Purpose: To investigate the adaptive responses of Salmonella Enteritidis to lethal pH (laboratory medium, mayonnaise) induced by exposure to sublethal acid conditions (low pH in the presence of undissociated acetic acid).
Methods: Tryptone Soy Broth without dextrose (TSB Glu(-)) was used for growth and adaptation of Salmonella. Acid challenge was performed in TSB adjusted to pH 2.5 with HCl and mayonnaise. For broth experiments, different concentrations of total acetic acid (AA) were used (15, 25 and 35 mM) and the pH was adjusted to 4.0, 4.5, 5.0, 5.5, 6.0 using HCl/NaOH. Non-AA adapted cells (0m M/pH 4.0, 4.5, 5.0, 5.5, 6.0) were also inoculated. Two of these acetic-adaptation cases showing the highest (15 mM/pH 6) and lowest reduction (35 mM/pH 5.5) together with non-AA adapted cells (0 mM/pH 5.5 and 6.0) were selected and used as adaptation inocula to commercial packages of mayonnaise (initial pH 3.9) stored at two temperatures. Non-adapted cells were those grown in non-acidified TSB Glu(-). Experiments were conducted twice in two replicates.
Results: In broth experiments, cells adapted with AA were countable for longer time compared to those pre-exposed to HCl. This was more profound within a range of undissociated AA (e.g. 35 mM/pH 5.5). Non-adapted cells had the fastest reduction. However, in mayonnaise experiments, non-adapted cells together with cells adapted to AA 35 mM/pH 5.5 remained countable for longer time compared to the other inocula. At the end of storage period, Salmonella was detectable (enrichment protocol) in all samples.
Significance: Results can provide new information regarding the impact of exposure to sublethal acid conditions on subsequent resistance of Salmonella Enteritidis and may help food industries develop safe food formulations.