Diffusion of tetracycline across the outer membrane of Escherichia coli K-12: involvement of protein Ia.

The possibility that passage of tetracycline across the outer membrane of $\text{E.}\text{coli}$ K-12 is controlled by one or more of the proteins I_a, I_b and II¹ (Henning's nomenclature) was investigated. A mutant lacking protein I_a (obtained by selection for resistance to phage TuI_a) was more resistant to tetracycline than wild-type strains or those lacking only proteins I_b or II¹. The envelope protein composition of a tetracycline-resistant mutant ($\text{cmlB}$) was altered in several respects, but the major change involved loss of protein I_a. These data support our previous suggestion 12] that tetracycline diffuses across the outer membrane through hydrophilic regions. Furthermore, they imply that only protein I_a plays a significant role in the passage of this antibiotic across the outer membrane.