Carbenicillin After Reversion from Spheroplast

Five of six strains of Pseudomonas aeruginosa had at least fourfold increase in resistance to carbenicillin after they reverted from carbenicillin-induced spheroplasts to rod forms. The most susceptible laboratory strain showed the greatest increase in resistance. There was a marked inoculum effect on the minimal inhibitory concentration of carbenicillin when high inocula (107 colony-forming units) or very low inocula (102 or 103 colony-forming units) were used. Population analysis of this reverted strain revealed increase in resistance of the entire population of cells to carbenicillin. However, the degree of resistance was not homogenous. Only a few cells were highly resistant. Comparison of the effect of carbenicillin on the growth of the parent and reverted strains confirmed the increase in resistance of the reverted strain. The resistance remained stable after 42 passages.