Changes in Populations of Listeria monocytogenes in a Medium with Internal pH Control Containing Streptococcus cremoris

Abstract

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. 103 cfu/ml and Streptococcus cremoris (.25%, 1.6 x 105 or 1.0%, 8.6 x 105 cfu/ml) and was incubated at 21 or 30°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 increase 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 106 to 107/ml at 21°C and 108/ml at 30°C. At 21°C inhibition of Listeria began after 18 and 24 h and at 30°C after 12 and 15 h of incubation in samples inoculated with 1.0 and .25% S. cremoris, respectively. In all trials with L. monocytogenes plus S. cremoris, populations of the pathogen increased to 104 to 105/ml before the start of inhibition; the pH of the medium at that time was approximately 5.5 (the value at which it is claimed to be ready for use). Complete inhibition of L. monocytogenes in IPCM-1 medium by S. cremoris was not seen when this pH was reached. © 1990, American Dairy Science Association. All rights reserved.