Midstream urine: ≥100 x 10 6 cfu/L – Serratia marcescens – Shipped February 20, 2017


A simulated midstream urine (MSU) collected from a 48 year old female patient in a MS (multiple sclerosis) rehabilitation facility was sent to category A, B, and C laboratories.

Participants were expected to report a colony count, isolate and identify Serratia marcescens, and perform susceptibility testing.


  1. S. marcescens is intrinsically resistant to ampicillin/amoxicillin/amoxicillin-clavulanate, 1st-generation cephalosporins (e.g., cefazolin, cephalexin), cefuroxime, cephamycins (e.g., cefoxitin, cefotetan), nitrofurantoin, and colistin.  Results of susceptibility testing should be reported as resistant to each of the above antimicrobial agents (if they are appropriate to report) regardless of how they test in vitro.
  2. Organisms known to produce inducible β-lactamases can develop resistance during therapy.  A comment should be included in the patient report indicating the potential for inducible resistance when reporting isolates initially susceptible to 3rd-generation cephalosporins.

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