Comparative antibacterial mode of action of purified alcalase- and tryptic-hydrolyzed palm kernel cake proteins on the food-borne pathogen Bacillus cereus

Purified alcalase- and tryptic-hydrolyzed palm kernel cake proteins, PAH and PTH, the value-added products derived from palm kernel cake, a co-product of palm kernel oil industry were proven to possess potent bacteriostatic effect against spore-forming bacteria particularly the Bacillus cereus. Comparatively, PAH exhibited slightly better inhibition effect against B. cereus than PTH. Both compounds inhibited the bacterial growth by altering the membrane permeability of the bacterial cells. Incubation for 30 min in the presence of PAH or PTH at minimum inhibitory concentrations (MIC, 250 μg/ml or 350 μg/ml respectively) rapidly induced K+ leakage of the cells. Intracellular ATP was also depleted after 30 min treatment with PTH at MIC level. The total ATP level of the cells was reduced by 5-fold after 30 min of treatment with PAH compared to control, thus suggesting that PAH tends to trigger ATP hydrolysis rather than causing ATP efflux as occurred in cells treated with PTH. Analysis of the biosynthesis of macromolecules demonstrated that RNA is the sole inhibition target of PAH and PTH. From this study, it could be concluded that both PAH and PTH appear to disrupt the membrane integrity, allowing efflux of K+, deplete the ATP molecules and inhibit the intracellular macromolecule metabolism especially the RNA of B. cereus.