Enhanced inactivation of Salmonella typhimurium and verotoxigenic Escherichia coli by nisin at 6.5 degrees C

While the use of natural antimicrobials such as nisin is useful to inhibit and/or inactivate Listeria monocytogenes, Bacillus cereus and Clostridium b otulinum, if is not effective on Salmonella and Escherichia coli, presumabl y due to the inaccessibility of target sites for nisin. Disruption of the o uter membrane permeability barrier by several meth ods including chelation of cations by ethylenediaminetetraacetic acid (EDTA) and various physical s tresses including osmotic shock, heat shock, acid and freezing, have been s hown to sensitize Gram-negative cells to nisin. We discovered that nisin en hanced inactivation of S. typhimurium and E. coli O157:H7 when stored at 6. 5 degrees C in peptone wafer. Cell numbers, enumerated on trypticase soy ag ar (TSA) decreased over 2 weeks in the presence of nisin more rapidly than in the absence of nisin. Salmonella typhimurium exhibited a 1.7 log decreas e in cell numbers (average of 17 trials), in the presence of 100 IU nisin c ompared to controls (P < 0.0001) Likewise, four strains of E, coli, includi ng O157:H7 also showed a significant (P < 0.001) 1.6 log decrease in cell n umbers after 7 days in the presence of nisin at 6.5 degrees C. This effect of nisin was completely reversed by the presence of 25 mM magnesium or 100 mM sodium or potassium. Nisin may be a means for providing an additional ba rrier to enhance the safety of refrigerated foods. (C) 1999 Academic Press.