Purpose: To investigate the inter-strain interactions on the growth of matrix-adapted and non-adapted L. monocytogenes on Ricotta and Camembert cheeses.
Methods: Ricotta (10g) and Camembert (6x2 cm2 pieces) cheeses were inoculated at approximately 2.5 log CFU/g or cm2 with four matrix-adapted and non-adapted L. monocytogenes strains (serotypes 1/2a, 1/2b, 4b), in single and two-strain cultures (1:1 strain ratio). Strains were selected as resistant to different antibiotics for their selective enumeration on TSA-YE+antibiotics. Adaptation of cells was performed in cheese broth (1:1 cheese in Maximum Recovery Diluent) for 48h at 7oC, before inoculation of cheeses. Adaptation aimed to evaluate exclusively the inter-strain interactions, eliminating the potential impact of cheese microenvironment on strain-to-strain interactions. Cheeses were stored aerobically at 7oC (n=2x2).
Results: Adaptation of L. monocytogenes strains on Ricotta and Camembert resulted in similar (p≥0.05) growth responses. Both single and co-cultured adapted strains, had shorter lag phase than the non-adapted ones. Particularly, adapted strain C5 showed growth of 1.5 log CFU/g or cm2 on the 2nd day of storage, compared to the non-adapted, which entered the exponential phase on day 4. Differences (p<0.05) in growth kinetics of some strains were observed when grown singly compared to the same strain in co-culture. Adapted and non-adapted singly cultured strain 6179 increased to 7.2 log CFU/g, whereas in co-culture remained at 5.6 log CFU/g, suggesting growth inhibition (p<0.05), when its competitor reached the maximum growth (7.9 log CFU/g). Matrix-adaptation enhanced the impact of inter-strain interactions resulting in increased differences, regarding growth kinetics, between singly and co-cultured grown strains.
Significance: The results reveal how cheese matrix determines the outcome of inter-strain interactions and therefore could assist in explaining the dominance of certain serotypes in foods of safety concern for L. monocytogenes.