effect of organic acids and nisin at sub-minimum inhibitory concentrations on growth of a strain of bacillus cereus isolated from cheese

ABSTRACT

One of the methods for preserving foods is decontamination by using food preservatives. Food preservatives exert antagonistic effects on microorganisms through interference with the structure and function of their cell membrane, enzyme action, or genetic structure. The effects of different preservatives on the growth of Bacillus cereus isolated from cheese were determined. First the growth curve of the bacteria was studied without addition of the preservatives. Then the curve was re-drawn after adding acetic acid and nisin as a chemical and a natural preservative, respectively, at sub-minimum inhibitory concentrations [sub-MIC]. Finally growth curves were drawn in the presence of both citric acid and potassium sorbate at different concentrations, the pH changes being determined. Bacillus cereus did not grow well in acetic acid at a concentration of half-MIC, and the pH was 5 and did not change. However, it caused changes in the pH at 1/4-MIC and 1/8-MIC of acetic acid and grew. Furthermore, it could increase the pH in the presence of nisin and grow after a lag phase of 12 hours. The simultaneous use of citric acid and potassium sorbate showed a synergistic preservative effect. Bacillus cereus isolated from cheese is resistant to sub-MIC concentrations of different preservatives. Therefore, it is recommended not to use preservatives at such concentrations