marA and sdiA overexpression was a common feature of multidrug-resistant mutants selected by growth in lomefloxacin and ceftazidime. To our knowledge, this report is the first to describe in vitro selection with a fluoroquinolone or ceftazidime triggering sdiA overexpression in E. coli isolates.

Ten bacteriophages were isolated from faeces and their lytic effects assayed on 103 pathogenic and non-pathogenic Enterobacteriaceae. Two phages (DT1 and DT6) were selected based on their host ranges, and their lytic effects on pathogenic E. coli strains inoculated on pieces of beef were determined. We evaluated the reductions of viable cells of Escherichia coli O157:H7 and non-O157 Shiga toxigenic E. coli strains on meat after exposure to DT6 at 5 and 24°C for 3, 6, and 24 h and the effect of both phages against an enteropathogenic E. coli strain. Significant viable cell reductions, compared to controls without phages, at both temperatures were observed, with the greatest decrease taking place within the first hours of the assays. Reductions were also influenced by phage concentration, being the highest concentrations, 1.7 × 1010 plaque forming units per milliliter (PFU/mL) for DT1 and 1.4 × 1010 PFU/mL for DT6, the most effective. When enteropathogenic E. coli and Shiga toxigenic E. coli (O157:H7) strains were tested, we obtained viable cell reductions of 0.67 log (p = 0.01) and 0.77 log (p = 0.01) after 3 h incubation and 0.80 log (p = 0.01) and 1.15 log (p = 0.001) after 6 h. In contrast, all nonpathogenic E. coli strains as well as other enterobacteria tested were resistant. In addition, phage cocktail was evaluated on two strains and further reductions were observed. However, E. coli bacteriophage insensitive mutants (BIMs) emerged in meat assays. BIMs isolated from meat along with those isolated by using the secondary culture method were tested to evaluate resistance phenotype stability and reversion. They presented low emergence frequencies (6.5 × 10−7–1.8 × 10−6) and variable stability and reversion. Results indicate that isolated phages were stable on storage, negative for all the virulence factors assayed, presented lytic activity for different E. coli virotypes and could be useful in reducing Shiga toxigenic E. coli and enteropathogenic E. coli viable cells in meat products.

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SUMMARY:The aim of this study was to determine the prevalence and virulence factors of Shigella species isolated from patients with diarrhea. Shigella species were isolated from 1,022 stool samples collected from different hospitals in Rosario, Argentina. The isolates were characterized using phenotypic tests, serotyping, and detection of virulence genes by PCR. One hundred strains (9.8z of samples collected) of Shigella were isolated. Shigella flexneri was the most frequently identified species (74z), followed by S. sonnei (26z). S. flexneri was also the predominant species isolated from children aged 6-14 years. These clinical strains of Shigella were then tested for the presence of ipaH, virA, ial, sen, and set using specific primers. virA was present in all strains, whereas ipaH was detected in 98z of strains and ial in 83z. sen was found in 71.6z of S. flexneri and 42.3z of S. sonnei isolates, and 41.9z of S. flexneri isolates were positive for set. Furthermore, 32.4z of S. flexneri isolates were positive for both set and sen. This study provides data on the prevalence and distribution of diverse Shigella strains.

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