338 Modern Food Microbiology Figure 13–7 Growth of S aureus in pure culture (C) and in association with L plantarum (L), P cerevisiae (P), and the mixture (M) in cooked mechanically deboned poultry meat (MDPM) at 15◦ C Lactic acid bacteria were added at a concentration of 109 cells/g Source: From Raccach and Baker,153 copyright c 1978, International Association for Food Protection Used with permission fermented products The repression of growth of S aureus by Pediococcus cerevisiae and L plantarum is illustrated in Figure 13–7 With respect to mode of action, nisin and subtilin appear to be identical The structural genes appear to be the same for nisin, subtilin, and other antibiotics The cell target for these agents is the cytoplasmic membrane, where they depolarize energized bacterial membranes (reduce transmembrane potential) and form voltage-dependent multistate pores.1,166 The result of a pore formation is the loss of accumulated amino acids and the inhibition of amino acid transport A nisin-resistant mutant of L monocytogenes has been shown to contain significantly less phospholipids in its membrane.130 On the assumption that the membrane targets for nisin are phospholipids, fewer would make membranes less susceptible to pore formation.139 Unlike nisin (a class I bacteriocin), class II bacteriocins, such as lactococcin B, possess narrow host ranges and their membrane activity leads to the leakage of ions, ATP depletion, and proton motive force depletion A vast research literature on bacteriocins has accumulated in the past decade, and it is beyond the scope of this text to provide adequate coverage of this field For more detailed information, see references 8, 86, 91, and 170 The ineffectiveness of nisin on fresh beef is due apparently to its inactivation by glutathione S-transferase.164 It was shown that three glutathione molecules conjugated with one nisin molecule In another study, synergism between nisin and CO2 was shown by the leakage of carboxylfluorescein from liposomes in wild-type strains of L monocytogenes (this same effect is caused by enterocin P that is produced by Enterococcus faecium P13 (see reference 82) after exposure to 2.5 ppm nisin in an atmosphere of 100% CO2 137 With a nisin-resistant strain, 2.5 ppm caused no reduction in numbers but a 2-log reduction was seen with a wild-type strain in air and a 4.1-log reduction in 100% CO2 137 Synergy Food Protection with Chemicals, and by Biocontrol 339 was observed also against L monocytogenes when zinc and aluminum lactates or zinc and aluminum chlorides were used with 100 IU/ml nisin, and the results showed that pretreatment with zinc lactate sensitized the organism to nisin.124 These findings point to the bacterial cell membrane as the target for nisin, and this is further supported by the finding that nisin and vancomycin use the same target, specifically, the membrane anchored cell wall precursor lipid II for which nisin has a high affinity.21 Nisin was combined with lysozyme and EDTA in a gelatin coating to assess its effect on the spoilage biota The products tested were cooked ham and bologna sausage, and they were coated with 0.2 g of 7% gelatin + 25.5 g/l lysozyme-nisin (1:3) + 25.5 g/l of EDTA Each treatment was inoculated (with 4–5 log10 cfu) with six bacterial species, vacuum packaged, and stored at 8◦ C for weeks.67 An immediate reduction in numbers occurred in the antimicrobial gels and up to log cfu/cm2 for the four Gram positives (Brochothrix thermosphacta, Lactobacillus sakei, Leuconostoc mesenteroides, and Listeria monocytogenes) and further growth was inhibited during the weeks E coli 0157:H7 was reduced by logs on ham, but the antimicrobials were ineffective against this species on bologna.67 Using a Doehlert design, it was found that nisin and aw were synergistic and that cell numbers could be reduced by 4–5 log cycles with 1,000 to 1,400 IU of nisin/ml at pH 5.5–6.5 and aw of 0.97 and 0.98.31 The noted effect was not solute specific relative to aw control Other Bacteriocins Two strains of Carnobacterium piscicola were added to cold-smoked salmon stored at 5◦ C and one was effective in reducing L monocytogenes from 103 to