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 9 log CFU/ml of stationary
phase cells of L. monocytogenes was exposed to these antimicrobials in tryptic soy broth, there was a 3 to
5 log CFU/ml reduction with phage P100 and a complete 9 log CFU/ml reduction with LAE but no measurable
reduction with PL–SD after 24 h at 4 °C or 30 °C. In QFC, the L. monocytogenes populations increased
from the initial 3.5 log CFU/cm2 to 7.7 log CFU/cm2 in 28 days at 4 °C. Treatment with 7.8 log PFU/cm2
of phage P100 or 200 ppm of LAE showed strong listericidal effect initially by reducing L. monocytogenes
counts by 2 to 3.5–4 log CFU/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 28 days at 4 °C. Only the combined treatment of listericidal phage P100 or
LAE with listeriostatic PL–SD reduced the initial L. monocytogenes counts by 2–4 log CFU/cm2 and also kept the
L. monocytogenes counts at that reduced level in QFC for 28 days at 4 °C.
© 2012