Does Bactrim Treat Gram-Positive Bacteria?
Yes, Bactrim (trimethoprim/sulfamethoxazole) is effective against many gram-positive bacteria, including streptococci, staphylococci, and pneumococci, though it is generally more potent against gram-negative organisms. 1, 2
Spectrum of Activity Against Gram-Positive Organisms
Bactrim demonstrates documented activity against the following gram-positive bacteria:
- Streptococcus pneumoniae - FDA-approved indication with proven clinical efficacy 1
- Beta-hemolytic streptococci - MIC90 between 2-4 mg/L in clinical isolates 2
- Staphylococci (including methicillin-sensitive strains) - MIC90 between 2-4 mg/L for most strains 2
- Staphylococcus epidermidis - Less susceptible than other gram-positive cocci, with higher MIC90 values 2
Clinical Context and Limitations
For community-acquired MRSA (CA-MRSA), Bactrim has in vitro activity but clinical data for severe infections like pneumonia is limited. 3 The IDSA/ATS guidelines note that while CA-MRSA strains are more susceptible to trimethoprim-sulfamethoxazole than hospital-acquired strains, previous experience with severe infections (endocarditis and septic thrombophlebitis) suggests TMP-SMX is inferior to vancomycin 3. Further study is needed for CA-MRSA pneumonia specifically 3.
For methicillin-sensitive S. aureus infections, neither Bactrim nor vancomycin is optimal - beta-lactam antibiotics (penicillinase-resistant penicillins or first-generation cephalosporins) are preferred 3.
Comparative Potency
In a large comparative study of 1,338 gram-positive strains from community practice, Bactrim achieved MIC90 values of 2-4 mg/L for most gram-positive organisms except S. epidermidis 2. This demonstrates good but not exceptional activity compared to its more potent effect against gram-negative bacteria, where MIC90 values were typically less than 2 mg/L 2.
Important Clinical Caveats
- Bactrim is not first-line for most serious gram-positive infections - beta-lactams, vancomycin, or linezolid are typically preferred depending on the organism and severity 3
- The drug is relatively nontoxic in immunocompetent patients but requires dose adjustment when creatinine clearance falls below 30 mL/min 4, 5
- Both components (trimethoprim and sulfamethoxazole) are excreted primarily renally, with 84.5% of total sulfonamide and 66.8% of free trimethoprim recovered in urine within 72 hours 1
- Resistance develops more slowly with the combination than with either agent alone 1