Serratia is a genus of Gram-negative, facultatively anaerobic, rod-shaped bacteria of the family Yersiniaceae. They are typically 1–5 μm in length, do not produce spores, and can be found in water, soil, plants, and animals. Some members of this genus produce a characteristic red pigment, prodigiosin, and can be distinguished from other members of the order Enterobacterales by their unique production of three enzymes: DNase (nucA), lipase, and gelatinase (serralysin). Serratia was thought to be a harmless environmental bacteria until it was discovered that the most common species in the genus, S. marcescens, is an opportunistic pathogen of many animals, including humans. In humans, S. marcescens is mostly associated with nosocomial, or hospital-acquired, infections, but can also cause urinary tract infections, pneumonia, and endocarditis. S. marcescens is frequently found in showers, toilet bowls, and around wet tiles as a pinkish to red biofilm but only causes disease in immunocompromised individuals. Aside from S. marcescens, some rare strains of the Serratia species – S. plymuthica, S. liquefaciens, S. rubidaea, and S. odoriferae – have been shown to cause infection such as osteomyelitis and endocarditis.

Various strains of Serratia occupy an eclectic range of habitats: soil, water, plants, insects, and others.

Currently, four species of Serratia have been found in seawater: S. marcescens, S. liquefaciens, S. plymuthica, and S. marinorubra. S. marcescens is the most abundant, comprising approximately half of all strains found. S. aquatilis is a novel species of Serratia found in drinking water.

The plant types with the highest Serratia prevalence are vegetables, mushrooms, mosses, grasses, and decaying plant material. Serratia has been consistently found in figs and coconuts. S. marcescens and S. ficaria are often found in Calimyrna figs. Several species related to Serratia have also been identified on Smyrna figs and its fig wasps. Only one species of Serratia, S. marinorubra, has been identified on coconuts from various regions of the world, including California, France, and Brazil. Both S. marcescens and certain strains of Enterobacter were also identified in the rhizospheres of coconut palms.

Serratia is found in over 70 species of healthy, dead, and diseased insects. These include crickets, grasshoppers, bees, aphids, and fruit flies. Most of them reside in insects as bacterial flora and some form mutualistic symbiotic relationships with their hosts. For example, in aphids, strains of S. symbiotica play a key nutritional role by providing the host with vitamins and amino acids.

In 2017 it was reported that Serratia can be genetically engineered to prevent malaria in mosquitos. Research showed 93% fewer Plasmodium parasites than in untreated counterparts.

S. marcescens is the most characterized species in this genus. During a hot summer in 1819 in Legnaro near Padua, Italy, the town people discovered that their polenta dish turned red. At first, the people believed that this incident was caused by the devil. A pharmacist named Bartolomeo Bizio was appointed to investigate the strange phenomenon. After several experiments, Bizzo presented his results. S. marcescens was first documented as a red-colored putrefaction of polenta by Bartolomeo Bizio in Padua. The bacterium was later named in honor of Italian physicist Serafino Serrati. In 1945, an experiment was designed to establish the pathogenicity of S. marcescens. Captain Tom Paine in the U.S. Army conducted an experiment at Camp Detrick, MD. In this experiment, he exposed four people to the bacteria in an enclosed space. The individuals soon developed symptoms such as body aches, malaise, green sputum production. A few of the individuals developed fever and chills while others still had a fever after 24 hours. Several other experiments were performed throughout the 50s, 60s, and 70s to test the pathogenicity of S. marcescens, but it was not until the 1970s that S. marcescens was confirmed as a human pathogen.

S. liquefaciens is the second best characterized species after S. marcescens. S. liquefaciens was first classified as Aerobacter liquefaciens in the Enterobacter genus by Grimes and Hennerty. The first documentation of S. liquefaciens was in 1971. Over 20 isolates of S. liquefaciens were recovered from different specimens, such as urinary and respiratory. Out of the isolates, six were believed to cause infection in humans. From the 1970s to the 1980s, this species was the cause of several hospital outbreaks. However, the most well-known outbreak occurred in Colorado at a hemodialysis center. During this outbreak, there were 10 S. liquefaciens bloodstream infections.