The resistance of Pseudomonas aeruginosa to chloramphenicol

A strain of Pseudomonas aeruginosa, which was resistant to 400 mug/ml of chloramphenicol (CM), was isolated. The generation time of the resistant strain was the same in the presence or absence of CM and similar to that of the parent strain growing in the absence of chloramphenicol. Resistance is eliminated by treatment with acridine dyes, mitomycin C, and sodium dodecyl sulfate, suggesting that resistance may be expressed by a plasmid. The resistant strain does not produce the pigment pyocyanine and the addition of pyocyanine to this strain eliminates the resistance factor. A strain sensitive to CM was isolated. This strain does not produce the enzyme acetyl CoA : chloramphenicol transacetylase whereas the resistant strain does. The sensitive strain accumulates 14C-CM at a greater rate and to a greater extent than the resistant strain grown in the presence of CM. The results suggest that the resistant strain inactivates CM by acetylation and, in.addition, develops a "permeability" barrier towards chloramphenicol.