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 LEE, and aerobic and anaero bic L-PALCAMY, were compared with aerobic and anaerobic Pennsylvania State University (PSU) broths for the detection of severely heat-injured (62.8 de greesC for 5, 10, or 15 min; no colony appearance after heat injury on aero bic Trypticase soy agar containing 0.6% yeast extract and modified Oxford m edium) Listeria monocytogenes Scott A. Anaerobic conditions were produced b y adding L-cysteine and then purging the headspace with N-2 The effect of u ninjured background flora (10(3) CFU/mL of Enterococcus faecium) on frequen cy of detection was examined. Anaerobic PSU broth resulted in the lowest fa lse-positive rate and the highest frequency of detection of severely heat-i njured L. monocytogenes compared with UVM, LEE, and modified LEE (P < 0.05) . The presence of E. faecium significantly enhanced the detection of heat-i njured (10 min at 62.8<degrees>C) L. monocytogenes in aerobic and anaerobic PSU and aerobic and anaerobic L-PALCAMY broths (P ( 0.05). The highest con centration of uninjured E. faecium (>10(6) CFU/ml) inhibited the detection of heat-injured L. monocytogenes (P < 0.05). A heat-resistant, LiCl-toleran t Lactobacillus isolate from raw milk increased the rate of both false-posi tive and false-negative reactions.