Structure and viability of 24- and 72-h-old biofilms formed by four pathogenic bacteria on polystyrene and glass contact surfaces

The biofilms formed by Salmonella Hadar (SH174), Listeria monocytogenes (LM6), methicillin-resistant Staphylococcus aureus (MRSA125) and vancomycin-resistant Enterococcus faecium (VRE61s) on polystyrene and glass after 24 h and 72 h of incubation at 37 °C were examined by confocal laser scanning microscopy (CLSM) after staining with SYTO9 and propidium iodide (PI). A lower average biovolume (P < 0.05) was observed for SH174 biofilms (73,073.61 ± 52,365.90 μm3 in the observation field of 14,161 μm2) than for LM6 (180,804.50 ± 32,554.62 μm3), MRSA125 (208,605.34 ± 57,534.93 μm3) and VRE61s (212,543.91 ± 39,718.62 μm3) biofilms. SH174 showed the greatest (P < 0.05) biovolume on glass, as compared with polystyrene. Biofilms of LM6, MRSA125 and VRE61s were produced at comparable levels on both contact surfaces. After 24 h, SH174 formed small scattered cell clusters, and biovolume of biofilms increased (P < 0.05) after 72 h. By contrast, LM6, MRSA125 and VRE61s formed compact biofilms quickly (24 h) on both contact surfaces. Seventy-two-hour-old biofilms showed the largest biovolumes of dead or damaged (PI-stained) cells, except for MRSA125 (polystyrene) and VRE61s (polystyrene and glass). Appropriate procedures for the disinfection of food processing surfaces immediately after use are required to prevent the formation of biofilm by pathogenic bacteria.