Sunday, July 29, 2007
P75

Evidence that Bacillus cereus resistance to nisin involves interactions between cells

Dm Wrigley, M. Khaliq, and R. Tymchuck. Biology, Minnesota State U Mankato, 242 S Trafton Science Center, Mankato, MN 56001

Resistance to the food biopreservative, nisin, is often associated with molecular changes within the bacterial cell, such as change in fatty acids in the phospholipid target for nisin.  Bacillus sp. produce nisinase, an enzyme which degrades nisin and confers resistance. Exposure of Bacillus cereus to sublethal concentrations of nisin increased minimal inhibitory concentrations from 200 μg /ml to 420 μg /ml (±45 μg /ml) nisin, presumably due to induction of nisinase.  However, dispersed cell populations of nisin-conditioned and unconditioned populations of B. cereus grown on tryptic soy agar with 200 μg/ml nisin had similar frequencies of resistance (0.27% for nisin-conditioned, 0.21% for unconditioned). Individual cells of nisin-conditioned B. cereus were no more resistant to nisin than unexposed cells. B. cereus grown in tryptic soy broth formed short chains of 2 to 4 cells.  After exposure and adaptation to nisin, the cells formed longer chains and visible flocs.  Thus, activation of resistance phenotype may involve more than the activation of nisinase expression and predisposes aggregate formation.

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