SENSITIVITY OF PSEUDOMONAS ÆRUGINOSA TO ANTIBIOTICS: EMERGENCE OF STRAINS HIGHLY RESISTANT TO CARBENICILLIN

Abstract

Sensitivity tests on 1452 strains of Pseudomonas œruginosa ( pyocyanea) isolated from burns between 1966 and 1969 showed a slight increase in the proportion of strains highly sensitive to polymyxin and to gentamicin; resistance to carbenicillin, on the other hand, increased progressively, and in 1969 highly resistant carbenicillinase-producing strains suddenly appeared and quickly displaced all other Ps. œruginosa from the ward. The highly resistant strains were fully virulent for mice. Phage and serological typing showed that two distinct types acquired this form of resistance within 24 hours. Sensitive strains of the same types found in the same patients acquired a high degree of carbenicillin resistance, with carbenicillinase production, on growth in presence of carbenicillin; no increase in resistance to carbenicillin was found when the sensitive strains were grown in ampicillin or cloxacillin. The instability of the resistant strains and their enhanced reversion to sensitivity when exposed to acriflavine suggests that an extrachromosomal factor was responsible for resistance; the transfer of this factor to another strain could account for the nearly simultaneous emergence of carbenicillin resistance in two different types of Ps. œruginosa in the burns unit.