LISTERIA-INNOCUA TRANSFORMED WITH AN ANTIBIOTIC-RESISTANCE PLASMID AS A THERMAL-RESISTANCE INDICATOR FOR LISTERIA-MONOCYTOGENES

Listeria innocua PFEI is a chloramphenicol- and erythromycin-resistant organism obtained by electrotransforming L. innocua ATCC 33091 with the plasmid pGK12. L. innocua ATCC 33091 and L. innocua PFEI were more heat resistant between 56 and 66-degrees-C than Listeria monocytogenes F5069 and Scott A when evaluated in sterile phosphate buffer or milk. Decimal reduction times of each L. innocua strain were 1.5 to 3 times longer in either heating menstruum than were D-values of the most heat resistant L. monocytogenes strain studied (F5069). L. innocua PFEI retained the plasmid during heating so that, of 300 survivors evaluated, 100% were resistant to chloramphenicol and 98% were resistant to erythromycin. Thus, L. innocua ATCC 33091 or PFEI would be useful indicator organisms to evaluate the lethality of thermal processes with respect to L. monocytogenes. L. innocua PFEI has the advantages that it could easily be selected and enumerated among a large, complex, background microflora, but it may not be appropriate for application in a food processing environment.