The enterohemolysin phenotype of bovine Shiga-like toxin-producing Escherichia coli (SLTEC) is encoded by the EHEC-hemolysin gene

Naturally occurring enterohemolysin negative variants were observed during studies on bovine Shiga-like toxin-producing E. coli (SLTEC). Examination of three strains (413/89-1 and 332, 026:H-, and 570/89, O111:H-) and their isogenic variants (413/89-6, 332-I and 570/89-I, respectively) showed, that in each strain loss of the enterohemolytic phenotype correlated with the loss of a large plasmid ranging from 94 to 104 kb in size. The hemolysin determinant present on the 94 kb plasmid of strain 413/89-1 was cloned and discovered by DNA and N-terminal aminoacid sequence analysis to be highly homologous to the recently published EHEC-hemolysin (HlyEHEC; Schmidt et al., 1994; 1995). When a recombinant plasmid harboring this determinant was reintroduced into the enterohemolysin negative isogenic mutant 413/89-6, the enterohemolytic phenotype was restored. Southern blot hybridization analysis was used to demonstrate that the HlyEHEC is plasmid-borne in SLTEC-strains. Our cumulative data suggest that the enterohemolytic phenotype of SLTEC is encoded by the plasmid-borne HlyEHEC. These results further demonstrate the close similarity between SLTEC-isolates from bovine and human.

Clonal relatedness of Shiga-like toxin-producing Escherichia coli O101 strains of human and porcine origin

Shiga-like toxin (SLT)-producing Escherichia coli (SLTEC) O101 has recently been associated with hemorrhagic colitis and hemolytic-uremic syndrome in humans. In this study, SLTEC O101 strains from humans and pigs were characterized for clonal relatedness by nucleotide sequence analysis of their slt genes, DNA finger-printing of genomic DNA, and determination of virulence factors. The slt genes of five E. coli O101 strains were cloned and sequenced. For all strains, the deduced amino acid sequences of the B subunits were identical to those of the SLT-IIe present in the classical SLTEC O139 strains that cause edema disease in pigs. The A subunit revealed more than 99% homology to that of SLT-IIe. DNA fingerprinting revealed a high degree of genetic relatedness between the human and porcine O101 isolates. None of the O101 strains investigated had virulence factors frequently found in porcine (F107 fimbriae or heat-stable or heat-labile enterotoxins) or human SLTEC strains (eaeA or enterohemorrhagic E. coli hemolysin). The absence of virulence factors typical of SLT-I- and SLT-II-producing E. Coli together with the presence of SLT-IIe, a toxin previously seen only in porcine E. coli, suggests a new pathogenic mechanism for E. coli O101 infection of humans. For diagnostic purposes, we recommend the use of PCR primers and DNA probes complementary to slt-IIe to correctly identify such strains and to further evaluate their role in human diseases.

Association of enterohemolysin and non-fermentation of rhamnose and sucrose with Shiga-like toxin genes in Escherichia coli from calves

Fecal Escherichia (E.) coli strains from calves were tested for simply detectable phenotypical features associated with Shiga-like toxin (SLT) genes. DNA hybridization with SLT-specific oligonucleotide gene probes (detection of genes for SLT-I and SLT-II) was the "gold standard" for the evaluation of Vero cell cytotoxicity, fermentation of several saccharides, beta-D-glucuronidase activity and production of alpha-hemolysin (alpha-Hly) or enterohemolysin (E-Hly). While SLTEC and non-SLTEC did not significantly differ in production of alpha-Hly, beta-D-glucuronidase activity and fermentation of D-sorbitol, production of E-Hly and non-fermentation of L-rhamnose (Rha) and D-sucrose (Suc) were associated with SLT genes. Sensitivity and specificity of the E-Hly+ phenotype were 53% and 88% for identification of calf SLTEC. When three markers were combined to form the parameter ["E-Hly+ or (Rha- and Suc-)"], sensitivity was higher (65%) and specificity was almost the same (85%). Production of enterohemolysin and inability to ferment rhamnose and sucrose were more often associated with the SLT-I gene than with SLT-II genes. Approximately 71% SLT-I+ E. coli were positive in the enterohemolysin assay. The test combination "E-Hly+ or (Rha- and Suc-)" and Suc-)" was most valuable for the presumptive identification of SLT-I+ E. coli (sensitivity 85%, specificity 83%). These data suggest that the phenotype "E-Hly+ or (Rha- and Suc-)" may be a helpful marker for the detection of SLT-I+ E. coli in SLTEC associated diarrhoea of calves.

