Purpose: To investigate the impact of exposure to sublethal acid conditions (low pH in the presence of undissociated acetic acid) on the resistance of Salmonella Enteritidis to lethal pH (laboratory medium, mayonnaise).
Methods: Growth and adaptation of Salmonella was performed in Tryptone Soy Broth without dextrose (TSB Glu(-)), while acid challenge in TSB adjusted to pH 2.5 with HCl and mayonnaise. For broth experiments, different concentrations of total acetic acid (AA; 15, 25 and 35mM) were used and the pH was adjusted to 4.0, 4.5, 5.0, 5.5 and 6,0 using HCl/NaOH. Non-AA adapted cells (0mM/pH 4.0, 4.5, 5.0, 5.5 and 6.0) were also used. Based on the above experiments, two acetic-adaptation inocula, those showing the highest (15mM/pH 6) and lowest (35mM/pH 5.5) reduction together with non-AA adapted cells (0mM/pH 5.5 and 6.0), were inoculated into commercial packages of mayonnaise (initial pH 3.9). Samples were stored at 5oC and 10oC. Non-adapted cells were grown in non-acidified TSB Glu(-). Experiments were conducted twice in two replicates.
Results: In broth experiments, cells adapted to AA, and especially those adapted within a range of undissociated AA (e.g. 35mM/pH 5.5), were countable for longer time than those pre-exposed to HCl. Non-adapted cells had the fastest reduction. However, in mayonnaise experiments, non-adapted cells together with cells adapted to AA 35mM/pH 5.5 remained countable for longer time compared to the other inoculum preparations. Salmonella was detectable in all samples by enrichement at the end of storage period.
Significance: Our findings can provide new insight concerning the acid response of Salmonella Enteritidis after exposure to sublethal acid conditions and may help food industries improve food safety strategies.