Abstract
Acid adaptation enhances survival of foodborne pathogens under lethal acid conditions that prevail in several food-related ecosystems. In the present study, the role of undissociated acetic acid in inducing acid resistance of Salmonella Enteritidis Phage Type 4 both in laboratory media and in an acid food matrix was investigated. Several combinations of acetic acid (0, 15, 25, 35 and 45 mM) and pH values (4.0, 4.5, 5.0, 5.5, 6.0) were screened for their ability to activate acid resistance mechanisms of pathogen exposed to pH 2.5 (screening assay). Increased survival was observed when increasing undissociated acetic acid within a range of sublethal concentrations (1.9-5.4 mM), but only at pH 5.5 and 6.0. No effect was observed at lower pH values, regardless of the undissociated acetic acid levels. Three combinations (15mM/pH5.0, 35mM/pH5.5, 45mM/pH6.0) were selected and further used for adaptation prior to inoculation in commercial tarama (fish roe) salad, i.e., an acid spread (pH 4.35 ± 0.02), stored at 5°C. Surprisingly and contrary to the results of the screening assay, none of the acid adaptation treatments enhanced survival of Salmonella Enteritidis in the food matrix, as compared to non-adapted cells (control). Further examination of the food pH value, acidulant and storage (challenge) temperature on the responses of the pathogen adapted to 15mM/pH5.0, 35mM/pH5.5 and 45mM/pH6.0 was performed in culture media. Cells adapted to 35mM/pH5.5 were unable to induce acid resistance when exposed to pH 4.35 (tarama salad pH value) at 37°C and 5°C, whereas incubation under refrigeration (5°C) at pH 4.35 sensitized 45mM/pH6.0 adapted cells against the subsequent acid and cold stress. In conclusion, pre-exposure to undissociated acetic acid affected the adaptive responses of Salmonella Enteritidis Phage Type 4 in a concentration- and pH-dependent manner, with regard to conditions prevailing during acid challenge.
Highlights
It is well documented that pre-exposure of microorganisms to sublethal stress conditions may induce adaptive responses that enhance resistance to subsequent lethal factors [1] of the same or multiple stresses
It has been demonstrated that different protocols used to stimulate acid resistance can diversify the survival of Escherichia coli in apple juice stored under refrigeration [8]
The second part of this study aimed to evaluate whether the results from the broth media could be extrapolated in foods, in an acid food matrix, stored under refrigeration
Summary
It is well documented that pre-exposure of microorganisms to sublethal stress conditions may induce adaptive responses that enhance resistance to subsequent lethal factors [1] of the same or multiple stresses (cross protection; [2]). The acid induced phenotypic responses are highly affected by factors such as the selected strain [5,6] and conditions prevailing during adaptive and subsequent lethal challenge treatments, e.g., acidulant, temperature and composition [3]. Different adaptation protocols are used in order to formulate suitable sublethal acid conditions. Acid adaptation can be achieved by supplementation of growth media with glucose [6,7] or by long- or short-term pre-exposure of cells to various organic or inorganic acidulants [8,9,10,11,12,13]. It has been demonstrated that different protocols used to stimulate acid resistance can diversify the survival of Escherichia coli in apple juice stored under refrigeration [8]
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