|Hofmann, Christopher - Former ARS Employee|
|Nguyen, Ly Huong|
Submitted to: PLoS One
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/20/2013
Publication Date: 12/27/2013
Citation: Chen, C., Hofmann, C.S., Cottrell, B.J., Strobaugh Jr, T.P., Paoli, G., Nguyen, L.T., Yan, X., Uhlich, G.A. 2013. Phenotypic and genotypic characterization of biofilm forming capability in non-O157 Shiga toxin-producing Escherichia coli strains. PLoS One. 8(12):e84863.
Interpretive Summary: Shiga toxin-producing Escherichia coli (STEC) are major food-safety pathogens in the United States and abroad. Serotype O157:H7 remains the most important STEC in many parts of the world but six other serogroups (O26, O145, O103, O121, O45, and O111) are also considered important. Bacteria growing on surfaces can encase themselves in a secreted polysaccharide matrix (biofilm formation) to increase their resistance to harsh environmental conditions. In the STEC, the role of biofilms in causing disease and promoting attachment to food processing surfaces remains in question because biofilm expression is sporadic in many strains and because of our general lack of knowledge regarding how and why biofilms are formed. We have shown previously that interruption of a biofilm regulatory gene by the insertion of a bacteriophage (virus that infects bacteria) and sporadic changes (mutations) in a second regulatory gene are responsible for limiting biofilm formation in serotype O157:H7. In this study we tested the six important non-O157 STEC, plus serogroup O113, for similar regulatory impediments. We found that non-O157 STEC were more likely to form biofilm than O157:H7 strains and bacteriophage insertions were less common than in serotype O157:H7. In addition, we found that lack of motility was also associated with biofilm failure in non-O157 strains. These results expand our knowledge of the mechanisms controlling biofilm formation in the major serogroups of STEC food-borne pathogens and will help identify control points for intervention strategies to reduce STEC persistence in the food production chain.
Technical Abstract: 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 Escherichia 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 those strains from O103 and O113 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 near 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 are not 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.