Quantification of mature Listeria monocytogenes biofilm cells formed by an in vitro model: A comparison of different methods

The presence of biofilms in food industrial environments is one of the main causes associated with food product contamination by L. monocytogenes. Biofilm control in the food industry is very relevant to public health and finding reliable and realistic quantification methods is essential. The aim of this study is to compare five L. monocytogenes biofilm quantification methods - conventional plate count, TEMPO, DEM, VIDAS and qPCR - and to examine a biodetector to visually detect biofilms in industrial settings. Results show that depending on the biofilm matrix production, the recovery of cells that conform the biofilm can be low and therefore, if it is an indirect method, microbial counts can be underestimated. At a species level, the methods that did not present significant differences were plate count, TEMPO (P = 0.998), DEM and qPCR (P = 0.508), so correlation studies were performed which established high correlation for plate count and TEMPO, but not for DEM and qPCR. The VIDAS method was adjusted so that it could quantify the biofilms, but the standard curve only allowed counts from 7 Log CFU cm^-2. Results also revealed that the different strains of L. monocytogenes possess different biofilm-forming abilities, although it was not possible to correlate the capacity to produce these structures with the distinct serotypes. Last, visually detecting biofilms on stainless steel coupons proved that in industrial environments nowadays they can be rapidly and qualitatively detected so that relevant decisions can immediately be taken.