Deletion of the Group 1 Capsular Gene wza in Escherichia coli E69 Confers Resistance to the Antibiotic Erythromycin on Solid Media but not in Liquid Media

It has been suggested that the presence of a capsule influences the level of antibiotic resistance of a bacterium, although there have been contradicting studies in the literature. Previous research demonstrated that the deletion of three key group 1 capsular biosynthesis genes (wza, wzb and wzc), components of the Wzy-dependent polymerisation system, from Escherichia coli E69 leads to resistance to macrolide antibiotics. It is not known if this is the effect of deleting a single gene or the entire gene cluster. Wza forms a channel in the outer membrane suggesting that macrolides may diffuse into the cell through the Wza pore. In this study, we asked whether a deletion of the wza gene results in the macrolide resistant phenotype observed in the Wzy triple deletion strain. Antibiotic disc diffusion and microdilution assays were performed on an E. coli wza knockout strain and a Wzy triple gene deletion (wza, wzb, wzc) E. coli strain. Both strains exhibited resistance to erythromycin in antibiotic disc diffusion assays compared to wild type E. coli strain E69. In contrast, the wza knockout E. coli strain was more
sensitive to erythromycin than the wild type E. coli stain E69 and the Wzy triple deletion mutant in microdilution assays. We conclude that the deletion of wza is sufficient to induce resistance toward erythromycin on solid media but not on liquid media.