Evaluation of antimicrobials combined with hyperbaric oxygen in a mouse model of clostridial myonecrosis.

The efficacy of hyperbaric oxygen (HBO) alone and in combination with several antimicrobial agents was evaluated in a lethal model of gas gangrene in mice. Intraperitoneal administration of penicillin, imipenem, clindamycin, or metronidazole immediately followed inoculation of > 10(9) CFU of Clostridium perfringens type A in mice. Mice treated with hyperbaric oxygen were exposed twice a day to 100% oxygen at 303 kilopascals (kPa) pressure for 90 minutes. The total exposure time to HBO for surviving animals was 9 hours. Control (saline-injected) mice treated with HBO alone did not have an enhanced survival rate when compared with mice exposed to air at ambient pressure. Survival of infected mice treated with either clindamycin or metronidazole was significantly longer than that of groups treated with penicillin or imipenem (p < 0.05). Hyperbaric oxygen alone or in combination with the four antimicrobial agents evaluated did not statistically improve survival of mice infected with a lethal dose of C. perfringens.

Hyperbaric hyperoxia enhances the lethal effects of amphotericin B in Leishmania braziliensis panamensis.

Leishmania braziliensis panamensis promastigotes were exposed in vitro to amphotericin B (AmB), menadione, or phenazine methosulfate under normoxic conditions. Promastigotes were also exposed to hyperoxia alone (100% O2 at total pressures of 101.3 or 253.3 kPa), or combined with drugs. After incubation for 24 h at 27 degrees C, viable promastigotes were stained with fluorescein diacetate and counted using epifluorescence microscopy. Hyperbaric hyperoxia alone (PO2 = 229.3 kPa) was as effective as AmB alone (0.2 microM); both reduced the number of viable promastigotes to approximately 13% of the original inoculum. In addition, AmB in a hyperbaric hyperoxic environment killed more promastigotes (97% of the original inoculum) than AmB in normoxic (PO2 = 21.1 kPa) or hyperoxic conditions (PO2 = 91.7 kPa). Finally, AmB in hyperbaric hyperoxia killed significantly more (75%) promastigotes than hyperbaric hyperoxia alone. High oxygen tensions did not significantly alter the lethal effects of either menadione or phenazine methosulfate. In conclusion, the lethal effects of low dose AmB in Leishmania promastigotes were augmented by hyperbaric hyperoxia in vitro, but only at oxygen doses too high to be tolerated by human patients.

Hyperbaric oxygenation decreases blood flows in normal and septic rats.

The purpose of the study reported here was to characterize the effects of acute exposure to 100% oxygen at a pressure of 202.6 kPa on hemodynamics and organ blood flow in antibiotic-controlled bacterial sepsis. An abscess containing known numbers and strains of live Escherichia coli and Bacteroides fragilis was established in the peritoneal cavity of rats. Sham-operated rats were used as controls. Cardiac output, fractional blood flow distribution, and blood flow were calculated from data obtained using the radiolabeled microsphere technique. Myocardial and renal blood flows were decreased (20-30%) in both experimental groups during hyperbaric oxygen (HBO) exposure. Renal blood flow remained diminished for at least 20 min after rats were removed from the hyperbaric chamber. Adrenal gland perfusion in abscess-containing rats was reduced both during and after HBO exposure. Skeletal muscle blood flow (quadriceps and gastrocnemius muscles) was reduced by one third in both experimental groups 20 min after acute exposure to HBO. The results of this study indicate that changes in organ perfusion induced by HBO are similar in abscess-containing and normal rats. We conclude that HBO does not alter the regulation of blood flow in antibiotic-controlled sepsis.

Hyperoxia prolongs the aminoglycoside-induced postantibiotic effect in Pseudomonas aeruginosa.

