Synergistic Effect of Citric Acid and Pediocin K1, a Bacteriocin Produced by Pediococcus sp. K1, on Inhibition of Listeria monocytogenes

Pediocin K1, a bacteriocin produced by Pediococcus sp. K1 isolated from Korean traditional fermented flatfish, inhibited certain strains of Lactobacillus, Streptococcus, and Listeria monocytogenes. Pediocin K1 was found to be stable at <TEX>$90^{circ}C$</TEX> for 30 min. Among the organisms tested, Listeria monocytogenes was the most sensitive to pediocin K1 and was completely killed when the initial inoculum size of L.monocytogenes cells was equal to or less than <TEX>$10^3 CFU/ml$</TEX>. The degree of inibition of Listeria monocytogenes by pediocin K1 increased 10-fold on the addition of citric acid (<TEX>$0.2\%$</TEX>) to the medium, thereby showing the synergistic effect of citric acid. Listeria monocytogenes cells resistant to pediocin K1 appeared at a frequency of about <TEX>$10^-4$</TEX>/cells. Once developed after exposure to pediocin K1, the resistant phenotype still persisted in the absence of pediocin K1 in successive cultures. This infers that resistance may be attributable to genetic change(s) in the resistant cells.