P-033. Growth and Survival of Acid-Resistant and Non-Acid Resistant Shiga - Toxin Producing Escherichia coli (STEC) Strains during the Manufacture and Ripening of Microfiltered Milk Lactic Cheeses

M-P. Montet1, E. Jamet2, S. Ganet1, M. Dizin2, S. Miszczycha1, D. Thevenot1, C. Vernozy-Rozand1;
1ENVL, Marcy l'Etoile, FRANCE, 2ITFF, La Roche sur Foron, FRANCE.

Background: The ability of E. coli strains O157:H7 to survive in acidic conditions have been studied extensively but there are few reports about the tolerance to organic acids in foods by other Shiga-toxin producing E.coli (STEC) serotypes. The first objective of the present study was to investigate the growth and survival of acid resistant (AR) and non acid resistant (NAR) Shiga-toxin producing E.coli (STEC)strains during the manufacture and ripening of microfiltered milk lactic cheeses. The second objective was to check whether microfiltered milk lactic cheese could induce acid resistance in inoculated NAR STEC strains. Methods: For that purpose, six cocktails of AR and/or NAR STEC strains were inoculated separately, in microfiltered milk at a level of 103 CFU/g. Artisanal microfiltered cow’s milk lactic cheeses were prepared in the laboratory (P3 safety level) following industrial specification of the French Institute of Cheeses. Results: There were no biological differences between counts of NAR and AR STEC strains during the manufacture and ripening of the microfiltered milk lactic cheeses. To give general trends, whatever the cocktail of STEC strains used for the inoculation of the microfiltered milk, STEC counts increased by a range of 1 to 2 log10 CFU/g during the early steps of the cheeses manufacturing. Then the counts of STEC remained relatively stable from the salting until the drying stages of production and decreased throughout the ripening to reach 102 - 104 CFU/g at day 20. Exposure to SGF (pH: 2.0) reduced the number of NAR strains to undetectable levels within 40 min against 120 min for the AR STEC strains. These results suggest the absence of induction of acid-resistance of the 4 strains NAR by the mild acid conditions (4.65 - 6.46) noted throughout the manufacture and ripening of microfiltered milk lactic cheeses. Conclusion: The AR and NAR STEC were equally able to survive during the manufacture and ripening of microfiltered milk lactic cheeses. Moreover, this food matrix did not increase the acid tolerance of NAR STEC strains inoculated into microfiltered milk prior to the manufacture of cheeses.