Is Keflex (cephalexin) effective against heavy Staphylococcus aureus growth?

From the Guidelines

Keflex (cephalexin) is generally effective against Staphylococcus aureus infections, including heavy growth, but its effectiveness may be limited against methicillin-resistant Staphylococcus aureus (MRSA) strains. For most uncomplicated S. aureus infections, the typical adult dosage is 250-500 mg taken orally every 6 hours, or 500-1000 mg every 12 hours, for 7-14 days depending on infection severity, as recommended by the Infectious Diseases Society of America 1. Children's dosing is typically 25-50 mg/kg/day divided into four doses. However, effectiveness may be limited against MRSA strains, which are increasingly common. For this reason, culture and sensitivity testing is important to confirm the bacteria's susceptibility to cephalexin, as suggested by the guidelines 1. If treating a suspected S. aureus infection empirically with Keflex, patients should watch for signs of treatment failure such as worsening symptoms or lack of improvement within 48-72 hours, which might indicate MRSA or other resistant organisms requiring alternative antibiotics. Some key points to consider when using Keflex for S. aureus infections include:

• Cephalexin is a first-generation cephalosporin antibiotic that works by inhibiting bacterial cell wall synthesis.
• The decision to administer antibiotics should be based on the presence or absence of systemic inflammatory response syndrome (SIRS) and the severity of the infection, as outlined in the guidelines 1.
• Alternative antibiotics, such as vancomycin or linezolid, may be necessary for MRSA infections, as recommended by the guidelines 1. It's also important to note that the guidelines recommend using the most recent and highest-quality evidence available, and that the choice of antibiotic should be based on the specific circumstances of the patient and the infection, as suggested by the guidelines 1.

From the FDA Drug Label

Cephalexin has been shown to be active against most strains of the following microorganisms both in vitro and in clinical infections as described in the INDICATIONS AND USAGE section Aerobes, Gram-positive: Staphylococcus aureus (including penicillinase-producing strains)

The MIC values should be interpreted according to the following criteria: MIC (mcg/mL)Interpretation ≤ 8Susceptible (S) 16Intermediate (I) ≥ 32Resistant (R)

Keflex (cephalexin) is effective against Staphylococcus aureus, including penicillinase-producing strains, with an MIC of ≤ 8 mcg/mL indicating susceptibility 2. However, it is essential to note that methicillin-resistant staphylococci are resistant to cephalosporins, including cephalexin.

• The effectiveness of cephalexin against heavy Staphylococcus aureus growth depends on the susceptibility of the specific strain and the achievement of adequate drug concentrations in the blood and at the site of infection.
• Culture and susceptibility tests should be performed to guide therapy and ensure the selected antibiotic is effective against the causative organism 2.

From the Research

Effectiveness of Keflex Against Heavy Staph Growth

• Keflex, also known as cephalexin, is a first-generation cephalosporin antibiotic used to treat various bacterial infections, including those caused by Staphylococcus aureus 3.
• According to a study published in 2005, cephalexin is effective against methicillin-susceptible Staphylococcus aureus (MSSA) infections, but its use is limited in patients with immediate penicillin hypersensitivity 3.
• Another study from 1983 found that cephalexin remains an effective antibiotic for the treatment of streptococcal and staphylococcal skin infections, with cure rates of 90% or higher 4.

Minimum Inhibitory Concentrations (MICs)

• A 2022 study compared the MICs of cefadroxil and cephalexin against MSSA isolates from pediatric musculoskeletal infections, finding that both antibiotics had similar in vitro activities against MSSA, with MIC50 values of 2 μg/mL and MIC90 values of 4 μg/mL 5.
• The study suggested that cefadroxil could be an alternative to cephalexin for the treatment of MSSA infections, although further research is needed to determine the optimal dose and frequency of these antibiotics 5.

Clinical Efficacy

• A 2006 study compared the efficacy and safety of cefdinir and cephalexin in adolescents and adults with mild to moderate uncomplicated skin and skin structure infections (USSSI), finding that both antibiotics had similar clinical cure rates, with 89% of patients achieving clinical cure at the test-of-cure visit 6.
• The study also found that cefdinir was more highly rated than cephalexin in a composite usefulness assessment, primarily due to the convenience of taking the study medication 6.

Dosing Regimens

• A 2020 study developed a population pharmacokinetic model of cephalexin in children to determine twice-daily (BID) and thrice-daily (TID) dosing regimens for MSSA infections, finding that BID doses of 22-45 mg/kg and TID doses of 15-25 mg/kg were required to achieve the pharmacodynamic target for MSSA 7.
• The study proposed a BID and TID cephalexin dosing regimen that can be prospectively evaluated to reduce the medication burden for children and improve treatment compliance for MSSA infections 7.