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% O2), hyperoxia (100% O2, 101.3 kPa), or hyperbaric oxygen (100% O2, 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 less than 0.05). Exposure of P. aeruginosa ATCC 27853 to tobramycin and hyperbaric oxygen prolonged the time required for bacteria to increase 1 log10 CFU/ml compared with the time after exposure for this increase to occur in tobramycin-treated, normoxic or hyperoxic groups (P less than 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 less than 0.01) and protein synthesis (24%; not significant) compared with normoxia. Hyperbaric oxygen, however, delayed the growth recovery of bacteria (P less than 0.05). We conclude that hyperoxia enhances the bacteriostatic effects of tobramycin in a synergistic manner.+

Pathophysiology of sea nettle (Chrysaora quinquecirrha), envenomation in a rat model and the effects of hyperbaric oxygen and verapamil treatment.

The effect of lethal sea nettle envenomation on the morphology and blood flow in various rat organs was characterized and the influence of two antidotes (hyperbaric oxygen and verapamil) was compared. Either antidote slightly prolonged survival, but the protective effects were not statistically significant. The venom caused no histologic alterations in brain, heart, or lungs but induced hepatic and renal necrosis. Hepatocytes in mid-zonal regions and renal tubular epithelium were the cell types predominantly affected. Hyperbaric oxygen and verapamil did not decrease the hepatic injury. The venom did not influence central hemodynamics until preterminally and it diminished blood flow to brain, but not to liver or kidney. Hyperbaric oxygenation protected against venom-induced decreases in blood flow to the brain. These results add toxic hepatic and renal necrosis and cerebral ischemia to the pathophysiology of envenomation in this model.

Hyperoxia and the antimicrobial susceptibility of Escherichia coli and Pseudomonas aeruginosa.

We have tested the ability of hyperoxia (98% O2-2% CO2 at 2.8 atmospheres absolute [ca. 284.6 kPa]) to enhance killing of Escherichia coli (serotype O18 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 tests 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 log10 and P. aeruginosa by 2.7 log10 in cation-supplemented 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 reduction-oxidation-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.

Splanchnic blood flows in a rat model of antibiotic-controlled intra-abdominal abscess during normoxia and hyperoxia.

We have characterized splanchnic blood flows in a rat model of antibiotic-controlled, intra-abdominal abscess (ACIA) in normoxic and hyperoxic environments. Abscesses (1.5 ml average volume) were established by implanting a fecal-agar pellet containing known numbers and strains of live Escherichia coli and Bacteroides fragilis into the peritoneal cavity of rats via a midline incision. Sham-operated rats (SR) served as controls. Cannulas (PE-50) were placed into the left ventricle of both SR and ACIA rats for injection of tracer microspheres (MS) and into the tail artery for a reference withdrawal site. Blood flow measurements were made in conscious unrestrained rats before, during, and after hyperoxic exposure by using MS labelled with either 51Cr or 85Sr or 141Ce. No change in cardiac output (CO), heart rate (HR), or mean arterial pressure (MAP) was seen between ACIA and SR groups. The administration of hyperoxia diminished the fractional distribution of CO to the liver (portal), stomach, cecum, colon, and pancreas in both groups. The percent CO to the liver (arterial), small intestine, and spleen did not change in either group. The blood flow to all splanchnic organs, except liver (arterial), was diminished during hyperoxia in both groups and recovered after hyperoxia. In conclusion, central hemodynamics and splanchnic blood flows are not altered by the presence of a large intra-abdominal abscess controlled by antibiotics. Splanchnic blood flows are diminished by hyperoxia.

Fatal Fusarium solani infections in baby sharks.

The occurrence of fatal fusariosis in baby bonnethead sharks (Sphyrna tiburo) born at the National Aquarium, Baltimore, Maryland, is reported. An atypical strain of Fusarium solani was cultured from the tissues of two of the infected sharks following postmortem examination. Histopathology revealed an apparent predilection of the fungus for hyaline cartilage. Invasion of the cartilage resulted in hyphae with a distorted morphology. In slide culture the fungus displayed the unusual characteristic of terminal chlamydoconidium generation on macroconidia; this may be of some taxonomic significance.