless harbored virulence factors generally associated with APEC. This study represents the first characterization of an APEC isolated from a nonpoultry bird species.     

Characterizing the APEC pathotype

The purpose of this study was to compare avian pathogenic Escherichia coli (APEC) isolates to fecal isolates of apparently healthy poultry (avian fecal E. coli or AFEC) by their possession of various traits in order to ascertain whether APEC and AFEC are distinct and if the APEC strains constitute a distinct pathotype. Four hundred and fifty-one APEC and one hundred and four AFEC isolates were examined for possession of traits associated with the virulence of human extraintestinal pathogenic E. coli (ExPEC) as well as APEC. Several of the genes occurred in the majority of APEC and only infrequently in AFEC, including cvaC, iroN, iss, iutA, sitA, tsh, fyuA, irp2, and ompT. Of these genes, several have been found on large plasmids in APEC. Other genes occurred in significantly more APEC than AFEC but did not occur in the majority of APEC. Isolates were also evaluated by serogroup, lactose utilization, and hemolytic reaction. Twenty-nine and a half percent of the APEC and forty-two and three tenths percent of the AFEC were not serogrouped because they were not typeable with standard antisera, typed to multiple serogroups, were rough, autoagglutinated, or were not done. Around 65% of the typeable APEC (205 isolates) and AFEC (41 isolates) were classified into shared serogroups, and about a third of both fell into APEC- (113 isolates) or AFEC- (19 isolates) unique serogroups. Most were able to use lactose. No isolate was hemolytic. Overall, the majority of the APEC isolates surveyed shared a common set of putative virulence genes, many of which have been localized to an APEC plasmid known as pTJ100. This common set of genes may prove useful in defining an APEC pathotype.     

鸡大肠杆菌iss基因的克隆测序及原核表达

本实验对鸡大肠杆菌O2血清型菌株进行iss基因克隆测序，并在此基础上设计出两对套式引物，分别对含信号肽Miss基因序列和不含信号肽Miss基因序列进行扩增，并与原核表达载体pGEX-6p-1连接进行原核表达。iss基因克隆测序的结果与两组国外发表Miss基因序列进行比对，其同源性达100％。SDS—PAGE鉴定显示融合蛋白获得了较理想的表达，融合蛋白分子量分别约为36kD和33kD。     

Study on the porcinophilic foot-and-mouth disease virus I. production and characterization of monoclonal antibodies against VP1

Monoclonal antibodies (MAbs) reported here were produced against the porcinophilic foot-and-mouth disease virus (FMDV) that caused the devastating swine disease on 1997 in Taiwan. A panel (25) of MAbs were found to react with VP1 of O/Taiwan/97 (O/97) by ELISA with various potencies. The biological identities of these VP1 reacting MAbs, such as neutralization activity, isotype and capability to distinguish between two serotype O FMDVs, O/97 and O/Taiwan/KM1/99 (O/99), were further analyzed. Eleven out of the total eighteen O/97 neutralizing MAbs were able to neutralize heterologous O/99. Eight O/97 neutralizing and five non-neutralizing MAbs could differentiate two serotype O FMDVs by immunofluorescence assay (IFA) implied that these thirteen MAbs recognized O/97 specific epitope(s). Furthermore, reactivities of the VP1 reacting MAbs with a 29 amino acids synthetic peptide (P29) representing the betaG-betaH loop of VP1 were analyzed by ELISA and fourteen were found positive. MAb clone Q10E-3 reacting strongest with VP1 and P29, neutralizing both but not differentiating two serotype O viruses suggested that the antibody binding site might involve the RGD motif and its C terminal conserved region on betaG-betaH loop. MAbs with diverse characters presented in this study were the first raised against porcinophilic FMDV. The complete set of MAbs may be used for further studies of vaccine, diagnostic methods, prophylaxis, etiological and immunological researches on FMDV.     

Location of increased serum survival gene and selected virulence traits on a conjugative R plasmid in an avian Escherichia coli isolate

Avian colibacillosis is a costly disease for the poultry industry. The mechanisms of virulence employed by the etiologic agent of this disease remain ill defined. However, accumulated evidence suggests that complement resistance and the presence of the increased serum survival gene (iss) in an avian Escherichia coli isolate may be indicative of its ability to cause disease. This association of iss with the E. coli implicated in avian disease may mean that iss and/or, perhaps, the genes associated with it are important contributors to avian E. coli virulence. For this reason, we have begun a search for iss's location in the bacterial genome. Thus far, iss in an avian E coli isolate has been localized to a conjugative R plasmid and estimated to be about 100 kilobase (kb) in size, encoding resistance to tetracycline and ampicillin. Hybridization studies have revealed that this plasmid contains sequences with homology to tsh, a gene associated with virulence of avian E coli; intI 1, a gene encoding the integrase of Class 1 integrons; and certain genes of the aerobactin- and CoIV-encoding operons. Sequences homologous to merA, a gene of the mercury resistance operon, were not identified on this R plasmid. This plasmid, when transferred into an avirulent, recipient strain by conjugation, enhanced the transconjugant's resistance to complement but not its virulence, in spite of the plasmid's possession of several putative virulence genes and traits. Such results may reflect the multifactorial nature of virulence, the degree of the recipient's impairment for virulence, or an inability of the embryo assay used here to detect this plasmid's contribution to virulence. Additionally, this plasmid contains genes encoding antimicrobial resistances, which may provide a selective advantage to virulent E. coli in the production environment. Further study will be needed to determine whether this plasmid is widespread among virulent E. coli and to ascertain the implications that this link between virulence and antimicrobial resistance genes may have for poultry management.     

