Microbiological effects of prior vancomycin use in patients with methicillin-resistant Staphylococcus aureus bacteraemia.
BACKGROUND
We sought to determine whether prior vancomycin use (within 30 days) in patients who develop methicillin-resistant Staphylococcus aureus (MRSA) bacteraemia is associated with isolates of reduced vancomycin susceptibility and killing in vitro.


METHODS
Thirty-eight MRSA from previously vancomycin-treated patients and 43 MRSA from vancomycin-naive patients were evaluated by vancomycin and daptomycin CLSI broth microdilution and killing assays. PCR was used to determine accessory gene regulator (agr) type and staphylococcal cassette chromosome mec (SCCmec) type, and nucleotide sequencing was used to determine spa type.


RESULTS
Vancomycin MICs were 0.5, 1.0 and 2.0 mg/L for 19, 55 and 7 isolates, respectively. Daptomycin MICs were 0.25, 0.5, 1.0 and 2.0 mg/L for 4, 50, 26 and 1 isolate, respectively. The agr-type distribution was agr group II (59%), group I (25%) and group III (16%); 90% harboured SCCmec II. The genetic background extrapolated by spa-typing showed that 58% of the isolates were of clonal complex 5. MRSA bloodstream isolates from patients who had received vancomycin within the preceding 30 days had a significantly decreased vancomycin killing at 24 h in vitro (median log(10) decrease, 3.1 versus 2.2 cfu/mL; P = 0.021) and a significantly higher vancomycin MIC than isolates obtained from patients without that history (P = 0.002).


CONCLUSIONS
MRSA bloodstream isolates from patients recently treated with vancomycin may demonstrate reduced susceptibility and increased tolerance to vancomycin in vitro. Given that such microbiological phenotypes have been associated with reduced vancomycin efficacy, consideration may be given to alternative Gram-positive antimicrobial therapy in patients who have recently been treated with vancomycin.
University of the Pacific School of Pharmacy and VA San Diego Healthcare System, San Diego, CA 92161, USA.
Pamela A
PA
Moise
Davida S
DS
Smyth
Nadia
N
El-Fawal
D Ashley
DA
Robinson
Patricia N
PN
Holden
Alan
A
Forrest
George
G
Sakoulas
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000900:Anti-Bacterial Agents / Q000008:administration & dosage / Q000494:pharmacology / Q000627:therapeutic use*; D016470:Bacteremia / Q000188:drug therapy* / Q000382:microbiology; D004334:Drug Administration Schedule; D005260:Female; D006801:Humans; D007275:Injections, Intravenous; D008297:Male; D016106:Methicillin Resistance / Q000187:drug effects*; D008826:Microbial Sensitivity Tests; D008875:Middle Aged; D011446:Prospective Studies; D013203:Staphylococcal Infections / Q000188:drug therapy* / Q000382:microbiology; D013211:Staphylococcus aureus / Q000187:drug effects* / Q000302:isolation & purification; D013997:Time Factors; D014640:Vancomycin / Q000008:administration & dosage / Q000494:pharmacology / Q000627:therapeutic use*; D020713:Vancomycin Resistance / Q000187:drug effects*
D000900:Anti-Bacterial Agents; D014640:Vancomycin