Inter-strain Competition Affects Growth and Detection of Listeria monocytogenes or Salmonella in Foods by ISO Methods

Introduction: Classic pathogen detection methods in foods might not represent all strains initially present in samples, but those with competitive fitness, thereby introducing bias in detection.

Purpose: To investigate the competition among L. monocytogenes or Salmonella enterica strains during: (i) growth on laboratory media and foods, (ii) selective enrichment.

Methods: Growth of seven L. monocytogenes strains (serotypes 1/2a, 1/2b, 1/2c, 4b) was tested in single and two-strain cultures (1:1 strain ratio). The competitive potential of seven Salmonella enterica strains (serovars Typhimurium, Reading, Agona, Emek, Putten) was assessed in two and three-strain cultures. Strains were made resistant to different antibiotics for their selective enumeration on TSA+antibiotics. Pathogens were inoculated (3 log CFU/ml or CFU/cm²) in TSB, on TSA, and vacuum packed sliced ham (5x5 cm²) and stored at 10°C. According to ISO 11290:1:1996 and ISO 6579:1993 for L. monocytogenes and Salmonella spp. detection, respectively, two enrichment steps both followed by streaking on ALOA and XLD, were performed. Strain cultures were directly added in the enrichment broth or used to inoculate minced beef and sliced ham (3 log CFU/cm²). 60-100 colonies were used to determine the relative percentage of each strain recovered on plates.

Results: L. monocytogenes strains had similar (P > 0.05) growth rates when cultured singly or in mixtures in TSB and TSA. Conversely, growth on ham resulted in cases where a strain did not manage to increase in the presence of another strain. In laboratory media, some Salmonella strains dominated over others, suppressing their maximum populations by 2 log lower than the 9 log CFU/ml or cm² observed in single cultures, or reducing their growth rate 2-fold (P < 0.05) compared to single cultures. During enrichment, 4b serotype outcompeted (80-100% of total colonies) other serotypes, regardless of the food. In BPW, the dominance (80-100%) of Reading and Putten serovars was reversed in the second enrichment in RVS (0-20% of total colonies).

Significance: Understanding how strain competition of pathogens may affect their survival and growth may contribute to the improvement of detection methods and assist in outbreak investigations.