Growth, viability and architecture of biofilms of Listeria monocytogenes formed on abiotic surfaces

Abstract

The pathogenic bacterium Listeria monocytogenes can persist in food processing plants for many years, even when appropriate hygienic measures are in place, with potential for contaminating ready-to-eat products and, its ability to form biofilms on abiotic surfaces certainly contributes for the environmental persistence. In this research, L. monocytogenes was grown in biofilms up 8 days attached to stainless steel and glass surfaces, contributing for advancing the knowledge on architecture of mature biofilms, since many literature studies carried out on this topic considered only early stages of cell adhesion. In this study, biofilm populations of two strains of L. monocytogenes (serotypes 1/2a and 4b) on stainless steel coupons and glass were examined using regular fluorescence microscopy, confocal laser scanning microscopy and classic culture method. The biofilms formed were not very dense and microscopic observations revealed uneven biofilm structures, with presence of exopolymeric matrix surrounding single cells, small aggregates and microcolonies, in a honeycomb-like arrangement. Moreover, planktonic population of L. monocytogenes (present in broth media covering the abiotic surface) remained stable throughout the incubation time, which indicates an efficient dispersal mechanism, since the culture medium was replaced daily. In conclusion, even if these strains of L. monocytogenes were not able to form thick multilayer biofilms, it was noticeable their high persistence on abiotic surfaces, reinforcing the need to focus on measures to avoid biofilm formation, instead of trying to eradicate mature biofilms.

Keywords: Abiotic surfaces; Biofilms; Listeria monocytogenes; Persistence.

Fig. 1

Photomicrographs of biofilms of L. monocytogenes IAL 633 serotype 1/2a (A–C) and L. monocytogenes ATCC 19115 serotype 4b (D–F) acquired with CLSM (Leica SP5), ocular 10× and objective 63×, using immersion oil. Cells stained with SYTO 9 were green and cells stained with PI were red: 24 h (A and D); 96 h (B and E); 192 h (C and F). Estimates of viable and non-viable cells (G and H) respectively for L. monocytogenes ATCC 19115 and L. monocytogenes IAL 633 are also shown, expressed as fluorescence units per volume (FU μm⁻³). ^*#Denote significant statistical difference in viable cells.