Behavior of Escherichia coli O26:H11 in the presence of Hafnia alvei in a model cheese ecosystem
Résumé
This study was designed to evaluate the capacity of three Hafnia strains to inhibit the growth of an E. coli strain O26:H11 in an uncooked pressed model cheese, in the presence or absence of a microbial consortium added to mimic a cheese microbial community. Inoculated at 2 log CFU/ml into pasteurized milk without Hafnia, the E. coli O26:H11 strain reached 5 log CFU/g during cheese-making and survived at levels of 4 to 5 log CFU/g beyond 40 days. Inoculated into milk at 6 log CFU/ml, all three tested Hafnia strains (H. alvei B16 and HA, H. paralvei 920) reached values close to 8 log CFU/g and reduced E. coil O26:H11 counts in cheese on day 1 by 0.8 to 1.4 log CFU/g compared to cheeses inoculated with E. coil O26:H11 and the microbial consortium only. The Hafnia strains slightly reduced counts of Entero coccus faecalis (similar to-0.5 log from day 1) and promoted Lactobacillus plantarum growth (+02 to 0.5 log from day 8) in cheese. They produced small amounts of putrescine (similar to 1.3 mmol/kg) and cadaverine (similar to 0.9 mmol/kg) in cheese after 28 days, and did not affect levels of volatile aroma compounds. Further work on H. alvei strain B16 showed that E. coil O26:H11, inoculated at 2 log CFU/ml, was inhibited by H. alvei 816 inoculated at 6 log CFU/ml and not at 4.5 log CFU/ml. The inhibition was associated neither with lower pH values in cheese after 6 or 24 h, nor with higher concentrations of lactic acid. Enhanced concentrations of acetic acid on day 1 in cheese inoculated with H. alvei B16 (4 to 11 mmol/kg) could not fully explain the reduction in E. coil O26:H11 growth. A synergistic interaction between H. alvei B16 and the microbial consortium, resulting in an additional 0.7-log reduction in E. coli O26:H11 counts, was observed from day 8 in model cheeses made from pasteurized milk. However, E. coil O26:H11 survived better during ripening in model cheeses made from raw milk than in those made from pasteurized milk, but this was not associated with an increase in pH values. In vitro approaches are required to investigate the mechanisms and causative agents of this interaction. H. alvei B16 appears to be a. promising strain for reducing E. coil O26:H11 growth in cheese, as part of a multi-hurdle approach. (C) 2012 Elsevier B.V. All rights reserved.