INJURY, RESUSCITATION AND DETECTION OF LISTERIA SPP. FROM FROZEN ENVIRONMENTS

The ability of Listeria spp. to be reversibly freeze-injured was deter mined and methods for detection of freeze-injured Listeria were evalua ted. Strains of Listeria monocytogenes and Listeria innocua tested sho wed no significant variation as to the extent of injury sustained duri ng storage for 24 h at -9 to -11 degrees C. Average 24 h injury ranged from 44-64% for L. monocytogenes strains and 41-54% for L. innocua st rains. The most substantial increase in injury was seen in. the first 24 h and injury remained constant or: increased slightly throughout th e 14 day period. Freeze injury teas reversible in all Listeria strains tested when trypticase soy broth and Listeria repair broth (LRB) were used as repair media. In order to determine if nonselective pre-enric hment followed by selective enrichment using LRB, resulted in increase d detection of Listeria existing naturally in processing environments, samples were obtained from two dairy and one meat processing plant. U se of nonselective preenrichment followed by selective enrichment usin g LRB did not enhance recovery of Listeria from frozen environments. T his may be attributed to a limited amount of freeze-injury occurring i n a naturally existing population of Listeria.