Tobramycin (nebramycin factor 6): in vitro activity against Pseudomonas aeruginosa

Tobramycin (factor 6 of the nebramycin complex) is a new aminoglycoside antibiotic isolated from Streptomyces tenebrarius which is active against S. aureus, Enterobacteriaceae, and Pseudomonas aeruginosa. Susceptibility to tobramycin of 96 strains of P. aeruginosa, including 45 recent isolates from blood, was studied by using agar and broth dilution methods. The minimum inhibitory concentration (MIC) for 83 of 96 strains was 3.12 mug/ml or lower in Mueller Hinton agar; MIC values were two to eight times lower in Mueller Hinton broth tests. Agar dilution MIC values were generally lower than those obtained in parallel tests with gentamicin. Killing curves obtained from serial sampling of broth cultures showed a 100- to 10,000-fold decline in viability of log-phase organisms within 30 min of exposure to the drug. Two-dimensional agar dilution tests with carbenicillin and tobramycin with 79 strains showed additive or synergistic effects; no antagonism was documented. Seventy-eight of 79 strains were inhibited by a combination of 50 mug of carbenicillin per ml and 1.56 mug of tobramycin per ml, blood levels which seem attainable in man. Tobramycin appears to be a potent, rapidly bactericidal antibiotic against P. aeruginosa and merits clinical evaluation.

Agar-Gel Precipitating Antibody in Pseudomonas aeruginosa Infections

The human immune response to Pseudomonas aeruginosa infection was studied by using the double diffusion in agar-gel technique. Antigens from Fisher-Devlin-Gnabasik immunotypes were prepared by both trichloroacetic acid extraction and ultrasonic disruption. Serum from 72 of 168 patients (43%) from whom P. aeruginosa was isolated formed from one to eight precipitin bands. Precipitins were demonstrated in the sera of 60 of 66 (91%) patients recovering from bacteremia and deep infections; however, they were usually absent when Pseudomonas infection was fatal or when there was no clinical evidence of significant infection. Precipitating antibody was detectable at serum dilutions as high as 1:32, and appearance of single bands correlated with hemagglutinating antibody titers of ≥1:128. Antigen from sonically disrupted organisms usually resulted in stronger precipitin bands than trichloroacetic acid extracts, and antigen from the homologous infecting strain occasionally increased test sensitivity. None of 50 normal controls had Pseudomonas precipitins as was the case in patients convalescing from Escherichia coli (15 patients), Klebsiella-Enterobacter-Serratia (18), and Proteus (14) bacteremias. Measurement of agar-gel precipitins was useful and specific in evaluating the circulating antibody response to P. aeruginosa infections.