Cephalexin's Coverage of Gram-Negative Bacilli
Cephalexin has limited activity against gram-negative bacilli, covering only select species such as Escherichia coli, Proteus mirabilis, and Klebsiella pneumoniae, but lacks coverage against Pseudomonas, Acinetobacter, Enterobacter, and many other gram-negative bacilli.
Spectrum of Activity
- Cephalexin is a first-generation cephalosporin with FDA approval for treating genitourinary tract infections caused by specific gram-negative bacilli including Escherichia coli, Proteus mirabilis, and Klebsiella pneumoniae 1
- Cephalexin has no activity against Pseudomonas species or Acinetobacter calcoaceticus, and is not active against most strains of Enterobacter species, Morganella morganii, and Proteus vulgaris 1
- When compared to other cephalosporins, cephalexin demonstrates inferior activity against gram-negative bacilli, with studies showing it to be less effective than cefazolin against common gram-negative pathogens 2
Clinical Applications
- For mild infections where gram-negative coverage is needed, cephalexin is listed as an option primarily for uncomplicated urinary tract infections caused by susceptible strains of E. coli, P. mirabilis, and K. pneumoniae 1
- For moderate to severe infections involving gram-negative bacilli, cephalexin is inadequate and other agents such as third-generation cephalosporins, carbapenems, or combination therapy are recommended instead 3
- In diabetic foot infections, cephalexin is only recommended for mild infections primarily targeting gram-positive organisms, not for moderate or severe infections where gram-negative coverage is essential 3
Comparative Efficacy
- Studies have demonstrated that cephalexin has significantly less activity against gram-negative bacilli compared to other cephalosporins, with one study showing 57.5% of gram-negative isolates were resistant to cephalexin 4
- When gram-negative coverage is required, particularly for Enterobacteriaceae, other agents such as cefazolin, cefoxitin, ceftriaxone, or fluoroquinolones are preferred over cephalexin 3
- For serious infections involving gram-negative bacilli, particularly in critically ill patients, combination therapy with two antimicrobial agents of different classes with gram-negative activity is recommended initially 3
Common Pitfalls and Caveats
- A major pitfall is assuming cephalexin provides adequate coverage against all gram-negative bacilli, which could lead to treatment failure in infections caused by Pseudomonas, Acinetobacter, or Enterobacter species 1
- For infections potentially involving ESBL-producing Enterobacteriaceae, cephalexin and other cephalosporins should be avoided due to high rates of resistance 3
- When treating infections with potential gram-negative involvement, culture and susceptibility testing is essential before relying on cephalexin, as resistance patterns vary significantly 1
- In settings with high prevalence of multidrug-resistant gram-negative organisms, cephalexin is not an appropriate empiric choice 3
Summary of Coverage
- Cephalexin does cover: E. coli, P. mirabilis, K. pneumoniae, and Moraxella catarrhalis 1
- Cephalexin does not cover: Pseudomonas species, Acinetobacter species, most Enterobacter species, Morganella morganii, Proteus vulgaris, and other non-fermenting gram-negative bacilli 1
- For serious infections involving gram-negative bacilli, broader-spectrum agents or combination therapy should be selected 3