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AmpC Serratia Ceftriaxone Risk — SCE Infectious Diseases MCQ

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ModerateAntimicrobial StewardshipAmpC Serratia Ceftriaxone RiskSCE Infectious Diseases

A 50-year-old man develops Gram-negative bacteraemia with an organism identified as Serratia marcescens. It is producing an AmpC beta-lactamase. His clinician prescribed IV Ceftriaxone based on initial susceptibility. Why is this problematic?

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Correct answer: A — Serratia is in the 'AmpC group' — exposure to Ceftriaxone can select for stably derepressed AmpC mutants causing resistance during therapy; Meropenem or Cefepime is preferred for serious Serratia infections

Serratia marcescens (along with Enterobacter, Citrobacter freundii, Morganella, Providencia — the 'ESCPM' AmpC group) harbours an inducible chromosomal AmpC beta-lactamase. Third-generation cephalosporins (Ceftriaxone, Cefotaxime) can select for stably derepressed AmpC mutants during therapy — even if the initial isolate tests susceptible. This leads to treatment failure with resistance emergence during therapy. For serious infections with AmpC-group organisms, Meropenem (or Cefepime — which is stable to AmpC hydrolysis) is preferred.

Reference: BSAC 2023 – AmpC; EUCAST 2024 – AmpC expert rules