Εvaluation of oxygen availability on growth and inter-strain interactions of L. monocytogenes in/on liquid, semi-solid and solid laboratory media.

The coexistence and interactions among Listeria monocytogenes strains in combination with the structural characteristics of foods, may influence their growth capacity and thus, the final levels at the time of consumption. In the present study, we aimed to evaluate the effect of oxygen availability in combination with substrate micro-structure on growth and inter-strain interactions of L. monocytogenes. L. monocytogenes strains, selected for resistance to different antibiotics (to enable distinct enumeration), belonging to serotypes 4b (C5, ScottA), 1/2a (6179) and 1/2b (PL25) and were inoculated in liquid (Tryptic Soy Broth supplemented with Yeast Extract - TSB-YE) and solid (TSB-YE supplemented with 0.6% and 1.2% agar) media (2-3 log CFU/mL, g or cm), single or as two-strain cultures (1:1 strain-ratio). Aerobic conditions (A) were achieved with constant shaking or surface inoculation for liquid and solid media respectively, while static incubation or pour plated media corresponded to hypoxic environment (H). Anoxic conditions (An) were attained by adding 0.1% w/v sodium thioglycolate and paraffin overlay (for solid media). Growth was assessed during storage at 7 °C (n = 3 × 2). Inter-strain interactions were manifested by the difference in the final population between singly and co-cultured strains. Τhe extent of suppression increased with reduction in agar concentration, while the impact of oxygen availability was dependent on strain combination. During co-culture, in liquid and solid media, 6179 was suppressed by C5 by 4.0 (in TSB-YE under H) to 1.8 log units (in solid medium under An), compared to the single culture, which attained population of ca. 9.4 log CFU/mL or g. The growth of 6179 was also inhibited by ScottA by 2.7 and 1.9 log units, in liquid culture under H and An, respectively. Interestingly, in liquid medium under A, H and An, ScottA was suppressed by C5, by 3.3, 2.4 and 2.3 log units, respectively, while in solid media, growth inhibition was less pronounced. Investigating growth interactions in different environments could assist in explaining the dominance of L. monocytogenes certain serotypes.