Microtiter plate-based in vitro biofilm formation assay of E. coli and L. monocytogenes. The formation of biofilm (at 24 and 48 h post-inoculation, at 30°C) was measured by optical density readings at 550 nm. Values are presented as the mean ± SE of three experiments, repeated eight times. Columns mean, bars SE. Columns with different letters indicate significant differences (P < 0.05).