Microbial competition: effect of Pseudomonas fluorescens on the growth of Listeria monocytogenes

Abstract Listeria monocytogenes Scott A was cultured alone and in coculture with Pseudomonas fluorescens ATCC 33231 to characterize quantitatively the effects of microbial competition on the growth of this psychrotrophic pathogen. The bacteria were cultured in brain–heart infusion broth (BHI), using a 3×3×3×2 complete factorial design to assess the impact of temperature (4, 12, 19°C), initial pH (5·0, 6·0, 7·0), and sodium chloride content (5, 25, 45 gl −1 ) on the interaction between the two micro-organisms. Samples were periodically plated on BHI agar and Vogel Johnson agar to obtain total counts and L. monocytogenes counts, respectively. Growth curves were generated by fitting the data to the Gompertz equation, and the derived growth kinetics were compared. When P. fluorescens did influence the growth of L. monocytogenes , the primary effect was a suppression of the maximum population density (MPD) reached by the pathogen. Suppression of L. monocytogenes was generally associated with low incubation temperatures (4°C) and sodium chloride levels (5 and 25 gl −1 ). Slight increases ( −1 ) in the MPD attained by L. monocytogenes were observed when grown in the presence of P. fluorescens at higher temperatures (12 and 19°C) and sodium chloride levels (25 and 45 gl −1 ) when the pH was 5·0. The current study supports earlier work that indicates that reliance on microbial competition as a barrier to control L. monocytogenes in refrigerated foods will require detailed knowledge of how the interaction between the pathogen and the microflora is affected by environmental and food characteristics such as storage temperature, pH, and water activity.