Comparison of different enrichment broths and background flora for detection of heat-injured listeria monocytogenes in whole milk

Various primary enrichment broths, including University of Vermont medium (UVM), Listeria enrichment broth (LEB), modified LEB, and aerobic and anaerobic L-PALCAMY, were compared with aerobic and anaerobic Pennsylvania State University (PSU) broths for the detection of severely heat-injured (62.8°C for 5, 10, or 15 min; no colony appearance after heat injury on aerobic Trypticase soy agar containing 0.6% yeast extract and modified Oxford medium) Listeria monocytogenes Scott A. Anaerobic conditions were produced by adding L-cysteine and then purging the headspace with N₂. The effect of uninjured background flora (10³ CFU/ml of Enterococcus faecium) on frequency of detection was examined. Anaerobic-PSU broth resulted in the lowest false-positive rate and the highest freuqncy of detection of severely heat-injured L. monocytogenes compared with UVM, LEB, and modified LEB (P<0.05). The presence of E. faecium significantly enhanced the detection of heat-injured (10 min at 62.8°C) L. monocytogenes in aerobic and anaerobic PSU and aerobic and anaerobic L-PALCAMY broths (P<0.05). The highest concentration of uninjured E. faecium (>10⁶ CFU/ml) inhibited the detection of heat-injured L. monocytogenes (P<0.05). A heat-resistant, LiCl-tolerant Lactobacillus isolate from raw milk increased the rate of both false-positive and false-negative reactions.