749. Whole Genome Sequencing of Linezolid Resistant Coagulase Negative Staphylococci reveals limited genomic mutations associated with resistance
Session: Poster Abstract Session: Antimicrobials: Resistance Mechanisms
Friday, October 4, 2013
Room: The Moscone Center: Poster Hall C
Background: There are limited data regarding genomic mutations associated with development of linezolid resistance in coagulase negative staphylococcus (LRCNS) isolates.

Methods: We reviewed 28 cases of LRCNS (Linezolid MIC ≥8 ug/ml) identified at a tertiary center (01/2007-01/2012). CLSI broth microdilution was used to confirm MICs. Strain typing by multi-locus sequence typing and repPCR was performed to identify clones. Whole genome sequencing (WGS) was used to evaluate the putative resistance mechanisms present in these 28 LRCoNS isolates.

Results: Among the 28 LRCNS isolates, 21 (75%) were S. epidermidis and 7 (25%) were S. haemolyticus. We found 6 groups of clones (4 S. epidermidis and 2 S. haemolyticus) and 15 unique clones. Single nucleotide variants (SNVs) in 23S rRNA were identified in 13 (62%) of S. epidermidis and 7 (100%) of S. haemolyticus. Previously identified resistance mutations C2534T, G2576T, G2447A and T2504A were found in 19 (68%), 9 (32%), 8(28%) and 7(25%) isolates respectively. All S. haemolyticus and none of the S. epidermidis isolates harbored all 4 23S mutations. Only 2 S. epidermidis isolates harbored the most commonly reported linezolid resistance mutation, G2576T. The presence of a cfr gene was found in 10 isolates (36%), all S. epidermidis. Linezolid MICs were significantly higher for isolates that harbored cfr as compared to those that did not (modal MIC, >256 µg/mL vs. 64 µg/mL, p<0.01). Nearly all (19/21 isolates) S. epidermidis harbored one or more mutations to the L3 proteins. A157R, V154L, H146Q, D159L were found in 12 (57%), 11 (52%), 10 (48%), 7(33%) and isolates respectively. G139R was found in only one (14%) S. haemolyticus isolate. 12 (57%) S. epidermidis isolates harbored mutations to the L4 protein; 11 had an insertion of 71N and 1 had a K68N mutation.

Conclusion: While mutation to the 23S rRNA is the most common mechanism of resistance, mutations to the L3 and L4 proteins are increasingly associated with resistance to linezolid. Ribosomal methylation by cfr was a more significant contributor to linezolid MIC than ribosome-associated mutations in these isolates. The majority of isolates were unique clones suggesting the linezolid resistance among CoNS may be an emerging issue.

Theodoros Kelesidis, MD1, Bing Gu, MD2, Ryan Tewhey, PhD3, Carmen L. Giltner, PhD.4, April M. Bobenchik, PhD.4, Janet A. Hindler, MCLS, MT4 and Romney M. Humphries, Ph.D.4, (1)David Geffen School of Medicine at UCLA, Los Angeles, CA, (2)the First Affiliated Hospital of Nanjing Medical University, Nanjing, China, (3)Scripps Translational Science Institute, The Scripps Research Institute, La Jolla, CA, (4)Department of Pathology and Laboratory Medicine, University of California, Los Angeles, Los Angeles, CA

Disclosures:

T. Kelesidis, None

B. Gu, None

R. Tewhey, None

C. L. Giltner, None

A. M. Bobenchik, None

J. A. Hindler, None

R. M. Humphries, None

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