Synthesis and evaluation of a non-radioactive gene probe for the detection of C.perfringens alpha toxin

The synthesis and evaluation of a non-radioactive hybridization probe is described, specific detecting the Clostridium perfringens alphatoxin gene (plc) by colony blot hybridization assay. A vector free digoxigenin-dUTP-labelled probe was generated by polymerase chain reaction (PCR) targeting the cloned plc gene of C.perfringens strain ATCC 13124. In a colony blot hybridization assay 296 strains of C.perfringens were tested for plc. None of the strains failed in hybridization. Presence of plc was even demonstrated in C.perfringens strains reported to lack lecithinase activity. Specificity of the probe was shown with various strains of other bacterial species. None different Clostridia sp. tested, e.g. C.bifermentans, C.tertium, C.novyi, C.chauvoei, C.sporogenes, C.difficile, C.putrifucum, C.sordellii, C.botulinum, C. septicum and C.histolyticum, hybridized with the plc specific probe. Strains expressing an enzymatically related phospholipase like Bacillus cereus, Listeria monocytogenes and Staphylococcus aureus gave also negative results. Comparing the results of conventionally used egg yolk turbidity assay and those gained with DNA hybridization, the plc probe proved to be a much more sensitive and specific diagnostic tool for the detection of C.perfringens plc.

Neutralizing antibodies against Shiga-like toxins from Escherichia coli in colostra and sera of cattle

Previous or present infection with Shiga-like toxin producing E. coli (SLTEC) was detected by an indirect neutralization assay of antibody titer. Bovine colostra and sera blocked the cytotoxic effects of Shiga-like toxin on Vero cell monolayers. SLT neutralizing antibodies were present in 84.0% (189/225) of the colostrum samples from randomly chosen cows in Bavaria, Germany. While all of the colostra with neutralizing activity reacted with SLT-I, only 14.7% neutralized both SLT-I and -II. Approximately 93.0% (37/40) of sera from heifers had SLT neutralizing activity. To quantify the neutralizing antibodies, colostra were tested in the Vero cell assay for their capability to reduce the 50% cytotoxic dose (CD50) of SLT standards, where the neutralizing units/ml (nu/ml) equal the log10 of CD50 reduction. Almost half of reactive colostra (48.7%) reduced the CD50 of the SLT-I standard by 10(4) to 10(5) (4-5 nu/ml). Higher reactivity (5-7 nu/ml) was found in 46.5% of positive colostra. The remaining colostra samples had over 7 nu/ml. To determine if the colostra were blocking receptors for SLT on Vero cells, cells were preincubated with colostra, and SLT was later added. No neutralizing activity was detected, indicating the reactivity of colostra was directed against SLT. When the colostra were subjected to ammonium sulphate precipitation and DEAE anion exchange chromatography, high levels of neutralizing activity were found in the IgG1 containing fractions. Colostrum fractions were tested for SLT-I binding antibodies in a capture ELISA, based on the binding of SLT-I to the toxin receptor analogue P1-glycoprotein. Only fractions from colostra with over 5 nu/ml were reactive in this assay, indicating the ELISA was less sensitive than the Vero cell assay. The results support the theory that SLTEC exposure of cows in Germany is more widespread than expected from epidemiological studies based on bacterial isolation. This possibly indicates a higher risk of human SLTEC infection via beef and milk products.

Construction of recombinant Shiga-like toxin-IIv (SLT-IIv) and its use in monitoring the SLT-IIv antibody status of pigs

We constructed and purified recombinant B-subunits of the SLT-IIv as well as tested their usefulness in an immunoblot assay. The slt-IIvB gene amplified by PCR was ligated into the fusion vector pGEX-2T, and expressed in E. coli K 12 laboratory strains. Deletion of the signal sequence was necessary for optimal expression. High quantities of the fusion protein could be purified by affinity chromatography and subsequently used as antigen for immunoblot analysis with serum samples from diseased pigs and healthy controls. IgG antibodies against SLT-IIv were detected in the sera of 11 of 52 (21.15%) healthy pigs. By contrast, only in 1 of 28 (3.57%) serum samples of pigs with edema disease caused by SLT-IIv-producing E. coli we could demonstrate SLT-IIv-specific antibodies. During an outbreak of edema disease, sera from 10 pigs were taken at 4, 20, and 40 days after disease onset to investigate the immune response elicited by SLT-IIv. Immunoblot analysis with the recombinant SLT-IIv fusion protein revealed that the number of IgG-positive serum samples increased within this period of 40 days from one on day 4, to seven on day 20, to ten on day 40; the number of IgM-positive samples also increased from one after 4 days to eight after 20 days. Forty days after disease onset, IgM reactivity was no longer detectable. Since all animals seroconverted in the follow-up sera, the antigenicity of SLT-IIv during infection of pigs seems to differ from that of SLT-II in human hemolytic uremic syndrome where only a minority of patients are known to mount an immune response. The recombinant SLT-IIvB described here may be a possible candidate for vaccination trials.

