Listeria monocytogenes Strains Differ in Their Ability to Form Biofilms but Show Similar Swarming and Response to Sanitizers

Introduction:  Listeria monocytogenes is composed of two subpopulations that differ in their likelihood and ability to cause invasive listeriosis.  Epidemic clone (EC) strains have been linked to most listeriosis outbreaks and sporadic cases in several countries, yet are infrequently isolated from foods. Nearly 50% of L. monocytogenes isolates from foods and 40% of isolates from food-associated environments carry one of 18 described mutations leading to a premature stop codon (PMSC) in the key virulence factor Internalin A (InlA, encoded by inlA). 

Purpose:  The purpose of this study was the evaluate the hypothesis that L. monocytogenes isolates carrying a PMSC mutation in inlAadapted to food-associated environments as a trade-off for loss of full virulence.  

Methods:  We assembled 13 EC strains and 15 strains carrying the most common PMSCs and tested the ability of these strains to swarm, form biofilms, and survive recommended and sub-inhibitory concentrations of two sanitizers.  Swarming assays were performed at 30 °C in semi-soft agar.  Biofilm experiments were performed under biofilm promoting conditions in polyvinyl chloride microplates, followed by crystal violet staining to quantify biofilm formation.  Lastly, broth microdilution assays determined the susceptibility of strains to recommended and sub-inhibitory concentrations of multi-quaternary ammonium compound (QAC) and sodium hypochlorite (SHC) under nutrient rich and nutrient limiting conditions. 

Results:  EC and inlA PMSC strains did not differ in their ability to swarm after 72 h (P > 0.05).  inlA PMSC strains demonstrated enhanced biofilm formation compared to EC strains, after 3 and 5 days (P = 0.0199 and P = 0.0068, respectively).  Under nutrient rich conditions, all but one strain were found to be susceptible to all concentrations of QAC and all but three strains were resistant to all concentrations of SHC. However, under nutrient limiting conditions, all strains were susceptible to all concentrations of QAC and some concentrations of SHC.  

Significance:  inlA PMSC strains demonstrated increased biofilm formation when compared to EC strains, which may explain their common presence in food-associated environments.  EC strains showed similar swarming abilities when compared to inlA PMSC strains and both subpopulations showed similar responses to sanitizers.