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Chloramphenicol resistance in Streptococcus pneumoniae: enzymatic acetylation and possible plasmid linkage
Abstract:
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.
Insights
First observed in 1973, chloramphenicol-resistant Streptococcus pneumoniae strains in France inactivate the drug via inducible chloramphenicol acetyltransferase. This resistance marker may be plasmid-borne, despite challenges in isolating plasmid DNA.
Area of Science:
- Microbiology
- Molecular Biology
- Antimicrobial Resistance
Background:
- Clinical isolates of Streptococcus pneumoniae exhibiting chloramphenicol resistance emerged in France in 1973.
- A 4-year survey revealed these resistant strains constituted 6% of 564 isolates in a general hospital, belonging to 13 distinct serotypes.
Purpose of the Study:
- To investigate the mechanism of chloramphenicol resistance in Streptococcus pneumoniae.
- To determine the genetic basis and transmissibility of chloramphenicol resistance in clinical isolates.
Main Methods:
- Characterization of chloramphenicol inactivation by resistant strain BM 6001.
- Induction studies of the inactivation process.
- Analysis of inactivation products and enzyme activity.
- Transformation experiments to assess marker transfer.
- Ethidium bromide treatment to induce loss of resistance.
- Mutagenesis and screening for resistant mutants.
- Attempts to isolate plasmid DNA.
Main Results:
- Strain BM 6001 demonstrated inducible inactivation of chloramphenicol, producing O-acetoxy esters.
- An inducible chloramphenicol acetyltransferase was identified as the enzyme responsible for drug inactivation.
- The chloramphenicol resistance marker was transferable via transformation at 1% the frequency of control markers.
- Ethidium bromide treatment enhanced the loss of resistance.
- No plasmid DNA was successfully isolated from the resistant strain.
Conclusions:
- The study identified an inducible chloramphenicol acetyltransferase as the mechanism for resistance in Streptococcus pneumoniae.
- Epidemiological data and the nature of the resistance gene suggest a potential plasmid-borne location for the chloramphenicol resistance marker.
- Further investigation is warranted to confirm the extrachromosomal nature of this resistance in S. pneumoniae.