Pch Genes Control Biofilm and Cell Adhesion in a Clinical Serotype O157:H7 Isolate

In a previous study, induction of the serotype O157:H7 SOS response decreased expression in the clinical isolate PA20 at 30°C but strongly induced genes in the horizontally transferred-DNA regions (HTR), including many known virulence regulators. To determine the role of HTR regulators in the control of and curli, specific regulators were plasmid-expressed in the wild-type and mutant strains of PA20 and its biofilm-forming derivative, 20R2R. At 30°C, plasmid over-expression of the O157:H7 group 3 homolog, , strongly repressed PA20 transcription (>7-fold) while the group 1 homologs, and , resulted in smaller reductions (6-fold) making group 1 , which are enhanced by the SOS response, the likely SOS-induced repressors. Plasmid-based over-expression also reduced 20R2R biofilm formation (>6-fold) and the curli-dependent, Congo red affinity of both PA20 and 20R2R. However, to properly appreciate the regulatory direction, expression patterns, and environmental consequences of these and other CsgD-controlled functions, a better understanding of natural regulation will be required. The effects of HTR regulators on PA20 and 20R2R adhesion to HEp-2 cell at host temperature were also studied. Under conditions where prophage genes were not induced, curli, rather than , contributed to host cell adhesion in strain 20R2R. High levels of expression in trans reduced curli-dependent cell adherence (>2-fold) to both 20R2R and the clinical isolate PA20, providing a host-adapting adhesion control mechanism. Expression of was also repressed by induction of the SOS response at 37°C, providing a mechanism by which curli expression might complement EspA-dependent intimate adhesion initiated by the group1 homologs. This study has increased our understanding of the O157 genes at both host and environment temperatures, identifying as a strong regulator of and CsgD-dependent properties.