Antimicrobial effects of D-3-phenyllactic acid on Listeria mnocytogenes in TSB-YE medium, milk, and cheese

D-3-Phenyllactic acid is a compound with anti-Listeria activity which is produced and secreted by the yeastlike fungus, Geotrichum candidum. This compound has a bactericidal effect independent of the physiological state of Listeria monocytogenes when added at a concentration of 7 mg/ml to tryptic soy broth supplemented with yeast extract (TSB-YE). An initial L. monocytogenes population of 10(5) CFU/ml was reduced 100-fold (2 log) after 4 days of culture at 25 degrees C in TSB-YE containing D-3-phenyllactic acid. The Listeria population was reduced 1,000-fold (3 log) when the compound was added during the exponential growth phase, and was reduced to less than 10 CFU/ml when it was added during the stationary phase. D-3-Phenyllactic acid had a bacteriostatic effect in UHT whole milk, reducing the population by 4.5 log, to give fewer cells than in the control after 5 days of culture. The results obtained with L. monocytogenes at concentrations of 10(5) and 10(3) CFU/ml in cheese curds were less conclusive. D-3-Phenyllactic acid was 10 times less active than nisin in our experimental conditions (TSB-YE at 25 degrees C).