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One hundred and eleven strains of Pseudomonas aeruginosa isolated from clinical material were studied for susceptibility to carbenicillin. Of the strains, 80% were inhibited by 125 μg/ml or more. The combination of carbenicillin and gentamicin was shown to have inhibitory and bactericidal synergistic effect on 15 of 16 strains of P. aeruginosa tested. There was poor correlation between the single-disc sensitivity method and the tube dilution method.

The in vitro activity of 6-[ d -α-(3 guanylureido)-phenylacetamido]-penicillanic acid (BL-P1654) was evaluated against 117 clinical isolates of Pseudomonas aeruginosa , many of which were known to be resistant to both gentamicin and carbenicillin. BL-P1654 was two to eight times more active than carbenicillin against P. aeruginosa . However, all 28 highly carbenicillin-resistant isolates (minimal inhibitory concentration [MIC] > 500 μg/ml) were also highly resistant to BL-P1654. When an MIC of 32 μg/ml or less and a zone of inhibition of 12 mm or more by a 10-μg disk were used as criteria indicating susceptibility to BL-P1654, the false-resistance rate by the disk test was 10.6% and the false-susceptibility rate was 4.2%. The combination of BL-P1654 and gentamicin was synergistic against 45 of 70 isolates of P. aeruginosa tested, but synergism was demonstrated against only 4 of 24 highly gentamicin-resistant (MIC > 63 μg/ml), 1 of 12 highly BL-P1654-resistant (MIC > 250 μg/ml) isolates, and none of nine isolates highly resistant to both of these antibiotics.