The effect of bacteriocin-producing Lactobacillus plantarum strains on the intracellular pH of sessile and planktonic Listeria monocytogenes single cells

A wide range of lactic acid bacteria (LAB) produce bacteriocins mainly active against other closely related LAB, but some bacteriocins are also active against the food-borne pathogen Listeria monocytogenes. With the aim of increasing food safety it has thus been considered to utilise bacteriocins and/or bacteriocin-producing LAB as natural food preservatives in foods such as cheese, meat and ready-to-eat products. Some strains of Lactobacillus plantarum produce bacteriocins termed plantaricins. Using a single-cell based approach, the effect on the intracellular pH as a measure of the physiological state of sessile and planktonic L. monocytogenes (strains EGDe and N53-1) during co-culturing with plantaricin-producing L plantarum (strains BFE 5092 and PCS 20) was investigated using fluorescence ratio imaging microscopy (FRIM). Mono-cultures of L monocytogenes were used as control. Expression levels of plantaricin-encoding genes by sessile and planktonic L plantarum were determined using qRT-PCR. L plantarum BFE 5092 possesses the genes for plantaricin EF, JK and N, while L plantarum PCS 20 contains the genes for plantaricin EF, although determination of the nucleotide sequence of the PCS 20 plantaricin E gene showed that this peptide is probably non-functional. When cultured as mono-culture, both L monocytogenes strains maintained pH, at a constant level around 7.2-7.6 throughout the experiment, independently of the matrix. On a solid surface, L plantarum BFE 5092 strongly affected pH, of L monocytogenes N53-1 with only 20% of the cells being able to maintain pH, in the physiological optimal range with pH>7 and 52% of the cells with pH-pH showing that the cells had no proton gradient towards the environment. The effect on L monocytogenes EGDe was less pronounced, but still notable. L plantarum PCS 20 left both strains of L monoc-ytogenes virtually unaffected when co-cultured on a solid surface. In liquid, both L plantarum strains strongly affected the physiological state of L monocytogenes EGDe as judged by pH,, whereas L monocytogenes N53-1 was left virtually unaffected after 5 h of co-culturing and after 8 h 50% of the cells still maintained pH(i) >= 7. Higher concentrations of lactic acid were produced in liquid compared to a solid surface, and the different response of EGDe and N53-1 to the activities of the two L plantarum strains probably reflect higher susceptibility of L monocytogenes EGDe to organic acids compared to L monocytogenes N53-1. Taken together, our results may be explained by the difference in the range of plantaricins produced by the two L. plantarum strains and matrix- and strain-related differences in the susceptibility of L monocytogenes to plantaricins and organic acids. In conclusion, the present study represents the first demonstration of the ability of a bacteriocin-producing LAB to dissipate the proton gradient of sessile and planktonic L monocytogenes. (C) 2010 Elsevier B.V. All rights reserved.