Assessment of the prevention of biofilm formation by (a,b) monoculture or (c,d) mixed cultures by chitosan nanoparticles (ChNP) or chitosan nanoparticles conjugated to ɛ-poly-L-lysine (ChNP-PL) on five foodborne pathogens, L. monocytogenes (Lm) F4244, S. aureus (Sa) 25923, P. aeruginosa (Pa) PRI199, S. Enteritidis (SE) 1344, and E. coli (Ec) O157:H7 EDL933. Panels are (a) bacterial counts, (b) crystal violet staining of monoculture biofilms in a microtiter plate, and (c) bacterial counts and crystal violet staining of mixed culture biofilms of Lm and Sa, or Lm and Pa with ChNP or ChNP-PL. Bacteria isolated from biofilms were quantified by the plating method. A pairwise Student’s t-test was used for statistical analysis. * p < 0.005, ** p < 0.005, *** p < 0.0005, **** p < 0.0001. Dotted lines represent the detection limit.