- G(-) rod, S. marcescens is the species encountered in clinical practice
- Naturally resistant to penicillin, aminopenicillins (amoxicillin, ampicillin), 1st generation cephalosporins, and macrolides
- Beta-lactam resistance due to production of "AmpC", a beta-lactamase which the organism can increase production of during therapy when exposed to certain beta lactams.
- Assume AmpC is being produced!
- Use cefepime, a carbapenem, or a non beta-lactam even if cultures show sensitivity to other beta-lactams (and especially if patient has received doses of other beta lactams during their stay), otherwise may lead to resistance development during therapy and treatment failure.
- Once sensitivities are available, de-escalation to a non-beta-lactam is possible if sensitive
- Rarely causes healthcare-acquired infections, more likely to cause pneumonia or UTI