Purpose: The present study was conducted to evaluate the antibacterial effectiveness of phenyllactic acid (PLA), produced by certain Lactobacillus species, against four foodborne pathogens.
Methods: Brain heart infusion broth (BHI) with added PLA at 0.0 (control) 0.312, 0.625, 1.25, 2.5, 3.75, 5.0, 7.5, 10 or 15 mg/ml, was inoculated with a multiple-strain cocktail of Listeria monocytogenes, Salmonella enterica, Staphylococcus aureus, or Escherichia coli O157:H7 to give a final concentration of 6.0 log CFU/ml. Growth of the pathogens for 24 hours in BHI (pH 6.0 or 7.2) at 35 °C was monitored using a Bioscreen C turbidometer to measure optical density of the cultures at 600 nm. To determine whether PLA exerted a bacteriostatic or bactericidal effect, viable counts in broth with the respective minimum inhibitory concentration (MIC) of PLA for each pathogen were evaluated by surface-plating serially diluted (10-fold) broth samples on tryptic soy agar supplemented with 0.6% yeast extract and counting bacterial colonies after 24 hours of incubation (35 °C).
Results: Increasing levels of PLA extended the lag phase and decreased the growth rate of all four pathogens. The MICs of PLA at pH 6.0 and pH 7.2 were 2.5 and 5.0 mg/ml, respectively, irrespective of the target organism. PLA exhibited a bactericidal effect at its MIC for each pathogen with reductions in viable counts ranging from ~0.6 log CFU/ml (E. coli O157:H7) to 2.0 log CFU/ml (S. aureus) after 24 hours of exposure to PLA.
Significance: Based on these results PLA exhibits good potential as a natural antimicrobial for controlling growth of foodborne bacterial pathogens. Further research of the antimicrobial activity of phenyllactic in appropriate food systems is warranted.