Two porins, OmpK36 and OmpK35, have been described previously in Klebsiella pneumoniae, and they are homologous to the Escherichia coli porins OmpC and OmpF, respectively, at both the DNA and amino acid levels. Optimal resolution of the two K. pneumoniae porins by electrophoresis on polyacrylamide gels is not achieved using gel systems already described for E. coli and requires modifications of the bisacrylamide content of the resolving gels. Once resolved, identification of porins OmpK36 and OmpK35 cannot be based solely on their apparent molecular masses since in some strains the OmpK36 porin migrates faster than the OmpK35 porin, whilst in other strains OmpK35 is the faster-migrating porin. Expression of OmpK35 porin is increased in low-osmolarity medium and, combined with Western blot analysis, this allows for the identification of both porins. Application of this identification system showed that most isolates lacking expression of extended-spectrum /?-lactamases express the two porins, whereas most isolates producing these /?-lactamases express only porin OmpK36, and the OmpK35 porin is either very low or not expressed.
Sialylation of bacterial capsules has been proposed as an important virulence factor for several species of encapsulated pathogens, including group B Streptococcus. We have constructed a transposon mutant strain of type III group B Streptococcus that expresses a capsular polysaccharide differing from the wild type only in that the mutant strain's capsule lacks sialic acid. The mutant polysaccharide is antigenically identical to the capsular polysaccharide of type 14 Streptococcus pneumoniae, as predicted by the structures of the type III group B Streptococcus and S. pneumoniae polysaccharides. Loss of capsular sialic acid was associated with loss of virulence in the mutant strain in a neonatal rat model of lethal group B Streptococcus infection. These studies demonstrate directly that capsular sialic acid is a critical virulence determinant for type III group B Streptococcus and support the general hypothesis that surface sialylation aids pathogenic microorganisms in evading host defenses.
OmpK35 from Klebsiella pneumoniae is the homologue of Escherichia coli OmpF porin. Expression of OmpK35 in K. pneumoniae strain CSUB10R (lacking both OmpK35 and OmpK36) decreased the MICs of cephalosporins and meropenem >128-fold and decreased the MICs of imipenem, ciprofloxacin, and chloramphenicol >8-fold. MIC reductions by OmpK35 were 4 times (cefepime), 8 times (cefotetan, cefotaxime, and cefpirome), or 128 times (ceftazidime) higher than those caused by OmpK36, but the MICs were similar or 1 dilution lower for other evaluated agents.
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