Reduction of Listeria monocytogenes in queso fresco cheese by a combination of listericidal and listeriostatic GRAS antimicrobials

Abstract

Single and combined effects of three GRAS (generally recognized as safe) antimicrobials including, bacteriophage P100 (phage P100), lauric arginate (LAE), and potassium lactate–sodium diacetate mixture (PL–SD) were evaluated against Listeria monocytogenes cold growth in queso fresco cheese (QFC). The fate of phage P100 when exposed to LAE (200ppm) or PL–SD (2.8% PL and 0.2% SD) was determined at 4°C and 30°C in a broth model. Phage P100 was found to be stable in the presence of these antimicrobial agents as plaque forming units (PFU) did not vary between control, LAE or PL–SD treatments. When 9logCFU/ml of stationary phase cells of L. monocytogenes was exposed to these antimicrobials in tryptic soy broth, there was a 3 to 5logCFU/ml reduction with phage P100 and a complete 9logCFU/ml reduction with LAE but no measurable reduction with PL–SD after 24h at 4°C or 30°C. In QFC, the L. monocytogenes populations increased from the initial 3.5logCFU/cm2 to 7.7logCFU/cm2 in 28days at 4°C. Treatment with 7.8logPFU/cm2 of phage P100 or 200ppm of LAE showed strong listericidal effect initially by reducing L. monocytogenes counts by 2 to 3.5–4logCFU/cm2 while there was a subsequent regrowth of L. monocytogenes at 4°C. Treatment with PL–SD showed strong listeriostatic effect without decreasing L. monocytogenes counts but growth was prevented for 28days at 4°C. Only the combined treatment of listericidal phage P100 or LAE with listeriostatic PL–SD reduced the initial L. monocytogenes counts by 2–4logCFU/cm2 and also kept the L. monocytogenes counts at that reduced level in QFC for 28days at 4°C.