Bacillus cereus and its Resistance to the Antimicrobial, Nisin

Bacillus cereus is a gram positive microorganism which causes mild food borne illnesses. Nisin, naturally produced during some food fermentations, is an effective antibiotic against gram positive pathogens. Elevated use of nisin may contribute toward increased bacterial resistance to such treatment. This study examines changes in microbial cell surface charges, as an effect of acquired resistance. B. cereus was cultured with nisin and without nisin and compared for changes in cell surface charges by their binding of the cationic dye, Congo Red. The two cultures were suspended in Congo Red and briefly incubated. Then the bacteria were removed by centrifugation, and the dye remaining in solution was measured. The difference between the original dye absorbance and the dye mixed with bacteria was used to quantify a percentage of absorbed dye. Optimal conditions for pH and dye concentrations were determined. Under the best conditions, nisin-conditioned B. cereus bound 70.55% of the Congo Red, while the unconditioned B. cereus culture bound 51.9%. Clearly, the unconditioned B.cereus culture reduced Congo Red binding activity. Thus phenotypic expression of acquired nisin resistance strongly correlates with a change in the cell surface charge