Purpose: This study was conducted to evaluate the effect of oxygen availability and sub-optimal pH in a structured medium, on the growth of immobilized L.monocytogenes, and subsequent resistance under lethal acid stress.
Methods: Tryptic Soy Broth supplemented with 0.6% w/V Yeast Extract (TSBYE) solidified with gelatin (10% w/V) served as growth medium. The pH was adjusted (HCl), post-autoclaving, to 6.2 (common pH in foods) and 5.5 (adaptation inducing level). Two L. monocytogenes strains (C5; 6179) were surface or pour plated (three log CFU/mL) corresponding to aerobic and hypoxic conditions, respectively, and incubated at 10°C for 15 days. Anoxic conditions were achieved by adding 0.1% w/V sodium thioglycollate and a paraffin overlay. Growth, followed by survival at lethal acidity, in TSBYE (pH 2.0, HCl, 37oC) were assessed on day 7 and 15 (n=2x2). A biphasic inactivation model was used to fit survival data (GinaFiT tool) and calculate time for 4D reduction (t4D).
Results: Anoxic conditions resulted in slower growth (P<0.05) of L. monocytogenes than aerobic and hypoxic environments; or in lower acid resistance, manifested by the immediate population reduction below the enumeration limit. Prolonged habituation of L. monocytogenes (15 days), at both pHs, increased acid tolerance. Cells grown under hypoxic or aerobic conditions, at pH 6.2, demonstrated similar (P≥0.05) acid tolerance responses after 7 (t4D=10-25 min) or 15 days (t4D=30 min) storage; while habituation at pH 5.5 resulted in acid resistance, only, after 15 days incubation (t4D=15-25 min).
Significance: Elucidating the role of oxygen limitation conditions, often encountered in structured food, on acid resistance of L.monocytogenes, would assist in assessing the capacity of L. monocytogenes originated from different food-related niches to withstand gastric acidity and initiate infection.