Chloramphenicol Resistance in Myxococcus xanthus

Derivatives of Myxococcus xanthus FB t resistant to chloramphenicol (25 μg/ml) arose spontaneously with a frequency of approximately 10 −7 . One of these organisms (FB t Cam 1 r ) was characterized. FB t Cam 1 r showed a unique type of phenotypic instability. After transfer from medium containing chloramphenicol to medium lacking the drug, resistance was lost after approximately one generation. The loss resulted in a sharp drop in the total number of chloramphenicol-resistant organisms and was not due to segregation of chloramphenicol-susceptible organisms during growth. Cell-free extracts of strain FB t Cam 1 r converted chloramphenicol to acetyl chloramphenicols in a fashion implicating activity of chloramphenicol acetyltransferase. This activity was lost simultaneously with the loss of chloramphenicol resistance after removal of the drug from cultures. Organisms with a similar phenotype to FB t Cam 1 r could be produced at high frequencies when strain FB t was exposed to low concentrations of chloramphenicol (2 to 5 μg/ml), to 3-acetylchloramphenicol (25 μg/ml), or to 1,3-diacetylchloramphenicol (25 μg/ml). Since strain FB t is capable of deacetylating acetyl chloramphenicols, these effects are probably all due to low concentrations of chloramphenicol. In the presence of chloramphenicol, FB t Cam 1 r produced fruiting bodies and myxospores on fruiting agar; however, glycerol-induced myxospore formation was inhibited. In the absence of the antibiotic, chloramphenicol resistance was maintained by glycerol-induced myxospores.