Serratia are Gram-negative bacteria classified in the tribeKlebsielleae. Serratia marcescens is the only pathogenic species causing human infection. S. marcescens—as a causative agent of a bloody discoloration in a cornmeal mush called polenta—was first identified by Bartolomeo Bizio, a pharmacist from Padua, Italy, in 1819. The bacterium was named Serratia in honor of an Italian physicist named Serrati, who invented the steam-boat. The species name marcescens is derived from the Latin word meaning decaying due to fast-deteriorating nature of the bloody pigment produced by the bacteria. Since the 1960s, S. marcescens been recognized as an opportunistic pathogencausing infection in humans. Some strains of S. marcescens typically produce a nondiffusible pigment called prodigi-osin, which varies in color from dark red to pink or magenta, depending on the age of the colonies. S. marcescens usually grows on starchy foodstuffs, where the production of pig-mented colonies is easily mistaken for drops of blood. S. marc-escens are pleomorphic with minute coccobacillary and normalbacillary forms.
In the hospital, Serratia usually colonizes the respiratory and urinary tracts of adult patients. The bacteria is responsi-ble for nearly 2% of nosocomial infections of the urinary tract, lower respiratory tract, surgical wounds, blood, and skin and soft tissues of these patients. S. marcescens has been associated with outbreaks of meningitis, wound infections, and arthritis in pediatric wards, and in the intensive care units. The bacteria also causes endocarditis and osteomyelitis in people addicted to intravenous drugs, such as heroin. Older people, patients with previous antibiotic treatment and chronic or debilitat-ing diseases are at increased risks for severe infections with Serratia.
S. marcescens is sensitive to amikacin and quinolonesbut is resistant to gentamicin and tobramycin. The bacte-ria are naturally resistant to ampicillin, macrolides, and first-generation cephalosporins. Therefore, treatment of S. marcescens is based on the results of antibiotics suscepti-bility testing.