Biological control of Listeria monocytogenes in soil model systems by Enterococcus mundtii strains expressing mundticin KS production

Abstract

The agricultural practices applied in pre-harvest greatly influence the presence and the levels of microorganisms in fresh produce. Among these, Listeria monocytogenes represents one of the most lethal foodborne pathogens associated with vegetables. The main hypothesis of this work is that bacteriocin producer Enterococcus mundtii strains can be effective against L. monocytogenes in soil. To this purpose, bacteriocin production by E. mundtii WFE3, WFE20 and WFE31, three strains showing a strong bacteriocin activity in terms of inhibitory power and inhibition spectra, was evaluated in sterile extracts from agricultural soil and peat moss, in organic nutrient solution (ONS) and mineral nutrient solution (MNS). ONS supernatants from E. mundtii WFE3 showed the highest inhibition of the strain L. monocytogenes ATCC 19114. Thus, this strain [104 colony forming units (CFU) g−1 dw] was co-inoculated with E. mundtii WFE3 (106 CFU g−1 dw) in the sterile extracts and solutions. A general increase of E. mundtii cell densities and the contemporary decrease of the levels of L. monocytogenes were observed, particularly in ONS. This solution was used to amend autoclaved soil to test in vivo the competition among bacteriocin producer and sensitive strain. L. monocytogenes decreased of almost 1.5 log CFU when E. mundtii was added to soil. The test was also carried out with basil plants showing that the anti-Listeria effect of E. mundtii is exerted during the very first days from inoculation. After 2 days, the levels of NO3−-N in soil increased for all trials, while the concentrations of ammonium and α-amino N decreased. The lowest concentrations of NH4+-N were found in presence of L. monocytogenes. At harvest, the plants were analysed for the presence of E. mundtii and L. monocytogenes, but none of the two bacterial species deliberately added to soil was transferred to plants. Although a biocontrol based on bacteriocin application in soil has to be properly set, this study provides useful insight for the future development of chemical-free agricultural strategies.