Aim: To determine the critical component(s) of skim milk for biofilm formation of Cronobacter species.
Methods and results: Biofilm forming ability of 72 Cronobacter strains in skim milk preparation was assayed by crystal violet staining. The results revealed that whey protein and casein are more important determinants of skim milk for biofilm formation than lactose, although there was a wide variation in biofilm forming ability. Biofilm structure and capsular material of six strains exhibiting different biofilm forming ability was investigated via electron microscopes. Scanning electron microscopy showed visually that while the strong biofilm formers (E27B, FSM 30 and 2.82) resulted in almost complete coagulation of skim milk, the weak biofilm formers (55, FSM 290 and 2.84) caused less coagulation. No capsule was clearly delineated in transmission electron micrographs of either strong or weak biofilm formers.
Conclusion: These results indicate that, for biofilm formation of Cronobacter species in skim milk, nitrogen source is probably a more important determinant than carbohydrate, and that strong biofilm formers are responsible for substantial coagulation of skim milk.
Significance and impact of the study: This study provides information for better understanding of the underlying mechanisms by which Cronobacter species form biofilm in infant formula milk.