Comparison of resistance of biofilm and planktonic Bacillus cereus against organic acids and ethanol

Abstract: To compare the resistance of biofilm and planktonic cells of B. cereus to the environmental stresses, the effects of four organic acids (namely, acetic acid, citric acid, lactic acid, and malic acid), and ethanol on the cell survival were investigated in this paper. Upon 8% acetic acid treatment for 30 min, the survival of the cells in biofilms which were incubated for 24 h was found to be much higher than that of the planktonic bacteria cultured for the same period; besides, the planktonic bacteria in the stationary phase (incubation of 24 h) showed greater resistance to acetic acid treatment than those in the logarithmic phase (incubation of 4-12 h). The scanning electron microscopic analysis revealed that upon acetic acid treatment, the surfaces of the planktonic cells were markedly damaged, whereas those of the biofilm cells were only slightly damaged. Likewise, the biofilm cells were found to be more resistant than the planktonic cells in the presence of citric acid, lactic acid, malic acid, and ethanol; especially at higher concentrations (16%-20% for organic acids, and 50%-60% for ethanol), this phenomenon appeared to be more significant. Moreover, the planktonic cells in the stationary phase were more resistant to the environmental stresses tested than those in the logarithmic phase. Among the four organic acids and ethanol, malic acid showed the highest inhibitory activity against planktonic cells of B. cereus, while acetic acid was found to be the best for inhibition of biofilm cells. These findings suggested the risk of food spoilage by B. cereus biofilms, and demonstrated the importance of controlling its biofilms in the food industry. The results provided theoretic guidance for controlling the formation of biofilms and planktonic cells of B. cereus in the food industry.