ACCLIMATIZATION RESISTANCE OF A PSEUDOMONAS-AERUGINOSA PROTOTYPE STRAIN TO IMIPENEM

A prototype sensitive strain of Pseudomonas aeruginosa (IR(s)) was acclimatized in vitro to imipenem by serial transfers in broth containing increasing concentrations of the antibiotic up to 32-mu-g/ml. Serial subculture of the resistant progeny (IRI(r)) in antibiotic-free solid media resulted in a "revertant" progeny (IR2(r)) that retained resistance to imipenem. Acclimatization resistance to imipenem and the attempted reversion procedure affected colony morphology, growth rate, pigment production, growth at 42-degrees-C and glucose oxidation. SDS-PAGE analysis of the outer membrane proteins revealed in strain IR1(r) complete loss of 6 proteins that were present in IR(s) (85 kdaltons, 46 kdaltons or porin D1, 45.5 kdaltons or porin D2, 43 kdaltons or porin E, 21 kdaltons or protein H1 and 20.5 kdaltons or lipoprotein H2) and in strain IR2(r) a complete loss of 3 proteins (46 kdaltons, 43 kdaltons, 20.5 kdaltons), while three others were found only in trace amounts (75 kdaltons, 45.5 kdaltons and 21 kdaltons). In the outer membranes of strains IR1(r) and IR2(r) an acquisition of a 56-kdalton protein was noted. Lipopolysaccharide chemical analysis revealed a marked, partially reversible increase in 2-keto-3-deoxyoctonate, total hexose and heptose constituents; readily extractable lipid chemical analysis and TLC, revealed a marked, partially reversible, increase in the phospholipid content of the outer membrane. Acclimatization resistance to imipenem was accompanied by cross-resistance to gentamicin, by partially reversible cross-resistance to moxalactam, carbenicillin and ticarcillin and by fully reversible cross-resistance to aztreonam. Sensitivity to azlocillin and polymyxin B remained unaltered. To explain this type of resistance to imipenem, an irreversible, non-inducible "loss" mutation mechanism, working in concert with a partially reversible mechanism, is proposed.