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

The ability of Listeria spp. to be reversibly freeze-injured was determined and methods for detection of freeze-injured Listeria were evaluated. Strains of Listeria monocytogenes and Listeria innocua tested showed no significant variation as to the extent of injury sustained during 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 strains. The most substantial increase in injury was seen in. the first 24 h and injury remained constant or: increased slightly throughout the 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-enrichment followed by selective enrichment using LRB, resulted in increased detection of Listeria existing naturally in processing environments, samples were obtained from two dairy and one meat processing plant. Use of nonselective preenrichment followed by selective enrichment using LRB did not enhance recovery of Listeria from frozen environments. This may be attributed to a limited amount of freeze-injury occurring in a naturally existing population of Listeria.