Luminescent Salmonella strains as real time reporters of growth and recovery from sublethal injury in food

LuxA and luxB genes from V. harveyi carried by a Tn-5 containing plasmid were introduced into S. enteritidis by either conjugation or electroporation. Lux genes were used as a reporter to indicate the effects of heat treatment (50 °C, 55 °C and 65 °C) and pH (from 1 to 7) on the survival, growth and recovery of Salmonella cells. When a luminescent S. enteritidis strain, obtained by electroporation, was subjected to a plasmid curing procedure, the resulting culture lost the plasmid but remained luminescent. Southern hybridization was performed to determine the location of lux genes in S. enteritidis cells. The light output from recombinant cultures with different gene locations was compared and found to be higher for strains in which the luciferase was plasmid-mediated. The luminescent Salmonella culture was inoculated in food samples such as homogenized chicken meat, whole liquid eggs and fluid milk and development of luminescence with time was monitored. The minimum number of Salmonella cells required for positive observation was ~1 × 102 CFUml in broth and homogenized meat cultures, 2 × 103 CFUml in fluid milk and 7 × 103 CFUml in whole liquid egg.