Listeria is Naturally Resistant to Cephalosporins
Listeria monocytogenes is naturally resistant to most modern cephalosporins, particularly third-generation cephalosporins such as ceftriaxone and cefotaxime. 1
Natural Resistance Pattern of Listeria
- Listeria species demonstrate natural resistance to most "modern" cephalosporins including cefotaxime, ceftriaxone, cefuroxime, ceftazidime, and other third-generation cephalosporins 1
- This resistance is intrinsic (natural) to the organism and not acquired through mutation or horizontal gene transfer 2
- MIC testing has shown that cefuroxime and cefotaxime have no clinically relevant activity against Listeria monocytogenes, with MIC90 values greater than 128 μg/ml 3
Clinical Implications
- Due to this natural resistance, cephalosporins should never be used as monotherapy for Listeria infections 4
- When treating patients at risk for Listeria meningitis (elderly, immunocompromised, patients with cancer, or those using immunosuppressive therapy), empiric therapy must include coverage for Listeria 4
- The ESCMID guideline specifically states that Listeria is resistant to third-generation cephalosporins, which should therefore be avoided as empiric monotherapy in risk groups 4
Recommended Treatment for Listeria Infections
- The current therapy of choice for listeriosis is ampicillin or penicillin G, often combined with an aminoglycoside (usually gentamicin) 4, 2
- For patients with penicillin allergies, trimethoprim-sulfamethoxazole is the recommended alternative 4, 5
- Meropenem is also considered an acceptable alternative therapy according to treatment guidelines 4
Common Pitfalls in Treating Listeria Infections
- A major pitfall is initiating empiric therapy with only a third-generation cephalosporin in patients at risk for Listeria meningitis 4
- For patients over 50 years of age or those with risk factors for Listeria (diabetes, immunosuppression, cancer), guidelines recommend adding ampicillin to cephalosporin therapy to ensure coverage 4
- Failure to consider Listeria in at-risk populations can lead to treatment failure, as the organism will not respond to cephalosporin monotherapy 4
Susceptibility Pattern
- While resistant to cephalosporins, Listeria remains generally susceptible to penicillins, aminoglycosides, tetracyclines, macrolides, glycopeptides, carbapenems, and trimethoprim-sulfamethoxazole 1, 5
- High prevalence of clindamycin resistance (96.2%) has been reported 5
- Listeria species show varying susceptibility to quinolones, with some species being naturally resistant 1
Understanding this natural resistance pattern is crucial when selecting empiric antimicrobial therapy for patients at risk for Listeria infections, particularly in cases of meningitis or systemic infections.