Purpose: The study investigated: i) growth, ii) expression of virulence genes and iii) in vitro virulence potential of L. monocytogenes in the presence of four different food-related microorganisms.
Methods: Growth of L. monocytogenes (ScottA) was evaluated as monoculture or in co-culture with L. innocua, Bacillus subtilis, Lactobacillus plantarum and Pseudomonas aeruginosa in Tryptic Soy Broth (10°C/10 days and 37°C/24 hours). The transcription of 9 key virulence genes (inlA, inlB, inlC, inlJ, sigB, prfA, hly, plcA, plcB), in addition to invasion efficiency (45 min) and intracellular growth (4 h) in Caco-2 cells, were determined for L. monocytogenes grown singly or in co-culture previously incubated for 3 days at 10°C or 9 hours at 37°C.
Results: Significant differences in growth between single and co-cultures of ScottA were observed when grown with L. innocua at 37°C or 10°C (e.g., lower final populations) and B. subtilis at 37°C (e.g., growth cessation after 9 h). ScottA revealed considerably increased invasion efficiency when co-cultured with L. innocua but attenuated, efficiency in the presence of B. subtilis. Intracellular growth of L. monocytogenes in Caco-2 cells was reduced up to 35 folds compared to monoculture, when grown in co-cultures. The key virulence genes of L. monocytogenes were under-expressed after co-cultivation with B. subtilis at both temperatures while co-cultivation with L. innocua at 37°C, increased the overall gene expression levels of ScottA (e.g., 7-fold increase of prfA).
Significance: Investigating the impact and mechanisms of microbial interactions on growth and virulence of L. monocytogenes expands our understanding on the survival and infection potential of the pathogen in the gastrointestinal environment.