Escherichia coli O26:H11 growth in co-culture with an antagonistic strain of Hafnia alvei according to pH and temperature

This study examines the combined effects of temperature (33 °C, 27 °C, 22 °C, 18 °C) and pH (5.3, 5.8, 6.3, 6.8), within the range of values encountered during cheese-making, on the growth of an Escherichia coli O26:H11 strain (21765) co-cultivated with an antagonistic strain of Hafnia alvei (B16) at different initial concentrations (4, 5, 6, 7 Log CFU/mL). Experimental designs including these parameters were applied to a liquid medium (Brain Heart Infusion) and to uncooked pressed model cheeses. In both these environments, the polynomial models gave a good prediction of E. coli O26:H11 levels after 6-24 h of incubation. In both predictive models, ANOVA analysis showed that temperature and pH both had a major positive effect on the level of E. coli O26:H11 at 6 h and the initial concentration of H. alvei had a negative effect. In BHI medium and in uncooked pressed model cheese at 24 h, temperature had a weaker positive effect when B16 was inoculated at 7 Log rather than 5 Log CFU/mL. In cheese at 24 h, pH also had a more marked positive effect on E. coli O26:H11 levels when H. alvei was inoculated at 7 Log. The predictive model established in BHI gave a good prediction of the levels of E. coli O26:H11 in uncooked pressed cheeses at 6 h but overestimated them at 24 h. H. alvei B16 inoculation at 7 Log CFU/mL associated with pH 5.3 and a temperature of 22 °C may help to control E. coli O26:H11 at 24 h in cheeses. Further studies with cheese production will be necessary to manage pH (via starter culture) in combination with temperature, without modifying the sensorial qualities of the cheese.