Large, unstable inserts in the chromosome affect virulence properties of uropathogenic Escherichia coli O6 strain 536

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Abstract
The hemolytic, uropathogenic Escherichia coli 536 (O6:K15:H31) contains two inserts in its chromosome (insert I and insert II), both of which carried hly genes, were rather unstable, and were deleted spontaneously with a frequency of 10(-3) to 10(-4). These inserts were not found in the chromosome of two nonhemolytic E. coli strains, whereas the chromosomal sequences adjacent to these inserts appeared to be again homologous in the uropathogenic and two other E. coli strains. Insert I was 75 kilobases in size and was flanked at both ends by 16 base pairs (bp) (TTCGACTCCTGTGATC) which were arranged in direct orientation. For insert I it was demonstrated that deletion occurred by recombination between the two 16-bp flanking sequences, since mutants lacking this insert still carried a single copy of the 16-bp sequence in the chromosome. Both inserts contained a functional hemolysin determinant. However, the loss of the inserts not only affected the hemolytic phenotype but led to a considerable reduction in serum resistance and the loss of mannose-resistant hemagglutination, caused by the presence of S-type fimbriae (sfa). It is shown that the Sfa-negative phenotype is due to a block in transcription of the sfa genes. Mutants of strain 536 which lacked both inserts were entirely avirulent when tested in several animal model systems.