Pseudomonas aeruginosa: evidence for the involvement of lipopolysaccharide in determining the outer membrane permeability to carbenicillin and

The role of lipopolysaccharide (LPS) in determining the permeability of the outer mem-brane of Pseudomonas aeruginosa to carbenicillin and gentamicin was investigated. The susceptibility of P. aeruginosa isolates to smooth LPS-specific phages and to pyocin Rl gave indirect evidence of an altered LPS structure in strains resistant to carbenicillin, gen-tamicin, or both. Some secondary mutation, however, also appeared to be required for acquisition of the antibiotic-resistant phenotype. Phage- and pyocin-resistant variants demonstrating both wild-type and mutant responses to the drugs were subsequently iso-lated. Four-, eight-, and 16-fold increases in resistance to carbenicillin, supersusceptible responses to gentamicin, or both, were associated with a number of the LPS-altered mu-tants. The results supported the hypothesis that a primary mutation involving LPS, in combination with some undefined secondary mutation, determines the permeability of the outer membrane to carbenicillin and to gentamicin. Recent studies of Pseudomonas aeruginosa [1, 2] have indicated that lipopolysaccharide (LPS) may interact with porin proteins in such a way as to de