Methicillin-resistant Staphylococcus aureus (MRSA) staphylococcal cassette chromosome mec genotype effects outcomes of patients with healthcare-associated MRSA bacteremia independently of vancomycin minimum inhibitory concentration.
BACKGROUND
Recent evidence has shown that community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) is less virulent than traditional hospital-associated MRSA. We explored whether the antimicrobial susceptibilities of the different strains account for their disparity in clinical virulence.


METHODS
This 10-year retrospective cohort study enrolled 291 patients with community-onset, healthcare-associated MRSA bacteremia. The vancomycin minimum inhibitory concentration (MIC) and staphylococcal cassette chromosome mec (SCCmec) type were determined for all isolates. CA-MRSA was defined as an isolate possessing the SCCmec type IV or V genes, and hospital-associated MRSA (HA-MRSA) was defined as an isolate possessing SCCmec type I, II, or III genes. Low and high vancomycin MICs were defined as MICs of ≤1 and ≥2 μg/mL, respectively. Patients with bacteremia due to CA-MRSA with a low vancomycin MIC (n = 111), due to HA-MRSA with a low vancomycin MIC (n = 127), or due to HA-MRSA with a high vancomycin MIC (n = 47) entered the outcome analysis. The outcomes of the 2 HA-MRSA bacteremia groups were compared to those of the CA-MRSA bacteremia group.


RESULTS
Treatment failure was observed in 35 (31.5%), 59 (46.5%), and 27 (57.4%) of patients with low-vancomycin-MIC CA-MRSA, low-vancomycin-MIC HA-MRSA, and high-vancomycin-MIC HA-MRSA bacteremia, respectively. After adjustment for potential confounding factors, the risk of treatment failure was significantly higher among patients with low-vancomycin-MIC HA-MRSA (adjusted odds ratio [aOR], 1.853; 95% confidence interval [CI], 1.006-3.413) and high-vancomycin-MIC HA-MRSA (aOR, 2.393; 95% CI, 1.079-5.309), compared with patients with low-vancomycin-MIC CA-MRSA.


CONCLUSIONS
The higher risk for treatment failure among patients with traditional hospital-associated MRSA infections, compared with patients with CA-MRSA infections, is independent of the vancomycin MIC, suggesting a potential intrinsic strain-specific virulence effect.
Department of Emergency Medicine, National Taiwan University Hospital, Taipei, Taiwan.
Shey-Ying
SY
Chen
Chun-Hsing
CH
Liao
Jiun-Ling
JL
Wang
Wen-Chu
WC
Chiang
Mei-Shu
MS
Lai
Wei-Chu
WC
Chie
Wen-Jone
WJ
Chen
Shan-Chwen
SC
Chang
Po-Ren
PR
Hsueh
D000368:Aged; D000369:Aged, 80 and over; D000704:Analysis of Variance; D000900:Anti-Bacterial Agents / Q000494:pharmacology; D016470:Bacteremia / Q000188:drug therapy / Q000382:microbiology*; D001426:Bacterial Proteins / Q000235:genetics*; D015331:Cohort Studies; D017714:Community-Acquired Infections / Q000188:drug therapy / Q000382:microbiology; D003428:Cross Infection / Q000188:drug therapy / Q000382:microbiology*; D005260:Female; D005838:Genotype; D006801:Humans; D008297:Male; D055624:Methicillin-Resistant Staphylococcus aureus / Q000187:drug effects / Q000235:genetics* / Q000302:isolation & purification; D008826:Microbial Sensitivity Tests; D008875:Middle Aged; D016017:Odds Ratio; D046915:Penicillin-Binding Proteins; D012189:Retrospective Studies; D013203:Staphylococcal Infections / Q000188:drug therapy / Q000382:microbiology*; D016896:Treatment Outcome; D014640:Vancomycin / Q000494:pharmacology*
D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D046915:Penicillin-Binding Proteins; C487789:mecA protein, Staphylococcus aureus; D014640:Vancomycin