Conditional function of autoaggregative protein cah and common cah mutations in Shiga toxin-producing Escherichia coli

Authors: Carter, Michelle; Brandl, Maria; Kudva, Indira; KATANI, ROBAB - Pennsylvania State University; MOREAU, MATTHEW - Pennsylvania State University; KAPUR, VIVEK - Pennsylvania State University

Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/20/2017
Publication Date: 12/15/2017
Citation: Carter, M.Q., Brandl, M., Kudva, I.T., Katani, R., Moreau, M.R., Kapur, V. 2017. Conditional function of autoaggregative protein cah and common cah mutations in Shiga toxin-producing Escherichia coli. Applied and Environmental Microbiology. 84(1):e01739-17. https://doi.org/10.1128/AEM.01739-17.
DOI: https://doi.org/10.1128/AEM.01739-17

Interpretive Summary: Shiga toxin-producing Escherichia coli (STEC) harbors genes encoding diverse adhesins and many of these are known to play an important role in bacterial attachment and host colonization. We demonstrated here that the autotransporter protein Cah confers E. coli DH5a cells with a strong autoaggregative phenotype that is inversely correlated with its ability to form biofilms, and plays a strain-specific role in plant and animal colonization by STEC. Although cah is widespread in the STEC population, we detected a mutation rate of 31.3% in cah, which is similar to that reported in rpoS and in fimH. Formation of cell aggregates due to increased bacteria-to-bacteria interactions may be disadvantageous to bacterial populations under conditions that favor a planktonic state in STEC. Therefore, loss-of-function mutation in cah is likely a selective trait in STEC when autoaggregative properties become detrimental to bacterial cells, and may contribute to the adaptability of STEC to fluctuating environments.

Technical Abstract: Cah is a calcium-binding autotransporter protein involved in autoaggregation and biofilm formation. Although cah is widespread in Shiga toxin-producing Escherichia coli (STEC), we detected mutations in cah at a frequency of 31.3% in this pathogen. In STEC O157:H7 super-shedder strain SS17, a large deletion results in a smaller coding sequence, lacking the C-terminal 71 amino acids compared with Cah in STEC O157:H7 strain EDL933. We examined the function of Cah in biofilm formation and host colonization to better understand selective pressures for cah mutations. EDL933-Cah played a conditional role in biofilm formation in vitro: it enhanced E. coli DH5a biofilm formation on glass surfaces under agitated culture conditions that prevented autoaggregation, but inhibited biofilm formation under hydrostatic conditions that facilitated autoaggregation. This function appeared to be strain-dependent since Cah-mediated biofilm formation was diminished when an EDL933-cah was expressed in SS17. Deletion of cah in EDL933 enhanced bacterial attachment to spinach leaves and altered the adherence pattern of EDL933 to bovine recto-anal junction squamous epithelial (RSE) cells. In contrast, in trans-expression of EDL933-cah in SS17 increased its attachment to leaf surfaces, and in DH5a, enhanced its adherence to RSE cells. Hence the ecological function of Cah appears to be modulated by environmental conditions and other bacterial strain-specific properties. Considering the prevalence of cah in STEC and its role in attachment and biofilm formation, cah mutations might be selected in ecological niches where inactivation of Cah would result in an increased fitness in STEC during colonization of plants or animal hosts.