matB, a common fimbrillin gene of Escherichia coli, expressed in a genetically conserved, virulent clonal group

A novel fimbrial type in Escherichia coli was identified and characterized. The expression of the fimbria was associated with the O18acK1H7 clonal group of E. coli, which cause newborn meningitis and septicemia when grown at low temperature; hence, it was named the Mat (meningitis associated and temperature regulated) fimbria. The fimbriae were purified from a fimA::cat sfaA::Gm fliC::St derivative of the O18K1H7 isolate E. coli THE 3034. The purified Mat fimbrillin had an apparent molecular mass of 18 kDa and did not serologically cross-react with the type 1 or S fimbria of the same strain. The matB gene encoding the major fimbrillin was cloned from the genomic DNA of the fimA::cat sfaA::Gm fliC::St derivative of THE 3034. The predicted MatB sequence was of 195 amino acids, contained a signal sequence of 22 residues, and did not show significant homology to any of the previously characterized fimbrial proteins. The DNA sequence of matB was 97.8% identical to a region from nucleotides 17882 to 18469 in the 6- to 8-min region of the E. coli K-12 chromosome, reported to encode a hypothetical protein. The 7-kb DNA fragment containing matB of THE 3034 was found by restriction mapping and partial DNA sequencing to be highly similar to the corresponding region in the K-12 chromosome. Trans complementation of the matB::cat mutation in the THE 3034 chromosome showed that matB in combination with matA or matC restored surface expression of the Mat fimbria. A total of 27 isolates representing K-12 strains and the major pathogroups of E. coli were analyzed for the presence of a matB homolog as well as for expression of the Mat fimbria. A conserved matB homolog was found in 25 isolates; however, expression of the Mat fimbriae was detected only in the O18acK1H7 isolates. Expression of the Mat fimbria was temperature regulated, with no or a very small amount of fimbriae or intracellular MatB fimbrillin being detected in cells cultivated at 37 degreesC. Reverse transcriptase PCR and complementation assays with mat genes controlled by the inducible trc promoter indicated that regulation of Mat fimbria expression involved both transcriptional and posttranscriptional events.