Antimicrobial effect of nisin and carvacrol and competition between Bacillus cereus and Bacillus circulans in vacuum-packed potato puree

A study was undertaken to investigate the growth and competition between Bacillus cereus and B. circulans strains in both BHI and ready-to-eat potato puree. In addition, antimicrobial potential of supplemented nisin and carvacrol was evaluated against inoculated B. cereus and B. circulans strains. The accomplished growth inhibition was observed for both B. cereus and B. circulans, where B. circulans strains were more sensitive. MIC values were decreased by lowering the incubation temperature in separate applications of nisin and carvacrol, while the effect of combined application of nisin and carvacrol appeared to be more obvious at higher temperatures. The overall effect of interactions between spoilage and pathogenic Bacillus spp. was dependent on the psychrotrophic character of both cultures. The complete inhibition of B. circulans in the co-culture experiments corresponded to B. cereus population density of approximately 6 log units. Microbial challenge testing in potato puree showed that, at both 7 and 10 °C, B. cereus TZ415 and B. circulans 4.1 could grow to high counts when no antimicrobial substances (nisin and carvacrol) were applied, while no visible spoilage occurred. A more pronounced antimicrobial activity of nisin and nisin-carvacrol combination was observed in potato puree compared to the BHI medium.