Serratia spp

Gram Stain

• Gram negative bacilli - fermenter

Clinical Significance

These organisms are found in a variety of environmental sources.

They may represent colonizers of the urinary and respiratory tracts in patients who have received broad spectrum antibiotic therapy.

These organisms are considered opportunistic pathogens and cause nosocomial infections (pneumonia, bacteremia, wound and urinary tract infections).

Serratia spp are associated with endocarditis, especially in intravenous drug users.

Usual Susceptibility Pattern

These organisms produce an inducible cephalosporinase (AmpC) and are resistant to ampicillin  and 1st/2nd generation cephalosporins.  Therapy with 3rd generation cephalosporins is not recommended as clinical failures may occur despite in vitro susceptibility.

Beta-lactamase inhibitor combinations are not recommended as the cephalosporinase is not inhibited by beta-lactamase inhibitors.

Meropenem has better activity than imipenem.

These organisms are resistant to nitrofurantoin, fosfomycin, tetracycline, colistin, and have variable susceptibility to aminoglycosides, TMP/SMX, and quinolones.

Some strains exhibit resistance to tobramycin while remaining susceptible to gentamicin.