[The immune response in edema disease of weaned piglets measured with a recombinant B subunit of shiga-like toxin II]

An outbreak of edema disease (ED) was monitored in 80 piglets after weaning over a period of 4 weeks. The shedding of Shiga-like toxin-IIe) producing Escherichia coli strains, the serum bactericidal activity (SBA) against SLTEC-IIe, and the antibody response against SLT-IIe were investigated. The antibody response was monitored by utilizing a glutathione-S-transferase (GST) + SLT-IIe B/SUB fusion protein (FRANKE et al., in press) for immunoblot assays. E. coli-strain GO15III (0141:K85ac) was diagnosed as SLT-IIe-producing E. coli by polymerase chain reaction, DNA hybridization and cytotoxicity assays. Maximum excretion of GO15III appeared between days 8 and 15 after weaning. On day 1 after weaning no piglet shed GO15III, while the number increased on day 8 to 53 (66.2%) and on day 15 to 59 (73.8%) of the piglets. 4 week after weaning, GO15III was only isolated from 23 (28.8%) of the piglets. In parallel, serum bactericidal activity against GO15III increased significantly in the sera of 73 (91.2%) piglets, reaching a stable maximum from day 15 on. During the first two weeks after weaning, no piglet yielded detectable SLT-IIe-IgG. However, the number of SLT-IIe-IgG positive piglets increased steadily from day 15. On day 15, 5 (6.2%) piglets were positive in SLT-IIe immunoblot analysis and 29 days after weaning the number increased to 31 (38.8%). These data represent the first serological monitoring of a natural outbreak of edema disease in piglets after weaning by using a recombinant fusion protein (GST+SLT-IIe B/SUB). The recombinant protein proved to be a useful diagnostical tool for monitoring the specific antibody status of piglets.

Nucleotide sequence analysis of enteropathogenic Escherichia coli (EPEC) adherence factor probe and development of PCR for rapid detection of EPEC harboring virulence plasmids

The 1-kb BamHI-SalI fragment from plasmid pMAR2 termed the enteropathogenic Escherichia coli (EPEC) adherence factor (EAF) probe was cloned in pUC19 and pK18. The nucleotide sequence of this fragment was determined, and a set of primers was designed to amplify a 397-bp region associated with pMAR2 by PCR. An analysis of the whole EAF sequence with database libraries indicated no significant homology to any known genes. However, between bases 701 and 787 of the fragment, an 82.8% homology between the EAF and the insertion sequence IS630 of Shigella sonnei exists. The results of PCR with primers of the EAF sequence demonstrated that all of the 151 EAF probe-positive EPEC strains with localized adherence to HEp-2 cells yielded positive EAF PCR results. In contrast, none of the 277 EAF probe-negative strains reacted to the EAF PCR. In addition, the PCR assay was successfully used to generate vector-free digoxigenin-labeled EAF fragments that gave valid results in colony blot hybridization assays. The EAF PCR appears to be a specific and efficient method for the detection of EPEC strains carrying the EAF plasmids.

[Comparative plasmid profile analysis of Salmonella typhimurium var. Copenhagen strains from a Salmonella outbreak in hospitalized horses]

From April 1990 through June 1991 clinical salmonellosis and asymptomatic faecal excretion of Salmonella spp. were seen in hospitalized horses at two veterinary hospitals. 76 Salmonella strains from hospitalized horses and 18 strains from horses without any clinical contact were characterized by serotyping and plasmid profile analysis. From April 1990 through January 1991 97.8% of the hospitalized horses were infected with strains of S. typhimurium var. Copenhagen, which were closely related according to their similar plasmid patterns. Other strains of S. typhimurium var. Copenhagen and serotype S. enteritidis were isolated not before February 1991. In the same period various plasmid profile types of S. typhimurium var. Copenhagen and strains of S. typhimurium, S. enteritidis and S. lexington were isolated from horses of the control group. Our results suggest that the high incidence of salmonellosis and latent salmonella infection in hospitalized horses was mainly due to the spread of one particular strain of S. typhimurium var. Copenhagen and that this strain was obviously acquired during hospitalisation.

Characterization of Shiga-like Toxin Producing Escherichia coli (SLTEC) Isolated from Calves with and without Diarrhoea

To determine if shiga-like toxin producing Escherichia coli (SLTEC) are involved in neonatal calf diarrhoea, isolated E. coli strains from diarrhoeic and non-diarrhoeic calves were characterized for shiga-like toxin (SLT) by colony blot hybridization and cytotoxicity assays. None of 150 E. coli strains isolated from diarrhoeic calves in 1985-1988 was positive for SLT, while 7/232 (3.0%) isolated in 1989 were positive for SLT. In contrast, samples collected during 1989 and 1990 from diarrhoeic calves were 21.9% SLTEC positive, and samples from non-diarrhoeic calves were 12.9% SLTEC positive. SLT I positive E. coli strains were isolated more often from diseased (17.8%) than from healthy animals (5.0%), while SLT II positive E. coli were more often detected in non-diarrhoeic (8.9%) than in diarrhoeic calves (4.1%). The mean percentage of SLT I positive E. coli in the whole E. coli flora of the samples was significantly higher in diarrhoeic than in healthy animals, implying a pathogenic role of SLT I producing E. coli in neonatal calf diarrhoea. Enterohemolysin was produced by 70.8% of the SLT I producing E. coli strains examined. Determination of O- and K-antigens of SLT positive E. coli revealed a highly diverse spectrum of SLTEC O-groups in calves. While no E. coli isolate belonged to serotype O157:H7, classical human enteropathogenic E. coli O-groups (O26, O111, O128) were detected. These results support the theory that cattle serve as a reservoir for human SLTEC infection.