The influence of lactic acid bacteria on the viability of the reference strain of Listeria monocytogenes 123 serotype I in plant foods

Abstract

Listeria (L.) monocytogenes is the causative agent of human listeriosis, the frequent source of which is food of animal origin. The aim of this study was to determine the influence of lactic acid bacteria (LAB) on the viability of Listeria in carrot juice and compound feed inoculated with L. monocytogenes. The effect of homogenous cultures of Streptococcus (Str.) lactis distaticus, Str. thermophilus and Lactobacillus (Lac.) lactis subsp. Cremoris and the combination of Str. thermophilus with Lac. bulgaricus in the carrot juice and compound feed samples on viability of inoculated L. monocytogenes were examined. There were no statistically significant differences in the results between the experimental groups. Regardless of used LAB, the results showed that the mean pH values in the carrot juice samples decreased from an initial pH of 6.7 to a mean value of 3.7 on 15 experimental day. The Listeria concentration in carrot juice samples decreased from average of 4.94 on day 5 to 3.24 log CFU/mL on day 10, and on day 15 achieved <0.01 log CFU/mL. In the compound feed trials, the pH decreased average from initial 6.5 to 3.7 on day 15. The concentration of Listeria decreased, similarly to the carrot juice samples, from average 5.0 on day 5 to 4.68 on day 10, and on day 15 achieved <0.01 log CFU/mL. In control samples, the number of Listeria increased throughout the study period and amounted to 9.2-9.84 log CFU/mL/g in all the samples. The activity of LAB has been shown to be antagonistic to L. monocytogenes. The results of the study did not show any clear differences between the used LAB strains in limiting the L. monocytogenes concentration. Based on the obtained results it can be conducted that the addition of LAB to animal food increases its microbiological safety.