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AmpC Induction Enterobacter — SCE Infectious Diseases MCQ

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ModerateAntimicrobial StewardshipAmpC Induction EnterobacterSCE Infectious Diseases

A 60-year-old woman with a recent hip replacement develops an acute wound infection at day 10. Wound cultures grow Enterobacter cloacae initially sensitive to Ceftriaxone. After 5 days of Ceftriaxone, cultures now show Ceftriaxone resistance (AmpC induction). What has happened?

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Correct answer: E — Chromosomal AmpC beta-lactamase induction — Enterobacter species can develop resistance to 3rd-generation cephalosporins during therapy through derepression of the chromosomal ampC gene

Enterobacter, Citrobacter freundii, Serratia, Morganella, and Providencia species (sometimes called the 'ESCPM' or 'AmpC group') harbour inducible chromosomal AmpC beta-lactamases. Exposure to 3rd-generation cephalosporins (Ceftriaxone, Cefotaxime) can select for stably derepressed AmpC mutants, resulting in resistance during therapy. This is why Ceftriaxone should be avoided for serious Enterobacter infections even if initially susceptible. Carbapenems, Cefepime, or Piperacillin/Tazobactam (with caution) are preferred. This is a classic ID/microbiology teaching point.

Reference: BSAC 2023 – AmpC beta-lactamases; EUCAST 2024 – Enterobacterales