Chloramphenicol resistance in Streptococcus pneumoniae: enzymatic acetylation and possible plasmid linkage.

Clinical isolates of Streptococcus pneumoniae resistant to chloramphenicol were observed in France for the first time in 1973. During a 4-year survey, these strains were found to represent 6% of a total of 564 isolates of S. pneumoniae in a general hospital and to belong to 13 different serotypes. One such strain, referred to as BM 6001, was shown to inactivate chloramphenicol, and the process was found to be inducible. The inactivated products were demonstrated to be O-acetoxy esters of chloramphenicol. The synthesis of an inducible chloramphenicol acetyltransferase was shown to be responsible for the inactivation of the drug. The resistant strain was able to transfer the chloramphenicol marker by transformation to competent strains of pneumococci at a frequency of 1% of that observed for control chromosomal markers. The loss of resistance was enhanced by ethidium bromide treatment, but no chloramphenicol-resistant mutant was isolated by mutagenesis of a "cured" clone or naturally susceptible isolates. All attempts to isolate plasmid deoxyribonucleic acid as covalently closed circular molecules from strain BM 6001 have been unsuccessful, but epidemiological evidence and the fact that the genes specifying chloramphenicol acetyltransferase synthesis are usually located on plasmids suggest that this marker may be plasmid-borne in S. pneumoniae.