Methicillin-resistant Staphylococcus aureus (SA) is a major pathogen causing mainly health-care associated infections and, lately, also community acquired infections. Few treatment choices exist to treat these infections. The currently recommended antibiotics for these infections are glycopeptides (vancomycin or teicoplanin). Glycopeptide treatment hs several disadvantages. It is a last resort antibiotic family that should be reserved for the future; Vancomycin is less effective that beta-lactam drugs for SA infections susceptible to both agents; treatment can only be given intravenously; and use of vancomycin has led to the development of SA strains with partial or complete resistance to vancomycin. Cotrimoxazole is an old antibiotic active against most strains of MRSA, depending on local epidemiology.

Study hypothesis: The purpose of this study is to show that cotrimoxazole is as effective as treatment with vancomycin for invasive MRSA infections.

We plan a randomized controlled trial comparing treatment with cotrimoxazole vs. vancomycin for invasive MRSA infections. The primary efficacy outcome we will assess will be Improvement or cure with or without antibiotic modifications, defined as: survival at 7 days post randomization with resolution of fever (<38 for two consecutive days) and resolution of hypotension (>90 systolic without need for vasopressor support); and physician's assessment that the primary infection was improved or cured. The primary safety outcome will be all-cause 30-day survival.

Staphylococcus aureus (SA) is a major pathogen causing community-acquired and health-care associated infections. In hospitals, SA infections are associated with a significant burden; in-hospital mortality during the last 15 years following SA bacteremia in Beilinson hospital was 38% and did not decrease in recent years. Resistance to beta-lactams is widely prevalent in hospitals (57% of all SA isolates causing bacteremia at our center). The drug of choice currently recommended for these infections is a glycopeptide (vancomycin or teicoplanin).

Cotrimoxazole (trimethoprim-sulfamethoxazole) is a relatively 'old' drug commonly used for urinary tract infections. Invitro, it is active against SA, including methicillin-resistance Staphylococcus aureus (MRSA) strains and its activity against SA is bactericidal. Trimethoprim alone is bactericidal against SA, while sulphamethoxazole alone is relatively inactive and their combination is synergistic both in-vitro and invivo. The prevalence of cotrimoxazole-susceptible SA varies locally. At our center, 97% of SA strains causing bacteremia in 2004 were susceptible to cotrimoxazole. Community-acquired MRSA, prevalent in the United States as a cause for severe skin and soft tissue infections, has not been described in Israel.

Several reasons exist to search for antibiotics other than vancomycin for MRSA infections. Vancomycin is less effective that beta-lactam drugs for SA infections susceptible to both agents. It is the last resort antibiotic for MRSA infections out of the currently recommended bactericidal antibiotics for invasive infections. Use of vancomycin has led to the development of SA strains with partial or complete resistance to vancomycin (VISA and VRSA, respectively). Vancomycin use is associated with the appearance of vancomycin-resistant enterococcus (VRE) species. Nosocomial infections with VISA and VRE are difficult to treat and may spread rapidly in the hospital. 10 Finally, vancomycin cannot be administered orally.

Limited evidence supports the efficacy of cotrimoxazole for MRSA infections, with paucity of data for high-burden invasive infections. Cotrimoxazole is probably inferior to vancomycin for methicillin-susceptible SA. ; thus we may infer indirectly its inferiority to methicillin and drugs alike for MRSA infections. Cotrimoxazole may be less effective than glycopeptides and oxacillin for left-sided endocarditis. No evidence exists to support the use of cotrimoxazole empirically for the treatment of suspected SA infections in the hospital.

We plan an open label single-center pragmatic randomized controlled trial to compare cotrimoxazole to vancomycin. We will include patients with documented or highly suspected MRSA infections, according to pre-defined risk factors. We chose to target this patient population to assess the efficacy of cotrimoxazole both empirically and for documented infections.

Inclusion Criteria:

• Adults >18 years
• providing signed informed consent or, if unable, having a legal guardian or a caretaker that will sign informed consent
• Patients with documented MRSA infections:
• MRSA bacteremia
• Other microbiologically documented MRSA infections defined as a clinical source of infection (CDC criteria) plus microbiological documentation of MRSA from the source of infection
• Patients with highly probable MRSA infections, prior to microbiological documentation of the pathogen:
• Suspected neurosurgical meningitis (including VP-shunt meningitis)
• Sepsis during hemodialysis
• Ventilator-associated pneumonia with prior antibiotic treatment within 48 hours
• Catheter-related or suspected catheter-related infections
• Surgical site infection in the presence of a foreign body

Exclusion Criteria:

Exclusion before randomization:

• Previous antibiotic treatment directed against MRSA >48 hours (including vancomycin, fucidic acid, rifampicin or cotrimoxazole)
• Known allergy to either study drug
• Acute leukemia and/ or BMT with neutropenia <500/mm3 or <1000/mm3 and expected to decrease below 500/mm3
• Pregnancy, lactation
• Previous enrollment in this study
• Concurrent participation in another trial

Exclusions after randomization:

• Documented Staphylococcal infection resistant to cotrimoxazole or VISA or VRSA
• Documented MSSA
• Documented left-sided endocarditis