Serratia marcescens is a motile, rod-shaped bacterium and a human pathogen that is frequently implicated in health care-associated infections. It is a cause of health problems in humans as well as other organisms such as some melons and corals.
In humans, Serratia marcescens is known to infect the urinary tract, eye and respiratory tract as well as wounds. In rare cases, it can cause endocarditis, osteomyelitis, pneumonia, gastroenteritis or meningitis. It is resistant to penicillin and many antibiotics, including ampicillin, gentamicin and cephalexin, due to the presence of R-factors that carry genes capable of encoding resistance.
Serratia marcescens are gram negative rods about 0.9 to 2 micrometers long and 0.5-0.8 micrometers in diameter. It is anaerobic and requires little nutrition. It prefers damp environments such as bathrooms where it feeds on phosphorus-containing or fatty substances such as soap residue and manifests as a pink, orange or red film due to its production of a pigment known as prodigiosin.
As of 2015, about 11 per 100,000 inhabitants are carrying the pathogen and about 1 per 100,000 people per year develop bacteremia as a result. The rate of infection is higher in people 60 and older and about twice as high in men as in women. The primary modes of transmission are direct contact and ingestion of contaminated foods.