Acquisition of a natural resistance gene renders a clinical strain of methicillin‐resistantStaphylococcus aureusresistant to the synthetic antibiotic linezolid

Abstract

SummaryLinezolid, which targets the ribosome, is a new synthetic antibiotic that is used for treatment of infections caused by Gram-positive pathogens. Clinical resistance to linezolid, so far, has been developing only slowly and has involved exclusively target site mutations. We have discovered that linezolid resistance in a methicillin-resistant Staphylococcus aureus hospital strain from Colombia is determined by the presence of the cfr gene whose product, Cfr methyltransferase, modifies adenosine at position 2503 in 23S rRNA in the large ribosomal subunit. The molecular model of the linezolid-ribosome complex reveals localization of A2503 within the drug binding site. The natural function of cfr likely involves protection against natural antibiotics whose site of action overlaps that of linezolid. In the chromosome of the clinical strain, cfr is linked to ermB, a gene responsible for dimethylation of A2058 in 23S rRNA. Coexpression of these two genes confers resistance to all the clinically relevant antibiotics that target the large ribosomal subunit. The association of the ermB/cfr operon with transposon and plasmid genetic elements indicates its possible mobile nature. This is the first example of clinical resistance to the synthetic drug linezolid which involves a natural resistance gene with the capability of disseminating among Gram-positive pathogenic strains.

Clinical and Microbiological Aspects of Linezolid Resistance Mediated by the cfr Gene Encoding a 23S rRNA Methyltransferase

“…As described previously, MRSA CM-05 did not exhibit any mutations in domain V of the 23S rRNA (28). The identification of the cfr gene, which encodes an rRNA methylase, in this isolate recovered from a patient after such a short exposure to linezolid (28) indicates that the gene was also most likely acquired by the isolate under a selective pressure that did not involve exposure to oxazolidinones. An alternative explanation is that the strain was selected in an unidentified patient exposed to linezolid and was then passed on to the case patient.…”

“…Recently, we reported that linezolid resistance in a clinical human MRSA isolate (designated CM-05) from Colombia was mediated by the presence of the same cfr gene (28). The MRSA CM-05 isolate was characterized, and it was found that, unlike the animal isolates, the gene was located in the chromosome but likely was a part of an integrated plasmid possibly capable of excision and mobilization (28).…”

“…Also, in MRSA CM-05, the cfr gene was clustered in the chromosome with the erm(B) gene (which encodes another rRNA methylase and which confers resistance to macrolide, lincosamide, and streptogramin B antibiotics), forming a transcriptional unit designated the mlr (for modification of the large ribosomal unit) operon (28). The presence of the mlr operon rendered the MRSA isolate resistant to all antibiotics whose target is the large ribosomal subunit (28).…”

“…The MRSA CM-05 isolate was characterized, and it was found that, unlike the animal isolates, the gene was located in the chromosome but likely was a part of an integrated plasmid possibly capable of excision and mobilization (28). Also, in MRSA CM-05, the cfr gene was clustered in the chromosome with the erm(B) gene (which encodes another rRNA methylase and which confers resistance to macrolide, lincosamide, and streptogramin B antibiotics), forming a transcriptional unit designated the mlr (for modification of the large ribosomal unit) operon (28). The presence of the mlr operon rendered the MRSA isolate resistant to all antibiotics whose target is the large ribosomal subunit (28).…”

Targeting Antibiotic Resistance

“…This resistance is more worrisome than the mutation‐based mechanism since it is horizontally transferable and carries a low fitness cost 32, 33. Mechanistically, the methyltransferase Cfr, through C8 methylation of the key residue A2503Ec in the 23S rRNA, greatly reduces susceptibility to a wide range of ribosome‐targeting antibiotics, including amphenicols, lincosamides, pleuromutilins, streptogramin A, 16‐membered macrolides, and linezolid 34.…”