L. monocytogenes, S. enteritidis, S. aureus and E. coli strains
were maintained frozen in broth until use. Prior to use, the cultures
were activated by three successive transfers in tryptic soy
broth (Difco Laboratories) at 37 C for 24 h. Cells were harvested
by centrifugation (10000g for 10 min at 4 C),