Cephalexin Coverage of Gram-Negative Rods

Spectrum of Activity

The FDA label and guidelines from the Infectious Diseases Society of America (IDSA) indicate that cephalexin demonstrates activity against a narrow subset of gram-negative bacteria, including susceptible strains of Escherichia coli, Klebsiella pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis 1
Cephalexin has no activity against most strains of Enterobacter species, Morganella morganii, and Pseudomonas species, which are common gram-negative rods 2, 1

The IDSA guidelines recommend cephalexin only for mild diabetic foot infections where the expected pathogens are primarily gram-positive, with the caveat that it requires QID dosing and is inexpensive 3
For infections requiring reliable gram-negative coverage, the IDSA guidelines explicitly recommend alternative agents, such as levofloxacin, ceftriaxone, ertapenem, or ampicillin-sulbactam 3, 2, 4
The IDSA guidelines also recommend against using cephalexin for moderate-to-severe infections with Enterobacteriaceae, animal bite wounds, and human bite wounds, due to its limited activity against gram-negative rods 3, 2, 4

When gram-negative coverage is uncertain but needed, the IDSA guidelines recommend choosing agents with broader and more reliable activity, such as levofloxacin, amoxicillin-clavulanate, or moxifloxacin for oral options, and ceftriaxone, ertapenem, or piperacillin-tazobactam for parenteral options 3

The IDSA guidelines advise against using cephalexin when gram-negative rods are the suspected or confirmed pathogen requiring definitive therapy, or when polymicrobial infections involving gram-negative organisms are present 1, 5
Cephalexin should also be avoided in cases where Pseudomonas or other non-fermenting gram-negative rods are possible, or when hospital-acquired infections are suspected due to the high risk of resistant gram-negatives 1