Changes in populations of Listeria monocytogenes in a medium with internal pH control containing Streptococcus cremoris.

Behavior of Listeria monocytogenes in a commercial starter culture medium with internal pH control was examined. The IPCM-1 medium was inoculated with L. monocytogenes (strain V7, Scott A, or California) at ca. 10(3) cfu/ml and Streptococcus cremoris (.25%, 1.6 x 10(5) or 1.0%, 8.6 x 10(5) cfu/ml) and was incubated at 21 or 30 degrees C for 30 h. The pH of the uninoculated medium and control (L. monocytogenes only) was between 6.8 and 7.0 before and after incubation. The area on a figure between control and treatment curves for numbers of L. monocytogenes was calculated, designated as area of inhibition, and used to quantitate inhibition of L. monocytogenes caused by S. cremoris. No significant difference was found in area of inhibition or pH values at 6, 24, and 30 h of incubation among the three strains of L. monocytogenes for a given set of conditions (percentage of S. cremoris added and temperature). Inhibition of the pathogen increased with an increase in amount of S. cremoris inoculum used and with higher rather than lower incubation temperature. Populations of L. monocytogenes in IPCM-1 medium without S. cremoris after 30 h of incubation were 10(6) to 10(7)/ml at 21 degrees C and 10(8)/ml at 30 degrees C. At 21 degrees C inhibition of Listeria began after 18 and 24 h and at 30 degrees C after 12 and 15 h of incubation in samples inoculated with 1.0 and .25% S. cremoris, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)