Porin expression in clinical isolates of Klebsiella pneumoniae

Two porins, OmpK36 and OmpK35. have been described previously in Klebsiella pneumoniae, and they are homologous to the Escherichia coli porins OmpC an d OmpF. respectively, at both the DNA and amino acid levels. Optimal resolu tion of the two K. pneumoniae porins by electrophoresis on polyacrylamide g els is not achieved using gel systems already described for E. coli and req uires modifications of the bisacrylamide content of the resolving gels. Onc e resolved, identification of porins OmpK36 and OmpK35 cannot be based sole ly on their apparent molecular masses since in some strains the OmpK36 pori n 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 syst em showed that most isolates lacking expression of extended-spectrum beta-l actamases express the two porins, whereas most isolates producing these bet a-lactamases express only porin OmpK36, and the OmpK35 porin is either very low or not expressed.