Survival of Listeria monocytogenes and Enterococcus faecium in sludge evaluated by real-time PCR and culture methods

Aims: This study evaluates the behaviour in spiked sludge of a pathogenic bacteria, Listeria monocytogenes, by cultural and molecular techniques, and compares its survival with the one of a faecal indicator, Enterococcus faecium. Methods and Results: Listeria monocytogenes strain Scott A and E. faecium T were followed for 17 days after inoculation in sludge. Kinetics of survival depended on the bacteria and on the technique used [ most probable number method, direct plate count or real-time quantitative PCR (qPCR)]. The concentration of L. monocytogenes decreased rapidly regardless of the technique, but the decrease was much more dramatic with culture techniques than with qPCR. On the contrary, the concentrations of culturable E. faecium T were stable. Conclusions: The results suggest that the cells of L. monocytogenes strain Scott A might have entered a viable, but nonculturable (VBNC) status, whereas cells of the indicator bacteria, E. faecium T, maintained themselves better and stayed culturable. Significance and Impact of the Study: The difference of survival kinetics in the sludge of a faecal indicator ( E. faecium) and a pathogenic bacterium ( L. monocytogenes) may be linked to the fact that they either enter or do not enter into a VBNC status.