Antibiotic treatment of Community Acquired Methicillin Resistant Streptococcus Aureus
Richard C. Walls Pharm.D. Candidate; Preceptor: Randolph E. Regal Pharm.D.
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Introduction to CA-MRSA
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Microbiology of CA-MRSA
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CA-MRSA Infection
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Coag+, Gram+ cocci in clusters
mecA gene expresses PBP2a, preventing binding of β-lactam antibiotics
More genetic variability than HA-MRSA, USA300 clone most common
Rapid rise in incidence from 1999-2006 (3.6% -> 28.2% of patients)
MRSA related infections plateaued from 2005-2009 (16.31 -> 17.68 P=0.22)
Primarily SSTI: 80% of CA-MRSA infections in 2006
Risk factors: immunosuppression, antibiotic use, IVDA, close proximity
Prevention: antibiotic restriction, hand hygiene
Antibiotic Treatment of CA-MRSA
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IDSA Treatment Guidelines
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B)
I&D primary treatment for simple abscess/boil
Add antibiotics if: severe/extensive disease, evidence of systemic illness,
immunosuppressed patient, extremes of age, difficulty draining, lack of response
Empiric treatment for CA-MRSA recommended for purulent cellulitis or
nonpruluent cellulitis nonresponsive to β-lactams
Trimethoprim/sulfamethoxazole
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Synergistic dual enzyme inhibition of folate synthesis
First line option for uncomplicated infections (95-100% in vitro susceptibility)
In vitro studies suggest TMP/SMX can increase the virulence of CA-MRSA strains
Large single site study suggest that increased use of TMP/SMX for CA-MRSA has
not been met with increased resistance
Risk of hyperkalemia and not recommended in the 3rd trimester of pregnancy
Clindamycin
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Binds 50S ribosomal subunit and prevents translocation
Treatment option for complicated infections and MRSA pneumonia (good
susceptibility, but with geographic variability)
Additional coverage for β-hemolytic strep without requiring additional agent
Inducible resistance: D-test for MLSB if erythromycin R and clinda S
20% incidence diarrhea, higher risk of C. difficile colitis than other antibiotics
Tetracyclines
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Binds 30S ribosomal subunit and prevents peptide elongation
tetK gene confers resistance, but spares minocycline susceptibility
tetM gene confers class-wide resistance
Pregnancy category D and not to be used in children <8 years of age
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Vancomycin
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Active against Gram+ only by preventing cell wall cross-linking
First line inpatient option for complicated infections
Lack of oral bioavailability makes unsuitable for outpatient
Other Options
Reserved for hospitalized patients with complicated infections that have failed other
therapeutic options
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Include linezolid, daptomycin, and telavancin
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Summary
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CA-MRSA infections have rapidly grown in incidence through the early 2000s
For non-complicated SSTI that require antibiotic treatment, TMP/SMX, clindamycin,
and tetracyclines are considered first line agents
For complicated infections, vancomycin is first line treatment
Local susceptibilities vary; consider local trends when selecting a patient’s therapy