Serratia marcescens
General Susceptibility

General Susceptibility

  • Carbapenem until susceptibility known, then step down to aminoglycoside or fluoroquinolone if susceptible.
  • These organisms usually produce inducible beta-lactamase which cause failure of 3rd generation beta-lactam therapy, despite in vitro susceptibility
  • SPICE organisms (Serratia spp, Providencia spp, "indole positive" Proteus spp (does not include mirabilis), Citrobacter, Enterobacter spp, Acinetobacter, Morganella) have inducible, chromosomal beta-lactamase genes known as AmpC. 
    • Resistance may not be detectable initially, but appears after a period of exposure to beta-lactam antibiotics including extended spectrum penicillins, B-lactam/B-lactamse inhibitor combinations. 
    • Resistance to third generation cephalosporins may develop in 3-4 days.