Escherichia coli O157:H7 isolated from feedlot-ground beef: bactericidal activity of AP7121 and synergistic effect with EDTA

Abstract

The emergence of multidrug-resistant Escherichia coli strains highlights a need for new therapeutic options. In Argentina, feedlot systems have acquired significant relevance as providers of meat products, such as ground beef. E. coli O157:H7 is one of the most important foodborne pathogens, with major implications in the meat products industry. AP7121 is a bacteriocin produced by the probiotic strain Enterococcus faecalis CECT7121. In this study, we aimed to investigate its bactericidal activity against E. coli O157:H7 (n=5) isolated from feedlot-ground beef and the synergistic effect with EDTA. 5 isolates of E. coli O157:H7 were isolated and characterized from feedlot-ground beef sold in Tandil County (Argentina) markets. Minimum Inhibitory Concentration (MIC) for AP7121, EDTA, and their combination against E. coli was performed (micro-dilution method). In vitro, the bactericidal activity of AP7121 alone or combined with EDTA (512 mg/L) was studied by carrying out time-kill curves to assess a synergistic effect. Samples were obtained for viable cell counts (0, 4, 8, and 24 h). MIC and time-kill curves were carried out three times. AP7121 (MICAP > 128 mg/L) alone did not show an inhibitory effect against E. coli; MIC EDTA was > 16000 mg/L. EDTA was not bactericidal. AP7121 combined with EDTA showed a lower MIC (MICAP 2-4 mg/L) with bactericidal activity against all E.coli isolates. A synergistic effect was observed at 4-8 and 24 h (-2.4 to -4.5 Δlog10 CFU/mL). AP7121 constitutes a candidate as a natural tool, combined with EDTA, against E.coli O157:H7. EDTA perturbs the cell membrane, allowing AP7121 to have a bactericidal effect against E.coli.