An agitated medium with internal pH control (IPCM-2) was inoculated to contain Listeria monocytogenes (strain V7, Scott A or California) at ca. 103 CFU/ml and Streptococcus cremoris (Lactococcus lactis subsp. cremoris) or Streptococcus lactis (Lactococcus lactis subsp. lactis) at 0.25 or 1.0% The inoculated medium was incubated with shaking in a waterbath at 30°C for 30 h. L. monocytogenes and lactic acid bacteria were enumerated and pH was determined at appropriate intervals. The area on a figure between curves for the control and treatment and designated as the area of inhibition (AI) was calculated and used to quantify inhibition of each strain of L. monocytogenes for a particular set of conditions in IPCM-2. Statistical analysis of AI values calculated from data obtained at 6, 24, and 30 h of incubation revealed no significant (p < 0.05) difference in inhibition among the three strains of L. monocytogenes for each type of lactic streptococcus present. Streptococcus cremoris was significantly (0.01 < p < 0.05) more inhibitory to all three strains of L. monocytogenes than was S. lactis at 24 and 30 h of incubation. IPCM-2 is considered ready for use at a pH of 5.4 or less, which was reached between 12 and 15 h of incubation in samples containing 0.25 or 1.0% S. cremoris. Populations of L. monocytogenes in such samples were ca. 104 to 106 CFU/ml regardless of strain of Listeria or percentage of S. cremoris added as inoculum. In samples initially containing 0.25 or 1.0% S. lactis, pH 5.4 was not reached until after 18–24 h of incubation. At this point all three strains of L. monocytogenes had grown to ca. 105 CFU/ml regardless of percentage of S. lactis added as inoculum. Despite the inhibition seen, substantial numbers of the pathogen were present when the medium was ready for use.

This content is only available as a PDF.