What is the coverage and dosage of Bactrim (trimethoprim/sulfamethoxazole) for various bacterial infections?

Bactrim Coverage and Dosing

Antimicrobial Spectrum

Bactrim (trimethoprim-sulfamethoxazole) provides broad-spectrum coverage against both Gram-positive and Gram-negative aerobic bacteria, making it effective for urinary tract infections, respiratory infections, skin and soft tissue infections including MRSA, Pneumocystis jiroveci pneumonia, shigellosis, and traveler's diarrhea. 1, 2, 3

Specific Pathogens Covered

• Gram-negative organisms: E. coli, Klebsiella, Proteus, Enterobacter, and other Enterobacteriaceae 4, 3
• Gram-positive organisms: Staphylococcus aureus (including community-acquired MRSA), Streptococcus pneumoniae 1, 5, 3
• Other organisms: Pneumocystis jiroveci, Shigella species 6, 2, 7

Standard Adult Dosing by Indication

Urinary Tract Infections

• Uncomplicated cystitis: 1 double-strength tablet (800mg/160mg) twice daily for 3 days 1, 8, 2
• Pyelonephritis: 1 double-strength tablet twice daily for 14 days (only when pathogen is known to be susceptible) 1, 8, 2
• Complicated UTI: 1 double-strength tablet twice daily for 10-14 days 8, 2, 7

Critical caveat: Do not use empirically if local E. coli resistance exceeds 20%, as treatment failure risk increases >17-fold with resistant strains 1. Clinical cure drops from 88% to 41-54% with resistant organisms 1.

Skin and Soft Tissue Infections (Including MRSA)

• Standard dosing: 1-2 double-strength tablets twice daily for 7-10 days 1, 5
• Hordeolum cellulitis: 1 double-strength tablet twice daily for 7-10 days 5
• Clinical improvement should be evident within 48-72 hours; if not, consider resistance or need for incision and drainage 5

Respiratory Infections

• Acute exacerbations of chronic bronchitis: 1 double-strength tablet twice daily for 14 days 2, 7
• Pertussis (alternative agent): Trimethoprim 320mg/sulfamethoxazole 1,600mg per day in 2 divided doses for 14 days (only for patients >2 months who cannot tolerate macrolides) 6

Pneumocystis Jiroveci Pneumonia

• Treatment: 15-20 mg/kg/day of trimethoprim component (75-100 mg/kg of sulfamethoxazole) in 3-4 divided doses for 21 days 6, 2, 7
• Prophylaxis: 1 double-strength tablet daily 2, 7

Gastrointestinal Infections

• Shigellosis: 1 double-strength tablet twice daily for 5 days 2, 7
• Traveler's diarrhea: 1 double-strength tablet twice daily for 5 days 2, 7

Pediatric Dosing (>2 months of age)

• UTI/Acute otitis media/Shigellosis: 40 mg/kg sulfamethoxazole and 8 mg/kg trimethoprim per 24 hours in 2 divided doses 2, 7
• PCP treatment: 75-100 mg/kg sulfamethoxazole and 15-20 mg/kg trimethoprim per 24 hours in 4 divided doses 6, 2, 7
• PCP prophylaxis: 750 mg/m²/day sulfamethoxazole with 150 mg/m²/day trimethoprim in 2 divided doses, 3 consecutive days per week 2, 7

Absolute contraindication: Infants <2 months of age due to kernicterus risk 6, 2, 7

Renal Dose Adjustment

• CrCl >30 mL/min: Standard dosing 2, 7
• CrCl 15-30 mL/min: 50% of usual dose 2, 7
• CrCl <15 mL/min: Use not recommended 2, 7

Alternative Agents When Bactrim is Inappropriate

For UTIs

• Nitrofurantoin: 100mg twice daily for 5-7 days (cystitis only, not pyelonephritis) 1, 8
• Fosfomycin: 3g single dose (uncomplicated cystitis) 1, 8
• Fluoroquinolones: Ciprofloxacin 500mg twice daily for 3 days (when local resistance <10%) 1, 8

For Skin/Soft Tissue Infections

• Clindamycin: 300-450mg orally three times daily 1, 5
• Doxycycline: 100mg orally twice daily 5
• Linezolid: 600mg orally twice daily 5

Contraindications and Precautions

• Pregnancy: Contraindicated in third trimester due to increased risk of birth defects and kernicterus 1, 5
• Nursing mothers: Contraindicated due to kernicterus risk in infant 6
• Hypersensitivity: To trimethoprim or sulfonamides 6
• Severe renal impairment: CrCl <15 mL/min 2, 7

Adverse Effects

Common

• Gastrointestinal disturbances, rash, photosensitivity 1
• Hematologic abnormalities (neutropenia, thrombocytopenia) 6

Serious (Rare)

• Stevens-Johnson syndrome, toxic epidermal necrolysis 6, 1
• Bone marrow suppression 1
• Hepatic necrosis 6
• Renal disorders (interstitial nephritis) 6

Monitoring: Regular complete blood count for prolonged therapy 1

Critical Clinical Pearls

• Resistance surveillance is mandatory: Rising TMP-SMX resistance among uropathogens limits first-line use in many regions 1
• Always obtain cultures: For pyelonephritis, recurrent UTI, or treatment failure before initiating therapy 8
• Adequate hydration: Maintain fluid intake to prevent crystalluria and renal stones 6
• Drug interactions: Monitor when used with methotrexate, oral anticoagulants, antidiabetic agents, thiazide diuretics, and anticonvulsants 6
• For severe infections: Consider initial IV therapy before transitioning to oral treatment 1