Alteration in expression of Serratia marcescens porins associated with decreased outer membrane permeability.

The porin expression of two clinical isolates of Serratia marcescens, which overproduced cephalosporinase and had decreased outer membrane permeability, were studied in comparison with those of reference strains. Separation of the porin proteins assessed by SDS-PAGE containing urea revealed that both clinical isolates overexpressed a single porin of 44 and 43 kilodalton (kDa), respectively. In contrast, the in-vitro porin deficient mutant, which was derived from the reference strain IFO3736 as latamoxef-resistant, showed decreased outer membrane permeability, but produced low levels of all three peptidoglycan associated porins of 45, 44 and 43 kDa, and overexpressed 39 kDa OmpA protein. These observations suggested that the clinical isolates had a different mechanism of latamoxef resistance compared with the mutant and overexpressed possible narrow transport channels of 44 or 43 kDa. The 45 kDa porin may facilitate a more effective channel than the other two proteins. Heterogeneity of porin profiles between biotypes was also suggested. The two isolates were also resistant to penicillins and cephalosporins tested, but imipenem was the most active agent and inhibited the isolates at 1.56 mg/L.