Varied influence of curli on Shiga toxin-producing Escherichia coli O157, O26, O111 biofilm formation and animal cell adherence
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Shiga toxin-producing Escherichia coli (STEC) persist in cattle and can colonize the bovine recto-anal junction, making it an important reservoir for transmission. Curli are extracellular amyloid fibers associated with biofilm formation and environmental persistence, but their contribution to STEC adherence to bovine epithelial cells is unclear. This study examined the relationship between curli production, biofilm formation, and adherence of O157, O26, and O111 STEC isolates to bovine recto-anal junction (RAJ) squamous epithelial (RSE) cells. Thirty isolates were evaluated for RSE adherence, curli-associated phenotypes, and biofilm formation under environmental and host-associated growth conditions. O157 and O111 isolates predominantly exhibited strong aggregative adherence to RSE cells, whereas O26 isolates showed more heterogeneous, primarily diffuse adherence. O111 isolates produced the strongest biofilms under environmental conditions and displayed relatively stable curli phenotypes, while O157 and O26 isolates showed greater variation. However, environmental biofilm formation did not consistently correlate with RSE adherence. Biofilm formation was also markedly reduced under host-simulated conditions in DMEM-LG at 26, 37, and 39°C, including among isolates that produced strong environmental biofilms. Differences in Shiga toxin genotype and expression likewise did not account for the major adherence patterns, as O111 isolates lacking stx 2 retained strong aggregative adherence. These findings indicate that curli-associated biofilm formation is primarily influenced by environmental conditions and does not directly predict STEC adherence to bovine RSE cells. The results suggest that environmental persistence and host-cell attachment represent distinct phenotypes and that STEC adherence to bovine epithelial cells is likely mediated by additional strain- and serotype-specific factors.
IMPORTANCE
Our results highlight a paradoxical role for curli in STEC biofilm formation versus host cell adherence that has implications in both diagnostics and intervention strategies. Standard laboratory biofilm assays may not accurately reflect true STEC-host cell adherence phenotypes, and curli targeted for environmental sanitation may not reduce persistence at the bovine RAJ suggesting a need for multi-target pre-harvest interventions. A complex interplay of curli and non-curli factors, besides growth conditions, appears to influence environmental versus bovine RSE cell attachment that do not always correlate with genotypes.
