TNPHOA-MEDIATED DISRUPTION OF K54 CAPSULAR POLYSACCHARIDE GENES IN ESCHERICHIA-COLI CONFERS SERUM SENSITIVITY

To assess whether non-K1, group 2 capsular serotypes are important in conferring serum resistance to extraintestinal isolates of Escherichia coli, a K54 blood isolate (CP9) was evaluated as a model pathogen. Tr ansposon mutagenesis (TnphoA) was used to generate isogenic capsule-ne gative mutants. CP9 was resistant to the bactericidal effects of serum , growing in 80% serum. In contrast, all of the capsule-negative mutan ts had an increased sensitivity to 80% normal human serum, undergoing a 2- to 3-log kill over 3 h when starting inocula of 10(4) to 10(7) CF U/ml were used. The killing of the capsule-negative strains was mediat ed through the alternative complement pathway and not by lysozyme or b eta-lysins. The protective effect of the K54 capsule against the bacte ricidal activity of serum was not through inhibition of the complement cascade, nor did it appear to be through a difference in the binding of C3.