Killing of Gram-positive cocci by fatty acids and monoglycerides

The susceptibilities of three Gram-positive cocci to medium-chain saturated and long-chain unsaturated fatty acids and their one-monoglycerides were s tudied. The bacteria were incubated with equal volumes of lipid solutions f or 10 min. Lauric acid, palmitoleic acid and monocaprin reduced the number of CFU by 6.0 log(10) or greater at 5 mM concentration for streptococci of group A (GAS) and group B (GBS). When further compared at lower concentrati ons and after longer incubation time monocaprin proved to be the most activ e. Capric acid showed the highest activity against Staphylococcus aureus at 10 mM. However, at lower concentrations monocaprin was the only lipid that showed significant activity against S. aureus. The mode of action of monoc aprin against GBS was studied by a novel two-color fluorescent assay of bac terial viability and by electron microscopy. The results, indicate that the bacteria are killed by disintegration of the cell membrane by the lipid, l eaving the bacterial cell wall intact. The highly lethal effect of monocapr in indicates that this lipid might be useful as a microbicidal agent for pr evention and treatment of infections caused by these bacteria.