Submitted to: American Society of Microbiologists Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: July 4, 2000
Publication Date: N/A
Technical Abstract: Some Escherichia coli strains express coiled protein surface fibers known as curli during low-temperature, low-salt, and stationary-phase growth conditions. Curli expression requires the gene products of two divergently transcribed operons. The csgBA operon encodes the curli subunit (CsgA) and the nucleator protein (CsgB). The csgDEFG operon encodes a transcriptional activator (CsgD) and putative assembly factors (CsgE, CsgF, CsgG). We identified two strains of E. coli O157:H7 (A1 and A2) which expressed curli in a phase variant manner. Curli fibers bind to congo red dye so that curli-expressing strains produce rough, dry, red colonies on agar containing congo red, while curli-negative strains produce white, smooth, moist colonies. Strains A1 and A2 produced both colony morphology variants. Serial passage of red (curliated) variants at 37C generated mixed red and white colonies. White (curli-negative) variants were stable under all growth conditions tested. PCR product amplified from the csgDEFG and csgBA operons of white and red colonies failed to identify insertions or deletions when compared by gel electrophoresis. However, comparison of the intergenic sequence between csgD and csgB of red and white variants identified a single base-pair difference at one of two locations in the -10 region of the csgDEFG promoter region. Also, the PCR amplified and plasmid- cloned csgDEFG operon from a strain A2 red colony transformed white colonies from both strains A1 and A2 into the rough, dry, red curliated phenotype. These results suggest that some E. coli O157:H7 strains undergo phase variation in colony morphology due to changes in curli expression. This variation may involve sequence changes in the promoter region of the csgDEFG operon.