The effects of salt and pH stress on the growth rates of persistent strains of Listeria monocytogenes collected from specific ecological niches

This study aimed to investigate the growth responses of persistent strains of Listeria monocytogenes to NaCl (5-10%) and pH (4-8.5) stress. Seven isolates of two persistent strains, collected from cheeses with 7 and 42 days of ripening, and from the shelf of one maturation room, and two reference strains were analysed. The experiments were carried out, in Tryptic Soy Broth (TSB), at 37 degrees C. Response surface methodology (RSM) was used to compare the effects of the selected variables on the growth rate of the isolates. Persistent strains showed, in general, higher growth rates than reference strains. The factor NaCl was highly significant to the isolate collected from the shelf. There was a different growth response between isolates from cheese with 7 and 42 days of ripening. These results suggest the adaptation of L. monocytogenes to specific environments within the dairies. (c) 2006 Elsevier Ltd. All rights reserved.