The objective of this study was to determine whether hyperoxia enhances aminoglycoside activity against Pseudomonas aeruginosa. The existence of tobramycin-oxygen synergy was determined by using the in vitro postantibiotic effect (PAE). P. aeruginosa strains were incubated for 1 h in medium containing tobramycin at four times the MIC in the following gas mixtures: normoxia (21% 02), hyperoxia (100% 02 101.3 kPa), or hyperbaric oxygen (100% 02, 274.5 kPa). Tobramycin was removed after 1 h and bacteria were incubated under normoxic conditions; growth rates were measured for 5 h. Exposure of three P. aeruginosa strains to hyperoxia prolonged the PAE of tobramycin approximately twofold compared with the PAE after exposure to normoxia (P < 0.05). Exposure of P. aeruginosa ATCC 27853 to tobramycin and hyperbaric oxygen prolonged the time required for bacteria to increase 1 loglo CFU/ml compared with the time after exposure for this increase to occur in tobramycin-treated, normoxic or hyperoxic groups (P < 0.02). Pulse-chase labeling of bacteria with L-[35S]methionine, immediately after removal of tobramycin, showed that protein synthesis rates were decreased compared with those in controls (P = 0.0001). Moreover, in tobramycin-treated groups, hyperoxia and hyperbaric oxygen induced 2-and 16-fold decreases, respectively, in protein synthesis rates compared with normoxia; these results did not achieve statistical significance. In the absence of tobramycin, hyperoxia increased bacterial growth (134%; P < 0.01) and protein synthesis (24%; not significant) compared with normoxia. Hyperbaric oxygen, however, delayed the growth recovery of bacteria (P < 0.05). We conclude that hyperoxia enhances the bacteriostatic effects of tobramycin in a synergistic manner. Hyperbaric oxygen also enhances the bacteriostatic effects of tobramycin against P. aeruginosa. Our findings may have relevance to the in vivo growth rates of P. aeruginosa in tissues of patients treated with high inspired oxygen tensions.The activities of many antimicrobial agents are altered by changes in oxygen tension. Anaerobic (anoxic) conditions markedly diminish aminoglycoside activity (32,36,37), because energy derived from quinone-associated electron transport is required to facilitate uptake of the antimicrobial agent into gram-negative bacteria (9). The loss of aminoglycoside bactericidal activity induced by an anoxic environment can be restored by hyperbaric oxygen (22). The activity of vancomycin against Staphylococcus aureus is also markedly diminished in an anaerobic environment, and this may account for the failure of the antimicrobial agent to sterilize infected tissues, such as bone, where low oxygen tensions are found (30). In addition, the bactericidal activity of ciprofloxacin is abolished under anaerobic conditions (35 kPa) and a sulfonamide-trimethoprim combination against Vibrio anguillarum in vitro. Hyperoxia enhanced trimethoprim activity, presumably by oxidizing enzymes or metabolic intermediates involved in folate metabolism in bacteria (1...

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We have tested the ability of hyperoxia (98% 02-2% CO2 at 2.8 atmospheres absolute [ca. 284.6 kPa]) to enhance killing of Escherichia coli (serotype 018 or ATCC 25922) by nitrofurantoin, sulfamethoxazole, trimethoprim, gentamicin, and tobramycin. We have also looked for interactions between hyperoxia and the aminoglycosides against Pseudomonas aeruginosa ATCC 27853. Hyperoxia significantly enhanced bacteriostatic activity of nitrofurantoin and trimethoprim as measured by MIC testing. The possibility exists that these effects might be due to the method required to test MICs under hyperoxic conditions rather than to the effect of hyperoxia itself. In addition, hyperoxia enhanced killing of bacteria by trimethoprim as measured by MBC testing. Hyperoxia decreased numbers of E. coli by 1.3 loglo and P. aeruginosa by 2.7 log1o in cationsupplemented Mueller-Hinton broth medium. The bacteriostatic effects of hyperoxia did not affect MICs of gentamicin or tobramycin. The lack of interaction between hyperoxia and gentamicin or tobramycin was confirmed by determining the number of viable bacteria remaining after 24 h of exposure to hyperoxia by using a pour plate method. We conclude that hyperoxia potentiates the antimicrobial activity of the reductionoxidation-cycling antibiotic tested (nitrofurantoin) and of one of the antimetabolites tested (trimethoprim). Hyperoxia does not enhance the bactericidal effects of gentamicin and tobramycin, which require oxidative metabolism for transport into bacterial cells.

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