Evaluation of differentially expressed proteins following serum exposure in avian pathogenic Escherichia coli

Colibacillosis, caused by avian pathogenic Escherichia coli (APEC), is an extraintestinal disease that causes great economic loss to the poultry industry each year. APEC must overcome host defenses, such as immune system components found in serum, in order to establish infection; however, the mechanism of such serum resistance has been elusive. In the present study, a proteomic approach was used to evaluate APEC proteins that were differentially expressed after exposure to chicken serum to identify specific proteins that may be involved in serum resistance of APEC isolates. Proteins were isolated and separated by two-dimensional (2D) gel electrophoresis, and 10 protein spots corresponding to differentially expressed proteins were chosen for sequencing using electrospray ionization tandem mass spectrometry. Eight proteins were identified among the spots, some of which have previously been associated with the virulence of E. coli. Significantly, an outer-membrane protein previously associated with serum resistance, OmpA, was among those proteins identified, further indicating that differential regulation of this protein may be involved in serum resistance. This study opens the door to future research using a proteomic approach to identify the key players in serum resistance of APEC.     

Complement resistance,as determined by viable count and flow cytometric methods,and its association with the presence of iss and the virulence of avian Escherichia coli

Previous work in our labs has shown that avian Escherichia coli virulence is correlated with resistance to complement. Also, our studies have revealed that the presence of the increased serum survival gene (iss), known to contribute to the complement resistance and virulence of mammalian E. coli, may predict the virulent nature of an avian E. coli isolate. This relationship warrants further research, but further clarification of the relationship among virulence, complement resistance, and iss sequences requires use of complement susceptibility assays. Such assays, unfortunately, are labor-intensive, expensive, and difficult to perform. In the present study, the results of two complement susceptibility assays for 20 E. coli isolates, 10 incriminated in avian colibacillosis and 10 from the intestinal tracts of apparently healthy birds, were compared in an attempt to determine if flow cytometric analysis was a reasonable alternative to a viable count assay. In addition, the virulence of these isolates for chick embryos was determined, and each isolate was examined for the presence of iss using amplification techniques. The flow cytometric method was found to be repeatable for most isolates, and its results showed moderate agreement with those obtained through viable counts. All intestinal isolates of healthy birds proved avirulent using the embryo lethality assay; however, not all isolates from sick birds were demonstrated to be virulent. Possible explanations of these results include that the methods originally used to isolate these organisms failed to detect the illness-inciting strains or that the virulence of these strains had declined following initial isolation. Additionally, we must consider the possibility that the embryo lethality assay of virulence used here might not be sensitive enough to detect differences between these two groups of isolates. Also, it should be noted that virulence assays, such as the one used here, fail to account for predisposing host or environmental conditions, enabling a less virulent isolate to cause disease under natural conditions. Interestingly, the complement resistance of a strain was significantly associated with its lethality in embryos, and iss-containing isolates were significantly more likely than those lacking iss to be classified as complement-resistant and virulent. Such results, at least for this group of avian E. coli, suggest that there is a compelling but imperfect relationship among complement resistance, virulence, and the presence of iss. These results also suggest that the flow cytometric assay may be a reasonable alternative to the viable count method of determining complement resistance.     

Virulence factors of Escherichia coli, with emphasis on avian pathogenic isolates

E. coli bacteria isolated from localized and systemic disease processes in poultry are designated as Avian Pathogenic E. coli (APEC). The disease-inducing potential of these isolates has been explained by the occurrence of specific virulence factors. Despite the extensive literature on virulence factors for E. coli, unambiguous markers of virulence have not been identified yet. The relationship between serotyping and virulence is not straightforward either and raises the question whether E. coli infections in poultry should mainly be considered as opportunistic. Investigations into the occurrence of certain (combinations of) virulence factors in APEC isolates as virulence markers should fulfil the molecular version of Koch's postulates if the former question is to be answered.     

鸡致病性大肠埃希菌素V质粒研究进展

大肠埃希菌素V(ColV)质粒是鸡致病性大肠埃希菌中重要的毒力质粒之一,能够编码大肠埃希菌素V、血清抗性、铁摄取系统等与致病相关的毒力基因。目前虽然对鸡的大肠埃希菌病研究的比较广泛,但其确切的发病机制仍需继续深入研究。文章综述了ColV质粒与鸡大肠埃希菌病的联系,及其3个表型与鸡致病性大肠埃希菌毒力的关系,为防控鸡大肠埃希菌病提供新的思路。