Serratia
Serratia is a genus of Gram-negative, facultatively anaerobic, rod-shaped bacteria of the family Yersiniaceae. According to the List of Prokaryotic names with Standing Nomenclature, there are currently 19 species of Serratia that are credibly published with accurate names as of 2020: S. aquatilis, S. entomophila, S. ficaria, S. fonticola, S. grimesii, S. liquefaciens, S. marcescens, S. microhaemolytica, S. myotis, S. nematodiphila, S. odoriferae, S. oryzae, S. plymuthica, S. proteamaculans, S. quinivorans corrig, S. rubidaea, S. symbiotica, S. ureilytica, S. vespertilionis. 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, lipase, and gelatinase. 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 wetted 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.
Habitat
Various strains of Serratia occupy an eclectic range of habitats: soil, water, plants, insects, and others.Water
Currently, four species of Serratia have been found in sea water: 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.Plants
The plant types with 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, have 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 rhizospheres of coconut palms.Insects
Serratia are 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.Isolation, identification, and metabolism
Isolation
S. marcescens is the most characterized species in this genus. During the summer in Padura, 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 Bizzo was appointed to investigate the strange phenomenon. After several experiments, Bizzo presented his results. S. marcescens was first documented as 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 wasn't 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, 6 of them were believed to cause infection in humans. From the 70s to 80s, 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.
S. ficaria is another species that can be harmful to humans. S. ficaria is a part of the fig tree community. In 1979, S. ficaria was first isolated from a patient who had a respiratory infection. The organism was isolated from the patient's sputum after she consumed a fig. The organisms continued to be isolated from several humans over the years. The last documented infection caused by S. ficaria was in Greece. A healthy man was bitten by a dog, the dog bite turned into an abscess. This was the first infection that was in a healthy individual.S. fonticola species was first found in human specimens in 1985. It is known to cause tissue infections following trauma to the area. The first reported infection caused by S. fonticola species was in 1989. The organism caused a leg abscess in a woman in France. In 1991, S. fonticola was the cause of a hand infection in another french woman. S. fonticola has been recovered from several other patients over the years.
There are not many reports of S. quinivoran causing infection in humans. A homeless man in France was admitted to the hospital with a mouth abscess. The man developed pneumonia and respiratory issues. S. quinivoran was recovered from a sample and was later identified as the cause of his organ failure and death. S. rubidaea, S. odorifera, and S. plymuthica are other Serratia species that are human pathogens. However, not all Serratia species are human pathogens. S. entomophia and S. proteamaculans are insect and plant pathogens.
Identification
Species of Serratia have been isolated in a variety of environments, including soil, water, plants, animals and even air. Several methods can be used to study the epidemiology of S. marcescens. Usual enrichment strategies involve the use of media containing antibiotic and antifungal substances. A caprylate-thallous media seems to be highly preferred for the selective growth of genus Serratia, as it can use caprylic acid as a carbon source.Serological typing and different types of polymerase chain reaction can be used to identify the Serratia. Biotyping, bacteriocin typing, phage typing, plasmid analysis, and ribotyping can also be used. Most strains of S. marcescens appear red on trypticase soy agar slants when grown at around 25 °C. S. marcescens and S. liquefaciens can be easily confused in the lab when using the analytical profile index system. They can both oxidise arabinose, but only S. liquefaciens can ferment arabinose in peptone water. The virulence of Serratia strains can also be identifiable by type 4 fimbriae, small hair-like projections.
Genome content
The average genome size of most Serratia species has not been documented except for that of S. marcescens, which is 3.57 * 109 Daltons. The range of G+C content of each species is as follows: S. marcescens 57.5 – 60.4%, S. liquefaciens 52.6-54.4%, S. plymuthica 53.3-56.3%, S. marinorubra 53.5-58.5%. S. macescens genome has the highest G+C content among all enterobacteria.Enzymes and biofilm
Serratia secrete a host of virulence factors including prodigiosin, biosurfactants, DNAse, lipase, protease, gelatinase, hemolysin, chitinase, chloroperoxidase, and alkaline phosphatase. Prodigiosin, a growth pigment, is often used as a phenotypic identification marker of Serratia species due to its red colorization. Biosurfactants have been isolated from Serratia marcescens, Serratia rubidaea and Serratia surfactantfaciens for their range of applications including emulsification, surface, antifouling, antitumor, and antimicrobial activity. Endonucleases, such as DNAse, may aid in scavenging activity, allowing them to exploit the environment and maximize availability of nutrients. Strains producing thermostable lipase, alkaline protease and gelatinase have been isolated from strains causing contact lens-related corneal ulcers in humans. Due to its short half-life and tendency to remain bound to cells upon secretion, hemolysin has scarcely been identified in Serratia. However, some studies employing more accurate detection techniques have evidenced hemolytic activity in almost all strains of Serratia. Plant chitinases are used as defense mechanisms against plant pathogens with which Serratia shares their plant habitat. Chloroperoxidase allows hydrolysis of phosphodiester bonds while alkaline phosphatases are involved in cell signaling processes.Metabolism
Serratia utilizes a metabolic enzyme ADP glucose pyrophosphorylase with distinct kinetic properties from those found in Enterobacteriaceae in that it is not greatly activated by fructose bisphosphate. ADP glucose pyrophosphorylase from strains of S. marcescens demonstrated optimal activity in buffer at pH 7.5 and 8.0, respectively. It is greatly activated by glycolysis intermediates such as phosphoenolpyruvate, 3-phosphoglycerate, fructose-6-phosphate, and 2-phosphoglycerate.Pathology
Most Serratia species are nonpathogenic, but those that are pathogenic typically cause infection in immunocompromised individuals. S. marcescens is the main pathogenic species, infecting animals and plants, but other species that have been reported to infect individuals includes Serratia plymuthica, Serratia liquefaciens, Serratia rubidaea, Serratia odorifera, and Serratia fonticola.Opportunistic human pathogen
S. marcescens is thought to be transmitted through hand-to-hand transmission by non-infected people via hand to hand transmission, hospital equipment, hospital personnel. 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 and other ailments that are commonly caused by other bacteria. Outbreaks of S. marcescens meningitis, wound infections, and arthritis have occurred in pediatric wards.Cases of Serratia arthritis have been reported in outpatients receiving intra-articular injections.