Two porins, OmpK36 and OmpK35, have been described previously in and they are homologous to the porins OmpC and OmpF, respectively, at both the DNA and amino acid levels. Optimal resolution of the two porins by electrophoresis on polyacrylamide gels is not achieved using gel systems already described for 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 β-lactamases express the two porins, whereas most isolates producing these β-lactamases express only porin OmpK36, and the OmpK35 porin is either very low or not expressed.


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