Cell viability of Listeria monocytogenes biofilms

Several authors have reported biofilm formation by Listeria monocytogenes, and it is suspected that biofilms form a unique niche for extended survival of this foodborne pathogen in food-processing environments. We have evaluated growth of two L. monocytogenes strains (Murray and 7148) in biofilms and analysed the relationship between culturable and viable-but-non-culturable (VBNC) cells. Biofilms were grown on glass slides in static conditions at 37 degrees C for up to 10 days. Culturable cells for L, monocytogenes Murray grew to 10(5) cfu cm(-2) within 2 days, while L. monocytogenes 7148 required 4 days to reach these cell numbers After 2 days, cell counts of L. monocytogenes Murray decreased, followed by another increase with cell numbers reaching almost 10(6) cfu cm(-2) on day 10. In contrast, cell counts of L. monocytogenes 7148 stayed close to 10(5) cfu cm(-2) until day 10. VBNC cells of L, monocytogenes Murray increased with biofilm age while this was not seen for strain 7148 Also, swabbing removed biofilms of strain Murray move easily than strain 7148. Comparisons of viable counts obtained for swabbed and in situ biofilms indicated that these strain differences are due either to variable composition of extracellular polymeric substances in the two biofilms or to different cell physiology of the two strains. (C) 2001 Academic Press.