Frontiers in Cellular and Infection Microbiology (Jun 2013)
Phage Biocontrol of Enteropathogenic and Shiga Toxin-Producing Escherichia coli in Meat Products
Abstract
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 previously characterized virulent strains of bacteria inoculated on pieces of beef were determined. We evaluated the reductions of viable cells (VC) of Escherichia coli O157:H7 and non-O157 Shiga toxigenic E. coli (STEC) 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 (EPEC) strain. Significant VC 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, 1.7x1010 plaque forming units (PFU/mL) for DT1 and 1.4x1010 PFU/mL for DT6, the most effective. When EPEC and STEC (O157:H7) strains were tested, we obtained VC 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 6h. In contrast, all the nonpathogenic strains of E. coli as well as the other enterobacteria tested were resistant. In addition, phage cocktail was evaluated on two strains and further biocontrol 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 (6.5x10-7 - 1.8x10-6) and variable stability and reversion. Results indicate that isolated phages were stable on storage, negative for all the virulence factors assayed, highly specific for virotypes of E. coli and could be useful in reducing STEC and EPEC VC in meat products.
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