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 degrees 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 (10(3) CFU/ml of Enterococcus faecium) on frequency of detection was examined. Anaerobic PSU broth resulted in the lowest false-positive rate and the highest frequency 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 degrees 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 (>10(6) 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.
Suh
J H
JH
Department of Food Science, The Pennsylvania State University, University Park 16802, USA.
Knabel
S J
SJ
D000332:Aerobiosis; D000693:Anaerobiosis; D000818:Animals; D001431:Bacteriological Techniques; D002417:Cattle; D015169:Colony Count, Microbial; D003470:Culture Media; D016984:Enterococcus faecium / Q000254:growth & development; D005188:False Negative Reactions; D005189:False Positive Reactions; D006358:Hot Temperature; D007778:Lactobacillus / Q000502:physiology; D008089:Listeria monocytogenes / Q000302:isolation & purification*; D008892:Milk / Q000382:microbiology*
D003470:Culture Media