Antilisterial Activity of Lactic Acid Bacteria as Influenced by Temperature, Incubation Period and Culture Media

Introduction: Listeria monocytogenes are regarded as a major safety problem because they can cause serious illness and death. Lactic acid bacteria (LAB) have been found to be effective in inhibiting or killing L. monocytogenes because of their antimicrobial metabolites.  

Purpose: The objective of this study was to determine the influence of temperature, incubation period and culture medium on the antilisterial activity of LAB. 

Methods: Three culture media (BHI, MRS and TSB broth) were inoculated with a L. monocytogenes cocktail at 10⁵ CFU/ml. Then 7 LAB strains previously selected (NP3, NP35, NP51, FS56, FS92, FS97 and RP1) were individually inoculated at a level of 10⁸ CFU/ml. The cultures were held anaerobically at 37°C for 72 hrs and at 5°C for 7 days, respectively. The viable counts of L. monocytogenes and LAB, as well as pH changes of media over time were measured.

Results: At 37°C, the highest L. monocytogenes reduction (9.2 log CFU/ml) was obtained by FS97 in BHI broth at hr 72, at 5 °C, FS56 led to the highest reduction (5.3 log CFU/ml) in BHI broth on day 7, and the lowest media pH (3.9) was obtained with NP35 in MRS broth at 37°C. In most cases, there were not significant (P > 0.05) changes in pH at 5°C compared to the control. The viable count changes of 7 LAB strains in three media under different temperatures were strain-specific for the bacteria.

Significance: Results from this study showed that LAB inhibited the growth of L. monocytogenes involving several factors and these factors played different role according to the growth conditions, and it seems that the mechanisms of these factors are not well understood and the consequences are not easily predictable. However these factors are able to significantly influence the microbiological safety and quality of food.