Highly selective medium for isolation of Listeria monocytogenes from food

Abstract

A new selective medium (Al-Zoreky-Sandine listeria medium [ASLM]) was formulated to recover Listeria monocytogenes from food specimens; the medium completely inhibited common food microflora. Recognition of Listeria colonies is evident by black discoloration of the medium due to esculin hydrolysis without need for special illuminating equipment. The medium contains acriflavin, ceftazidime, and moxalactam as selective agents. Compared with Listeria Selective Agar, ASLM was equally effective in recovering L. monocytogenes. However, ASLM inhibited micrococci, enterococci, and gram-negative bacteria, especially a strain that mimicked L. monocytogenes on Listeria Selective Agar. The new medium was able to recover heat injured cells with only 15% less count than the nonselective medium. Recent food outbreaks of listeriosis (6, 9, 12, 15, 20) have emphasized the need for a highly selective medium to detect Listeria monocytogenes, a human and animal pathogen. Listeriosis has a mortality rate of up to 33% (5, 17) and is highest for pregnant women, infants, and immunocompromised persons. Fifty percent lethal doses of L. monocytogenes are as low as 5 cells for immunocompromised mice, whereas the 50% lethal dose for normal mice is 105 to 107 * Corresponding author. t Technical paper no. 9220, Oregon Agricultural Experiment Station.

Comparison of cold enrichment and U.S. Department of Agriculture methods for isolating Listeria monocytogenes from naturally contaminated foods. The Listeria Study Group

“…The sublethally stressed cells were probably sensitive to the selective ingredients of LPM agar, which was used for the direct plating test (23,33). Sublethal injury in L. monocytogenes has been documented by several researchers and is an important consideration in the isolation of L. monocytogenes from foods (2,3,16,24).…”

Biocontrol of Listeria monocytogenes in a model cultured milk (lben) by in situ bacteriocin production from Lactococcus lactis ssp. lactis

“…Listeria spp. was counted on Al‐Zoreky‐Sandine Listeria Medium (ASLM) agar 22 after incubation at 37°C for 48–72 h.…”

Listeria monocytogenes, a food-borne pathogen.

“…Berrang et al (26) found that L. monocytogenes grew well at 15°C on fresh vegetables stored in air or under a controlled atmosphere, increasing in number by about 4 logs within 6 days on asparagus, broccoli, and cauliflower. It did not grow nearly as well at 4°C, however, increasing by only about 1 log after 14 days on asparagus and actually decreasing by about 0.5 log after 14 days on broccoli and cauliflower.…”