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The biofilm life style helps bacteria resist oxidative stress, desiccation, antibiotic treatment, and starvation. Biofilm formation involves a complex regulatory gene network controlled by various environmental signals. It was previously shown that prophage insertions in  mlrA  and heterogeneous mutations in  rpoS  constituted major obstacles limiting biofilm formation and the expression of extracellular curli fibers in strains of  Escherichia coli  serotype O157:H7. The purpose of this study was to test strains from other important serotypes of Shiga toxin-producing  E. coli  (STEC) (O26, O45, O103, O111, O113, O121, and O145) for similar regulatory restrictions. In a small but diverse collection of biofilm-forming and non-forming strains,  mlrA  prophage insertions were identified in only 4 of the 19 strains (serotypes O103, O113, and O145). Only the STEC O103 and O113 strains could be complemented by a trans-copy of  mlrA  to restore curli production and Congo red (CR) dye affinity. RpoS mutations were found in 5 strains (4 serotypes), each with low CR affinity, and the defects were moderately restored by a wild-type copy of  rpoS  in 2 of the 3 strains attempted. Fourteen strains in this study showed no or weak biofilm formation, of which 9 could be explained by prophage insertions or  rpoS  mutations. However, each of the remaining five biofilm-deficient strains, as well as the two O145 strains that could not be complemented by  mlrA , showed complete or nearly complete lack of motility. This study indicates that  mlrA  prophage insertions and  rpoS  mutations do limit biofilm and curli expression in the non-serotype O157:H7 STEC but prophage insertions may not be as common as in serotype O157:H7 strains. The results also suggest that lack of motility provides a third major factor limiting biofilm formation in the non-O157:H7 STEC. Understanding biofilm regulatory mechanisms will prove beneficial in reducing pathogen survival and enhancing food safety.

Phenotypic and Genotypic Characterization of Biofilm Forming Capabilities in Non-O157 Shiga Toxin-Producing Escherichia coli Strains