What is the difference in bacterial coverage between Bactrim (trimethoprim/sulfamethoxazole) and cephalexin?

Bacterial Coverage Comparison: Bactrim vs Cephalexin

Bactrim (trimethoprim-sulfamethoxazole) and cephalexin have different bacterial coverage profiles, with Bactrim offering superior gram-negative coverage including MRSA, while cephalexin provides better coverage against streptococci and is preferred for cellulitis without purulent drainage.

Spectrum of Activity

Cephalexin (First-Generation Cephalosporin)

• Excellent coverage against gram-positive organisms, particularly streptococci and methicillin-susceptible Staphylococcus aureus (MSSA) 1, 2
• Limited activity against gram-negative organisms, with poor coverage of Haemophilus influenzae 3
• No activity against methicillin-resistant Staphylococcus aureus (MRSA) 3
• No coverage of anaerobes 1
• No activity against atypical pathogens 3

Bactrim (Trimethoprim-Sulfamethoxazole)

• Good activity against many gram-positive and gram-negative organisms 3
• Effective against MRSA in community-acquired infections 3
• Active against many Enterobacteriaceae 3
• Limited activity against streptococci, including β-hemolytic streptococci 3
• No reliable activity against anaerobes 3
• No activity against Pseudomonas species 3

Clinical Applications

Skin and Soft Tissue Infections

• Cephalexin is preferred for uncomplicated cellulitis without purulent drainage, where streptococci are the predominant pathogens 3
• For purulent skin infections where MRSA is suspected, Bactrim is often preferred 3
• A randomized clinical trial showed no benefit of adding Bactrim to cephalexin for uncomplicated cellulitis 4
• Cephalexin achieves cure rates of 90% or higher for streptococcal and staphylococcal skin infections 2

Urinary Tract Infections

• Both agents are effective for uncomplicated UTIs 3, 5
• Cephalexin is effective against non-ESBL producing Enterobacteriaceae in UTIs 5
• Bactrim has historically been considered first-line for uncomplicated UTIs due to its efficacy against common uropathogens 3
• Increasing resistance rates to Bactrim may limit its use in some geographic areas 3

Respiratory Tract Infections

• Cephalexin has better activity against respiratory pathogens like Streptococcus pneumoniae 3
• Bactrim has poor coverage against S. pneumoniae and is not recommended for respiratory infections 3
• First-generation cephalosporins like cephalexin have inadequate activity against H. influenzae, limiting their use in some respiratory infections 3

Resistance Considerations

• Increasing resistance to Bactrim has been observed in many regions 3
• Bactrim use may select for resistant organisms more readily than cephalexin 3
• Local antibiogram data should guide empiric therapy decisions 3
• Cephalexin remains effective against many community-acquired infections despite decades of use 2

Dosing Considerations

• Cephalexin is typically dosed 500 mg 4 times daily or 3 times daily 3
• Bactrim is typically dosed as 1 double-strength tablet (160/800 mg) twice daily 3
• Cephalexin can be conveniently administered as 500 mg twice or three times daily 5
• Both medications require dose adjustment in renal impairment 1

Common Pitfalls and Caveats

• Bactrim should not be used as monotherapy for cellulitis without purulent drainage due to poor streptococcal coverage 3
• Cephalexin should not be used when MRSA is suspected 3
• Neither agent covers Pseudomonas aeruginosa 3
• Bactrim carries risk of adverse effects including rash, Stevens-Johnson syndrome, and bone marrow suppression 3
• Cephalexin is generally well-tolerated with fewer serious adverse effects 1
• Cross-reactivity between penicillins and cephalosporins should be considered in patients with penicillin allergy 1

In summary, the choice between Bactrim and cephalexin should be guided by the suspected pathogens, local resistance patterns, and the specific infection being treated.