Eight foodborne pathogenic and spoilage type gram-positive bacteria were evaluated for their spontaneous resistance frequencies to the peptide antimicrobial nisin. In brain heart infusion medium, spontaneous nisin resistance frequencies were in the range of 10−6 to 10−8 when exposed to nisin at concentrations 2 to 8 times the minimal inhibitory concentrations. A resistant mutant of Listeria monocytogenes Scott A (2000 U nisin per ml) was obtained by increasing stepwise exposure to nisin and was subsequently characterized. Nisin was not inactivated after exposure to mutant or parent cells growing in brain heart medium. Membrane fatty acid composition, phase transition temperature profiles (by differential scanning calorimetry), and specific growth rates of the resistant mutant and parent strain were compared. The resistant mutant had a higher phase transition temperature, higher percentage of straight-chain fatty acids, and a lower percentage of branched chain-fatty acids. The specific growth rate (k) of the resistant mutant was significantly decreased at the suboptimal temperature of 20°C where (k) was 40.9% of the parent strain k. In contrast, at 37°C, the mutant (k) was 87.6% of the parent (k). Collectively, these observations indicated that as a resistance response to nisin, fundamental changes occurred in bacterial membrane structure and function as opposed to a resistance response involving nisin degradation.

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