pulsed-field gel electrophoresis profile changes resulting from spontaneous chromosomal deletions in enterohemorrhagic Escherichia coli O157:H7 during passage in cattle.

A total of 905 enterohemorrhagic Escherichia coli (EHEC) O157:H7 isolates that were recovered from experimentally infected cattle, in addition to the inoculated strain, were analyzed by pulsed-field gel electrophoresis (PFGE). Twelve PFGE profiles other than that of the inoculated strain were observed. We successfully identified five distinct chromosomal deletions that affected the PFGE profiles using whole-genome PCR scanning and DNA sequencing analysis. The changes in PFGE profiles of EHEC O157:H7 isolates after passage through the intestinal tract of cattle were partially generated by deletion of chromosomal regions.

Changes in antimicrobial susceptibility in a population of Salmonella enterica serovar Dublin isolated from cattle in Japan from 1976 to 2005.

OBJECTIVES: We investigated the antimicrobial susceptibilities and resistance mechanisms of cattle-adapted Salmonella enterica serovar Dublin isolated in Japan in the past 30 years. This study is an example of evaluation of the impact of introduction of antimicrobials in veterinary medical practice on the selection of resistance in S. enterica. METHODS: The antimicrobial susceptibilities and prevalence of R-plasmids in Salmonella Dublin isolated in Japan from 1976 to 2005 were investigated. To evaluate the importance of gyrA mutation and active efflux, we derived the gyrA revertants and acrAB deletion mutants, and then compared with their parental strains the MICs of quinolone antimicrobials such as nalidixic acid, enrofloxacin, ofloxacin and ciprofloxacin. RESULTS: Salmonella Dublin isolates with R-plasmids and resistance to more than three antimicrobials were predominant between 1981 and 1995. From the latter half of the 1990s to the present, Salmonella Dublin isolates without R-plasmids became dominant. The introduction of nalidixic acid into the veterinary field in the mid-1980s was followed by the emergence of nalidixic acid-resistant isolates, which are now predominant. We found only a single gyrA mutation (Asp-87-->Tyr) among the nalidixic acid-resistant isolates. Although the reduced susceptibilities to the fluoroquinolones were observed among the nalidixic acid-resistant isolates, none of the isolates was resistant to the fluoroquinolones used in this study. The MIC data for the fluoroquinolones differed up to 4-fold. Results of the susceptibility test using gyrA revertants and acrAB mutants suggest that the isolates with the gyrA mutation were selected by the use of nalidixic acid, and the AcrAB-TolC system accounts for the decreased fluoroquinolone susceptibilities. CONCLUSIONS: These data suggest that the introduction of nalidixic acid in veterinary medicine seemed to affect the susceptibilities of Salmonella Dublin among the cattle population in Japan, whereas the introduction of enrofloxacin has not caused any additional effect. The prudent use of antimicrobials in the veterinary field should be continuously stressed.

Detection and characterization of variant Salmonella genomic island 1s from Salmonella Derby isolates.

The purpose of this study is to investigate the distribution and structure of Salmonella genomic island 1 (SGI1) among Salmonella enterica serovar Derby isolates from swine and their rearing environment. Three variants of SGI1s, specifically SGI1-A, C, and I, were identified by PCR mapping. The results of macro-restriction analysis and DNA sequencing of SGI1 flanking regions revealed that there are at least two genomic lineages of Derby strains bearing SGI1s.

Comparison of PCR-RFLP and PFGE for determining the clonality of enterohemorrhagic Escherichia coli strains.

We report here on a comparative evaluation of PCR-restriction fragment length polymorphism (PCR-RFLP) and pulsed-field gel electrophoresis (PFGE) assays, and ascertain the clonal relationship between 13 enterohemorrhagic Escherichia coli O157 : H7 strains isolated from fecal samples collected from three cows over a period of 2 months. PCR-RFLP analysis was carried out with either BglI or EcoRV digested LA-PCR amplicons, generated by targeting region V of the Stx-phage. While PCR-RFLP analysis placed these 13 strains into a single clonal type, pulsotyping analysis, as reported earlier, grouped these strains into four different PFGE subtypes of which three were closely related, while the other appeared to be different. The comparative analysis was extended further using two clonally different wild-type (3-0 and Sakai 215) strains and 17 derivative strains which had passed through an animal's gastrointestinal tract. The PCR-RFLP assay, which was not only able to differentiate the wild-type strains, but also placed the passaged derivative strains into their respective parental group, although PFGE patterns of the same set of strains resulted from different PFGE subtypes. These data indicate that PCR-RFLP is the more reliable and useful assay for a molecular epidemiological survey of enterohemorrhagic E. coli strains.