Clinical and microbiological aspects of linezolid resistance mediated by the cfr gene encoding a 23S rRNA methyltransferase

Abstract

The cfr (chloramphenicol-florfenicol resistance) gene encodes a 23S rRNA methyltransferase that confers resistance to linezolid. Detection of linezolid resistance was evaluated in the first cfr-carrying human hospital isolate of linezolid and methicillin-resistant Staphylococcus aureus (designated MRSA CM-05) by dilution and diffusion methods (including Etest). The presence of cfr was investigated in isolates of staphylococci colonizing the patient's household contacts and clinical isolates recovered from patients in the same unit where MRSA CM-05 was isolated. Additionally, 68 chloramphenicol-resistant Colombian MRSA isolates recovered from hospitals between 2001 and 2004 were screened for the presence of the cfr gene. In addition to erm(B), the erm(A) gene was also detected in CM-05. The isolate belonged to sequence type 5 and carried staphylococcal chromosomal cassette mec type I. We were unable to detect the cfr gene in any of the human staphylococci screened (either clinical or colonizing isolates). Agar and broth dilution methods detected linezolid resistance in CM-05. However, the Etest and disk diffusion methods failed to detect resistance after 24 h of incubation. Oxazolidinone resistance mediated by the cfr gene is rare, and acquisition by a human isolate appears to be a recent event in Colombia. The detection of cfr-mediated linezolid resistance might be compromised by the use of the disk diffusion or Etest method.

FIG. 1.

PFGE of MRSA isolates recovered after identification of the LR-MRSA isolate (isolate CM-05; arrow). The isolate nomenclature is the same as that in Table 1. Additional isolates include isolate P, MRSA CHL93, representative of the Chilean clone; isolate S, MRSA HDE3, representative of the pediatric clone; and isolate T, S. aureus NCTC 8325, used as a control.

FIG. 2.

Etest of LR-MRSA CM-05. The first halo of inhibition was evident at 24 h (interpreted as an MIC of 2 μg/ml; black arrow). After an additional 24 h of incubation, a second halo of growth was identified (MIC, 16 μg/ml; arrowhead).