Lactic Acid Bacteria as an Intervention Against Shiga Toxin-Producing Escherichia coli in Beef
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Shiga toxin-producing Escherichia coli (STEC) are Gram-negative bacteria that are able to cause disease in humans and animals. Beef is a significant transmission vehicle of foodborne outbreaks of STEC, which are commensal inhabitants of the gastrointestinal tracts of cattle and may be shed in the feces of animals and may be transferred onto the surfaces of beef during harvest via cross-contamination. Concern has been expressed surrounding the utility of studies describing the efficacy of antimicrobial interventions targeting the STEC that inoculate chilled versus non-chilled beef carcasses. The objectives of this study were to evaluate the effects of chilling on STEC attachment to brisket surfaces, and the effects of post-inoculation storage on STEC recovery. Paired briskets from split carcasses were separated; one brisket from each pair was kept non-chilled, while the other was chilled to a surface temperature of ≤5 °C prior to inoculation. Briskets were inoculated with a cocktail of eight STEC and then stored at 5 or 25 °C. At 0, 30, 60, 90 and 120 min post-inoculation, 30 cm² of tissue was aseptically excised, followed by selective enumeration of strongly and loosely attached STEC. A significant, though small (0.4 log10 CFU/cm²), difference in the numbers of strongly attached cells was observed between non-chilled and chilled briskets (p<0.05). Significant effects on cell attachment by the interaction of chilling and post-inoculation storage period, or chilling and post-inoculation storage temperature, were identified (p<0.05). Results indicate beef chilling and post-inoculation storage conditions influenced STEC attachment to beef. Lactic acid bacteria (LAB) produce antimicrobial substances such as organic acids, bacteriocins, and/or hydrogen peroxide that may have antagonistic activities against STEC. However, additional research is needed to assess the inhibitory potential of LAB interventions against non-O157:H7 STEC in beef. The objective of the subsequent study was to assess the antimicrobial efficacy of a commercial LAB mixture, applied via electrostatic or conventional spray, for the disinfection of STEC on beef strip loins during refrigerated aging in vacuum packaging for 14 or 28 d. Pre-rigor, non-chilled beef was inoculated with a cocktail of eight STEC, chilled for 24 h to ≤ 4 °C, and subsequently treated with a solution containing 8.7 ± 0.1 log10 CFU/ml LAB using either a hand-held pressurized tank air sprayer or an air-assisted electrostatic sprayer. Electrostatic and conventional sprays did not differ (p>0.05) in terms of STEC reductions, strip loin surface pH values, nor levels of applied LAB. Application of LAB (6.5 ± 0.1 log10 CFU/cm²) significantly reduced STEC populations (p<0.05) from pre-treatment levels (7.2 ± 0.01 log10 CFU/cm²), though pathogen numbers enumerated after 14 days of aging were not different (p≥0.05) from pre-treatment counts. Mean STEC numbers on beef sampled after 28 days of aging were significantly lower (p<0.05) than pre-treatment means, though the difference was numerically small (0.5 log10 CFU/cm²). Biopreservatives such as LAB can be useful for reducing pathogens on food surfaces, though optimization of the numbers and activity of such interventions must be completed to assure maximal food safety preservation.
Kirsch, Katie Rose (2015). Lactic Acid Bacteria as an Intervention Against Shiga Toxin-Producing Escherichia coli in Beef. Master's thesis, Texas A & M University. Available electronically from