Surface physiological changes induced by lactic acid on pathogens in consideration of pKa and pH

Antimicrobials affected the surface characteristics and membrane integrity of bacterial cells. The objective of this study was to investigate the physiological changes on the surface of Salmonella Enteritidis (ATCC 14028), Escherichia coli (CGMCC 1.2385), and Listeria monocytogenes (ATCC 19115) induced by lactic acid. Lactic acid was more effective than hydrochloric acid (HCl) at the same pH in inhibiting the growth of these pathogens. The results of zeta potential indicated that surface charges of three pathogens were significantly disturbed by lactic acid treatment. After exposure to 0.5% lactic acid, a rough cell surface with discrete ridges and concave collapses were observed by scanning electron microscopy (SEM). Meanwhile, the release of intracellular potassium (K+) also suggested the damage of cytoplasmic membrane. As an effective antimicrobial, lactic acid led to leakage of intracellular K+, damage of membrane permeability and integrity, and changes of surface charge in the pathogens.