Bessaiah, H. | Search bioRxiv

bioRxiv2022

Adaptive radioresistance of Enterohemorrhagic Escherichia coli O157:H7 results in genomic loss of shiga toxin-encoding prophages

Enterohemorragic Escherichia coli (EHEC) O157:H7 is a foodborne pathogen producing shiga toxins (Stx1 and Stx2), can cause hemorrhagic diarrhea, and life-threatening infections. O157:H7 strain EDL933 carries prophages CP-933V and BP-933W that encode shiga toxin genes (stx1 and stx2 respectively). The aim of this work was to investigate the mechanisms of adaptive resistance of EHEC strain EDL933 to a typically "lethal" dose of {gamma}-irradiation (1.5 kGy). Adaptive selection through six passages of exposure to 1.5 kGy resulted in the loss of CP-933V and BP-933W prophages from the genome and mutations within three genes: wrbA, rpoA, and Wt_02639 (molY). Three selected EHEC clones that became irradiation-adapted to the 1.5 kGy dose (C1, C2 and C3) demonstrated increased resistance to oxidative stress, sensitivity to acid pH, and decreased cytotoxicity to Vero cells. To confirm that loss of prophages plays a role in increased radioresistance, C1 and C2 clones were exposed to bacteriophage containing lysates. Although, phage BP-933W could lysogenize C1, C2, and E.coli K-12 strain MG1655, it was not found to have integrated into the bacterial chromosome in C1-{Phi} and C2-{Phi} lysogens. Interestingly, for the E. coli K-12 lysogen (K12-{Phi}), BP-933W DNA had integrated at the wrbA gene (K12-{Phi}). Both C1-{Phi} and C2-{Phi} lysogens regained sensitivity to oxidative stress, were more effectively killed by a 1.5 kGy {gamma}-irradiation dose and had regained cytotoxicity and acid resistance phenotypes. Further, the K12-{Phi} lysogen became cytotoxic, more sensitive to {gamma}-irradiation and oxidative stress and slightly more acid resistant. ImportanceGamma ({gamma})-irradiation of food products can provide an effective means of eliminating bacterial pathogens such as enterohemorrhagic Escherichia coli (EHEC) O157:H7, a significant foodborne pathogen that can cause severe disease due to the production of Shiga toxins. To decipher the mechanisms of adaptive resistance of the O157:H7 strain EDL933, we evolved clones of this bacterium resistant to a lethal dose of {gamma}-irradiation by repeatedly exposing bacterial cells to irradiation following a growth restoration over six successive passages. Our findings provide evidence that adaptive selection involved modifications in the bacterial genome including deletion of the CP-933V and BP-933W prophages. These mutations in EHAC O157:H7 resulted in loss of stx1, stx2, loss of cytotoxicity to epithelial cells and decreased resistance to acidity, critical virulence determinants of EHEC, concomitant with increased resistance to lethal irradiation and oxidative stress. These findings demonstrate that the potential adaptation of EHEC to high doses of radiation would involve elimination of the Stx encoding phages and likely lead to a substantial attenuation of virulence.

microbiology↗

bioRxiv2021

A newly identified group of P-like (PL) fimbriae from extra-intestinal pathogenic Escherichia coli (ExPEC) encode distinct adhesin subunits and mediate adherence to host cells

Fimbrial adhesins play a critical role for bacterial adherence and biofilm formation. Sequencing of avian pathogenic Escherichia coli (APEC) strain QT598 identified a fimbrial gene cluster belonging to the {pi} group that we named PL (P-like) fimbriae, since genetic organization and sequence are similar to Pap and related fimbriae. Screening of genomic databases indicated that genes encoding PL fimbriae located on IncF plasmids are present in a diversity of E. coli isolates from poultry, human systemic infections, and other sources. As with P fimbriae, PL fimbriae exhibit sequence divergence in adhesin encoding genes, and strains could be divided into 5 classes based on differences in sequences of the PlfG adhesin protein. The plf genes from two predominant PlfG adhesin classes, PlfG-I and PlfG-II were cloned. PL fimbriae were visualized by electron microscopy, promoted biofilm formation, demonstrated distinct hemagglutination profiles and promoted adherence to human bladder and kidney epithelial cell lines. Hybrid fimbriae comprised of genes from plfQT598 wherein plfG was replaced by papG encoding adhesin genes were also shown to be functional and mediate adherence to epithelial cells, further indicating similarity and functional compatibility between these two types of fimbriae. Although deletion of plf genes did not significantly reduce colonization of the mouse urinary tract, plf gene expression was increased over 40-fold in the bladder compared to during in vitro culture. Overall, PL fimbriae represent a new group of fimbriae demonstrating both functional differences and similarities to P fimbriae and may contribute to adherence to cells and colonization of host tissues. ImportanceFimbriae are important colonization factors in many bacterial species. The identification of a new type of fimbriae encoded on some IncF plasmids in E. coli was investigated. Genomic sequences demonstrated these fimbrial gene clusters have genetic diversity, particularly in the adhesin encoding PlfG gene. Functional studies demonstrated differences in hemagglutination specificity, although both types of Plf adhesin under study mediated adherence to human urinary epithelial cells. Such fimbriae may represent previously unrecognized adhesins that could contribute to host specificity and tissue tropism of some E. coli strains.

microbiology↗