Treatment with disinfectants may induce an increase in viable but non culturable populations of Listeria monocytogenes in biofilms formed in smoked salmon processing environments

The objectives of this study were 1) to evaluate the impact of two industrial disinfectants on the viability of Listeria monocytogenes populations in biofilm and 2) to investigate the viability state of L. monocytogenes cells present on contact surfaces in the smoked salmon processing environment. In the first step, we cultured mono species and mixed species biofilms containing L. monocytogenes on stainless steel or polyvinyl chloride (PVC) at 8 degrees C for 48h. The biofilms were then exposed to quaternary ammonium- and hydrogen peroxide-based disinfectants. Residual total populations of L. monocytogenes were measured by qPCR, and viable culturable (VC) cell populations were quantified using standard microbiological culture-based techniques and by a quantitative PCR (qPCR) assay coupled with a propidium monoazide treatment. Decreases in VC populations and the appearance of viable but non culturable (VBNC) populations were observed in response to treatment with the disinfectants. An 8 month sampling campaign in 4 smoked salmon processing plants was also carried out to detect L. monocytogenes in environmental samples. VBNC cells were detected mainly after the cleaning and disinfection operations. This study showed that industrial disinfectants did not inactivate all L. monocytogenes cells on inert surfaces. The presence of VBNC populations of L. monocytogenes in the smoked salmon processing environment is a public health concern.