Serratia species are opportunistic gram-negative bacteria classified in the tribe Klebsielleae and the large family Enterobacteriaceae. Serratia are widespread in the environment, but are not a common component of the human fecal flora. 
Serratia marcescens is the primary pathogenic species of Serratia.  Rare reports have described disease resulting from infection with Serratia plymuthica,  Serratia liquefaciens,  Serratia rubidaea,  Serratia odorifera, and Serratia fonticola. 
Some strains of S marcescens are capable of producing a pigment called prodigiosin, which ranges in color from dark red to pale pink, depending on the age of the colonies. The chemical structure of prodigiosin has been unveiled.  Serratia are capable of thriving in diverse environments, including water, soil, and the digestive tracts of various animals.  S marcescens has a predilection for growth on starchy foodstuffs, where the pigmented colonies are easily mistaken for drops of blood.
In 1819, Bartolomeo Bizio, a pharmacist from Padua, Italy, discovered and named S marcescens when he identified the bacterium as the cause of a miraculous bloody discoloration in a cornmeal mush called polenta. Bizio named Serratia in honor of an Italian physicist named Serrati, who invented the steamboat, and Bizio chose marcescens (from the Latin word for decaying) because the bloody pigment was found to deteriorate quickly. 
Since 1906, physicians have used S marcescens as a biological marker for studying the transmission of microorganisms because, until the 1950s, this bacterium was generally considered a harmless saprophyte. Only since the 1960s has S marcescens been recognized as an opportunistic pathogen in humans. 
Derivatives of prodigiosin have recently been found to have immunosuppressive properties and antitumor activity in vivo [11, 12] and are also currently being considered as a candidate treatment for Chagas disease. 
It appears that at least some Serratia isolates interfere with macrophage function or viability.  In the hospital, Serratia species tend to colonize the respiratory and urinary tracts, rather than the gastrointestinal tract, in adults.
Serratia infection is responsible for about 2% of nosocomial infections of the bloodstream, lower respiratory tract, urinary tract, surgical wounds, and skin and soft tissues in adult patients. An outbreak of S marcescens bloodstream infections was identified in patients receiving contaminated bags of parenteral nutrition.  Outbreaks of S marcescensmeningitis, wound infections, and arthritis have occurred in pediatric wards.
Cases of Serratia septic arthritis have been reported in patients receiving intra-articular injections, individuals with joint trauma, and patients with intravascular devices or who are undergoing intravascular procedures.
An outbreak of meningitis caused by S marcescens in patients who had undergone spinal anaesthesia for caesarean section has been ascribed to contaminated medications used for this purpose. 
Serratia species are responsible for 1.4% of nosocomial bloodstream infections.
The yearly incidence of Serratia bacteremia is 1.03 per 100,000 population, with 47% of episodes having their onset in the community. 
The prevalence of Serratia species as a cause of nosocomial infections is diminishing, but these bacteria are still able to cause hospital outbreaks, especially in intensive care units.
In the University Hospital of Heraklion, Crete, S marcescens was isolated in 65 (84.4%) of 77 patients with Serratia infection; the remaining 12 patients had infection with a nonmarcescens Serratia species. The most frequently observed infections were respiratory tract infection (32.5%) and keratitis/endophthalmitis (20.8%). 
In a population-based study of Serratia bacteremia, the 7-day and 6-month mortality rates were 5% and 37%, respectively. 
Serratia meningitis and Serratia endocarditis carry a high mortality rate.
Serratia species cause less than 6% of cases of hospital-acquired bacterial pneumonia. 
S marcescens causes 11% of burn-related surgical wound infections. 
Most (68%) episodes of Serratia bacteremia occur in males. 
Outbreaks of Serratia infection occur in neonates and infants. In adults, most Serratia infections are isolated, but occasional nosocomial outbreaks occur.
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