Purpose: In this study, we compared the biofilm-forming abilities of selected S. enterica and E. coli strains of fresh produce origin on polystyrene surface and their adherence to alfalfa and bean sprouts.
Methods: Overnight cultures (2 ml) of seven S. enterica and four E. coli strains in six different broths were placed into polystyrene tissue culture plates for biofilm development for 2 to 7 days at 28°C. The biofilms formed were quantified using the crystal violet binding assay. In a separate experiment, alfalfa and bean sprouts (5 g) were exposed to 25 ml of overnight cultures of S. enterica or E. coli at 22°C for 2h with shaking at 40 rpm. Contaminated sprouts were rinsed twice with sterile water and homogenized with a stomacher at normal speed for 30 sec. Bacteria attached to sprouts were enumerated.
Results: Biofilm mass on polystyrene surface increased with incubation time (P < 0.05) with some exceptions. Among the six microbiological media used, LB no salt broth better supported biofilm development (P < 0.05). Two E. coli strains formed more biofilms than the Salmonella and other two E. coli strains (P < 0.05). In the attachment study, more Salmonella (5.66 log CFU/g) than E. coli (3.46 log CFU/g) cells attached to both sprouts (P < 0.05). Cells of both pathogens attached in higher numbers to bean sprouts compared to alfalfa sprouts (P < 0.05).
Significance: Pathogens with relatively poorer initial adherence can sometimes develop more biofilms than the stronger colonizers. Since native biofilms are abundant on food surface, the potential public health threats posed by the poorer colonizers should be considered.