Acid-tolerant Listeria monocytogenes persist in a model food system fermented with nisin-producing bacteria

Aims: To investigate the induction of the acid tolerance response (ATR) in Listeria monocytogenes and to assess the persistence of the pathogen in broth fermented using a nisin-producing starter culture. Methods and Results: actic, acetic and hydrochloric acids were used to induce the ATR in L. monocytogenes growing at early exponential phase. Cells were then challenged in medium acidified to pH 3.5 with the same acid. Only lactic acid induced a detectable ATR. ATR(+) cells maintained their initial numbers after 1 h exposure while ATR(-) were reduced by c. 4 log(10) CFU. ATR(+) or ATR(-) cells were also inoculated in M17G broth fermented with nisin-producing (nis(+)) or control (nis(-)) Lactococcus lactis. When exposed to nisin, the numbers of ATR(+) cells were c. 2 log(10) CFU higher than non detectable ATR(-) cells at day 3. In the absence of nisin (nis(-) culture), L. monocytogenes was recovered from all ATR(+) and ATR(-) samples after 30 days. In contrast, no L. monocytogenes were recovered from any nis(+)ATR(-) samples but four of five nis(+)ATR(+) samples were positive for L. monocytogenes after 30 days. Conclusions: The ATR confers cross-resistance to nisin for at least 30 days in a system fermented by nisin-producing bacteria. Significance and Impact of the Study: The cross-resistance induced by the ATR should be considered for the safety of foods fermented with bacteriocin-producing cultures.