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Title: Escherichia coli O157:H7 acid sensitivity correlates with flocculation phenotype during nutrient limitation

item KAY, KATHRYN - North Carolina State University
item Breidt, Frederick
item Fratamico, Pina
item BARANZONI, GIAN MARCO - US Department Of Agriculture (USDA)
item KIM, GWANG HEE - Kangwon National University
item GRUNDEN, AMY - North Carolina State University
item OH, DEOG-HWAN - Kangwon National University

Submitted to: Frontiers in Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/11/2017
Publication Date: 7/26/2017
Publication URL:
Citation: Kay, K., Breidt, F., Fratamico, P.M., Baranzoni, G., Kim, G., Grunden, A., Oh, D. 2017. Escherichia coli O157:H7 acid sensitivity correlates with flocculation phenotype during nutrient limitation. Frontiers in Microbiology. 8:1404.

Interpretive Summary: We examined disease causing bacterial strains (Escherichia coli) to determine how acid resistant individual strains were. From previous research we knew that there is a wide range of acid resistance among these bacteria. Our research focused on determining the genetic mechanism(s) by which these important food pathogens are able to survive in acid conditions representative of acidic food products. We found that a particular regulatory mechanism involving regulatory RNA molecules inside the cells controlled not only acid resistance, but also how the cells respond to their environment by making a fibrous network that can aid in cells attaching to surfaces. This regulatory network is very complex and we have also identified some of the important components that respond to nutrients in the environment by using different kinds of growth media (with varying amounts of nutrients). By learning how the cells respond to acid and why some are better than others at survival in acid, we may be able to devise strategies for reducing the threat of these pathogens in foods, and also reduce survival of the bacteria in the stomach which is very acidic.

Technical Abstract: Shiga toxin producing Escherichia coli (STEC) strains vary in acid resistance; however, little is known about the underlying mechanisms that result in strain specific differences. Among 25 STEC O157:H7 strains tested, 7 strains flocculated when grown statically for 18 h in minimal salts medium at 37°C, while 18 strains did not. Interestingly, the flocculation phenotype (cells came out of suspension) was found to correlate with degree of acid sensitivity in an assay with 400 mM acetic acid solution at pH 3.3 targeting acidified foods. Strains exhibiting flocculation were more acid sensitive and were designated FAS, for flocculation acid sensitive, while the acid resistant strain designated PAR for planktonic acid resistant. Flocculation was not observed for any strains during growth in complex medium (Luria Bertani broth). STEC strains B201 and B241 were chosen as representative FAS (2.4 log reduction) and PAR (0.15 log reduction) strains, respectively, due to differences in acid resistance and flocculation phenotype. Results from electron microscopy showed evidence of fimbriae production in B201, whereas fimbriae were not observed in B241.Curli fimbriae production was identified through plating on Congo red differential medium, and all FAS strains showed curli fimbriae production. Surprisingly, 5 PAR strains also had evidence of curli production. Transcriptomic and targeted gene expression data for B201 and B241indicated that csg and hde (curli and acid induced chaperone genes, respectively) expression positively correlated with the phenotypic differences observed for these strains. These data suggest that FAS strains grown in minimal medium express curli, resulting in a flocculation phenotype. This may be regulated by GcvB, which positively regulates curli fimbriae production and represses acid chaperone proteins. RpoS and other regulatory mechanisms may impact curli fimbriae production, as well. These findings may help elucidate mechanisms underlying differences among STEC strains in relating acid resistance and biofilm formation.