Lactic acid permeabilizes gram-negative bacteria by disrupting the outer membrane

The effect of lactic acid on the outer membrane permeability of Escherichia coli O157:H7, Pseudomonas aeruginosa, and Salmonella enterica serovar Typh imurium was studied utilizing a fluorescent-probe uptake assay and sensitiz ation to bacteriolysis, For control purposes, similar assays were performed with EDTA (a permeabilizer acting by chelation) and with hydrochloric acid , the latter at pH values corresponding to those yielded by lactic acid, an d also in the presence of KCN. Already 5 mM (pH 4.0) lactic acid caused pro minent permeabilization in each species, the effect in the fluorescence ass ay being stronger than that of EDTA or HCl. Similar results were obtained i n the presence of KCN, except for P. aeruginosa, for which an increase in t he effect of HCl was observed in the presence of KCN, The permeabilization by lactic and hydrochloric acid was partly abolished by MgCl2. Lactic acid sensitized E. coli and serovar Typhimurium to the lytic action of sodium do decyl sulfate (SDS) more efficiently than did HCl, whereas both acids sensi tized P. aeruginosa to SDS and to Triton X-100. P. aeruginosa was effective ly sensitized to lysozyme by lactic acid and by HCl. Considerable proportio ns of lipopolysaccharide were liberated from serovar Typhimurium by these a cids; analysis of liberated material by electrophoresis and by fatty acid a nalysis showed that lactic acid was more active than EDTA or HCl in liberat ing lipopolysaccharide from the outer membrane. Thus, lactic acid, in addit ion to its antimicrobial property due to the lowering of the pH, also funct ions as a permeabilizer of the gram-negative bacterial enter membrane and m ay act as a potentiator of the effects of other antimicrobial substances.