SUITABILITY OF SELECTIVE MEDIA FOR RECOVERY AND ENUMERATION OF SUBLETHALLY HEAT- AND ACID-INJURED LISTERIA MONOCYTOGENES

The suitability of PALCAM and modified Oxford (MOX) agars for recovering sublethally heat- and lactic acid-injured Listeria monocytogenes was investigated. L. monocytogenes LM101M, LM103M (meat isolates), and Scott A were suspended in tryptose phosphate broth (TPB), heated for up to 40 min at 54C, and surface plated onto tryptose phosphate agar (TPA), TPA + 4% NaCl (TPAS), PALCAM, and MOX. TPA and TPAS were used to determine total viable and sublethally injured populations, respectively. Heat-injured LM103M was recovered in the highest numbers on all media, followed by Scott A and LM101M (P<0.01). TPA allowed best recovery of all test strains, followed by PALCAMand MOX which were not different, and TPAS (P<0.01). For acid-injury studies, uninjured and heat-injured (54C for 20 min) test strains were suspended in phosphate-buffered TPB + 0.85% lactic acid (bTPBLA) at 25C for up to 24 h and plated as described above. Uninjured and heat-injured L. monocytogenes were recovered better from bTPBLA on MOX than on PALCAM (P<0.05). Heat injured L. monocytogenes LM103M was recovered better than LM101M but similar to Scott A on MOX and PALCAM (P<0.05), whereas Scott A was recovered similarly to LM101M and LM103M on MOX and PALCAM (P>0.05). Acid-injury of L. monocytogenes LM103M was enhanced by prior heat stress.