Differences in pressure tolerance of Listeria monocytogenes strains are not correlated with other stress tolerances and are not based on differences in CtsR.

Thirty strains of Listeria monocytogenes were screened for their pressure tolerance phenotype at 400 MPa for 2 min at 21 degrees C. The strains exhibited reductions ranging from 1.9 to 7.1 log(10)CFU/ml in tryptic soy broth with 6% yeast extract (TSBYE). The 3 most and the 3 least pressure-tolerant strains were further tested for their thermal resistance (based on their ability to survive at 55 degrees C), acid tolerance (based on their ability to survive in acidified TSBYE; pH 2.0) and for their nisin sensitivity. No correlation between pressure tolerance and heat, acid or nisin resistances was found. Nucleotide sequence analysis of the ctsR region in these 6 strains demonstrated that this gene codes for a CtsR protein with identical predicted amino acid sequences. The sequences of the 200-bp region located immediately upstream of the ctsR start codon of the different strains were virtually identical and it is therefore likely that differences in pressure tolerance are based on factors other than the stress gene regulator CtsR. The pressure sensitivity of a cocktail of the 2 most pressure-resistant strains and a cocktail of the 2 most-sensitive strains was investigated when the cocktails were inoculated into a real food system consisting of ground chicken meat. We demonstrated that the nature of the suspending substrate or the temperature did not change the expected pressure tolerance of the cocktails.