This study assessed relationships among curli production, biofilm formation, and adherence of Shiga toxin-producing Escherichia coli (STEC) isolates from serotypes O157, O26, and O111 to bovine recto-anal junction squamous epithelial (RSE) cells. Thirty isolates were examined for curli-associated phenotypes, biofilm production under environmental and host-simulated conditions, and RSE-cell adherence. The objective was to determine whether curli-associated biofilm traits under laboratory conditions predict attachment to bovine epithelial cells relevant to cattle colonization.
Thirty STEC isolates representing serotypes O157, O26, and O111 were evaluated for three principal traits: curli-associated phenotypes, ability to form biofilms in defined environmental and host-like media, and patterns of adherence to bovine RSE cells. The study compared results across serotypes and under different growth temperatures and media to reflect environmental versus host-associated conditions.
Adherence assays on bovine recto-anal junction squamous epithelial cells revealed serotype-specific trends. Most O157 and O111 isolates exhibited strong aggregative adherence to RSE cells. In contrast, O26 isolates displayed more heterogeneity in adherence, with a predominance of diffuse adherence patterns. Thus, adherence to bovine epithelial cells varied by serotype and by strain within serotypes.
When cultured under environmental growth conditions, biofilm production varied between serotypes. O111 isolates generally produced the strongest environmental biofilms and showed relatively stable curli phenotypes. O157 and O26 isolates exhibited greater variation in curli-associated phenotypes and environmental biofilm formation. Importantly, strength of environmental biofilm formation did not consistently correlate with the observed RSE-cell adherence patterns: isolates that formed robust biofilms in environmental assays were not always strongly adherent to bovine epithelial cells.
Biofilm formation was substantially reduced when isolates were grown under host-simulated conditions using DMEM-LG at three temperatures evaluated (26°C, 37°C, and 39°C). This reduction occurred even among isolates that produced strong biofilms in environmental assays, indicating that conditions simulating the host environment suppress biofilm formation relative to environmental settings. These observations support the conclusion that curli-mediated biofilm phenotypes are highly context-dependent.
The study examined whether differences in Shiga toxin genotype or expression could account for adherence phenotypes. Findings indicate that Shiga toxin genotype and expression did not explain the major adherence patterns observed. For example, O111 isolates lacking stx2 retained strong aggregative adherence to RSE cells. Therefore, variation in Shiga toxin status did not predict the adherence phenotype in this collection of isolates.
Collectively, the data indicate that curli-associated biofilm formation is primarily influenced by environmental conditions and does not directly forecast STEC adherence to bovine RSE cells. Environmental persistence traits and host-cell attachment appear to be distinct phenotypes. Adherence to bovine epithelial cells likely depends on additional strain- and serotype-specific factors beyond curli-mediated biofilm capacity. The dissociation between environmental biofilm strength and epithelial adherence suggests separate regulatory or structural determinants govern these behaviors.
Because environmental persistence (curli-mediated biofilms) and epithelial colonization are not reliably linked in this dataset, interventions or surveillance strategies aimed at reducing STEC in cattle reservoirs should consider both phenotypes independently. Targeting only biofilm-associated environmental survival may not reduce bacterial adherence to bovine recto-anal junction cells, and vice versa. Further work is needed to identify the other strain- and serotype-specific factors that mediate attachment to bovine epithelial cells and to clarify how environmental signals and host conditions differentially regulate curli and other adhesins.
Note: The source reports the results summarized here; methodological details beyond those stated (for example, specific isolate origins, quantitative biofilm assay metrics, or statistical analyses) were not reported in the source abstract and therefore are not included in this summary.