<i>Pseudomonas aeruginosa</i> Resistant to Carbenicillin and Gentamicin

Abstract

Since November 1969 the Baltimore Cancer Research Center has used the combination of carbenicillin (C) and gentamicin (G), with or without cephalothin, as therapy for granulocytopenic patients with severe Gram-negative bacillary infection, especially Pseudomonas aeruginosa. Since 1971 an increasing percentage of isolates have been resistant to carbenicillin or gentamicin, or both, by disc-susceptibility testing. Measurements of minimum inhibitory concentrations showed that, during 11 months, 14 patients, 8 with acute leukemia, had developed resistant strains. From these 14 patients, 40 distinct serotypes or resistance patterns of P. aeruginosa could be delineated: 15 were C- and G-susceptible; 12, C-resistant and G-susceptible; 5, G-resistant and C-susceptible; and 8, C- and G-resistant. The appearance of resistant organisms correlated significantly with the administration of carbenicillin or gentamicin, or both. The induction of gentamicin resistance was particularly associated with oral G intake. Carbenicillin resistant variants and strains susceptible to carbenicillin and gentamicin were virulent, but those with gentamicin resistance seemed to be less invasive.