Testing matrix, inoculum size, and incubation temperature affect monolaurin activity against Listeria monocytogenes

Current literature reports conflicting minimal inhibitory concentrations (MIC) (range 10-96 mu g ml(-1)) of glycerol monolaurate (monolaurin) against Listeria monocytogenes. To resolve these disagreements, the individual MIC of monolaurin against eight L. monocytogenes strains was determined using the standard agar dilution technique on three commercial media (trypticase soy agar, plate count agar, and Oxford modified medium), and a catfish-based medium using three inocula sizes and three incubation temperatures. Mean MIC was 16 mu g ml(-1) on commercial media at 25 and 35 degrees C. Endpoints at 15 degrees C were twofold lower (8 mu g ml(-1)) than at 25 and 35 degrees C. On catfish-based medium, MIC values were four- to eightfold higher (64-128 mu g ml(-1)) compared with commercial media and depended on incubation temperature and size of inoculum tested. Poor solubility of monolaurin (85 mu g ml(-1)) in aqueous solutions and its reduced activity in the presence of food components limits its application as an antimicrobial agent in foods. (C) 1996 Academic Press Limited