Growth interferences between bacterial strains from raw cow's milk and their impact on growth of Listeria monocytogenes and Staphylococcus aureus.

AIMS: The purpose of this study was to detect growth enhancing or inhibiting activity between bacterial populations from raw milk under different conditions (temperature, medium). METHODS AND RESULTS: The interference of 24 raw milk isolates on growth of each other and on Listeria monocytogenes, Staphylococcus aureus, Bacillus subtilis and Micrococcus luteus was screened by drop assay and for selected pairs in co-cultivation experiments. By drop assay, antibacterial activity was observed for 40% of the strains. About 30% of the strains showed growth-enhancing activity on other strains. Most of the isolates were well adapted to cold temperatures and showed consistent or even increased inhibiting or enhancing effects on growth of other strains at 10°C. The growth of L. monocytogenes DSM 20600T and S. aureus DSM 1104T was significantly (P < 0·05) reduced in co-cultivation with Pseudomonas protegens JZ R-192. CONCLUSIONS: Growth interferences between bacterial populations have an impact on the structure of raw milk microbiota, especially when it develops under cold storage, and it may have an effect on the prevalence of certain foodborne pathogens. SIGNIFICANCE AND IMPACT OF THE STUDY: This study demonstrates growth-inhibiting and also growth-enhancing interactions between raw milk bacteria, which must be considered when predicting bacterial growth and spoilage in food. A Ps. protegens strain isolated from raw milk showed an antagonistic effect on growth of L. monocytogenes in refrigerated raw milk.