Serratia marcescens

Serratia marcescens is a species of Gram-negative bacterium in the family Enterobacteriaceae. A human pathogen, S. marcescens is involved in nosocomial infections, particularly urinary tract infections and wound infections.

S. marcescens (bacillus), motile organism and can grow in temperatures ranging from 5–40°C and in pH levels ranging from 5 to 9. S. marcescens is differentiated from other Gram-negative bacteria, as it is able to perform casein hydrolysis. Performing casein hydrolysis allows for S. marcescens to produce extracellular metalloproteases which are believed to function in cell-to-extracellular matrix interactions. S. marcescens also exhibits tryptophan- and citrate-degradation. One of the end products of tryptophan degradation is pyruvic acid, which is then incorporated into different metabolic processes of S. marcescens. A final product of citrate degradation is carbon. Thus, S. marcescens can rely on citrate as a carbon source. In identifying the organism one may also perform a methyl red test, which determines if a microorganism performs mixed-acid fermentation. S. marcescens results in a negative test. Another determination of S. marcescens is its capability to produce lactic acid via oxidative and fermentative metabolism. Therefore, it is said that S. marcescens is lactose O/F+.

Due to its ubiquitous presence in the environment, and its preference for damp conditions, S. marcescens is commonly found growing in bathrooms (especially on tile grout), where it manifests as a pink discoloration. Once established, complete eradication of the organism is often difficult, but can be accomplished by application of a bleach-based disinfectant.S. marcescens may also be found in environments such as dirt, supposedly "sterile" places, and the subgingival biofilm of teeth. Due to this, and the fact that S. marcescens produces a reddish-orange pigment called prodigiosin, S. marcescens may cause extrinsic staining of the teeth. The biochemical pathway illustrating the production of prodigiosin by S. marcescens is unknown except for the final two steps. In these steps, a monopyrrole (MAD) and a bipyrrole (MBC) undergo a condensation reaction by way of a condensing enzyme to ultimately form the tripyrrole pigment, prodigiosin.

Pathogenesis
S. marcescens can cause conjunctivitis, keratitis, endophthalmitis, and tear duct infections. It is common in the respiratory and urinary tracts of adults and the gastrointestinal system of children. Most S. marcescens strains are resistant to several antibiotics because of the presence of R-factors, which are a type of plasmid that carry one or more genes that encode resistance.

In coral, S. marcescens is the cause of the disease known as White pox. In silkworms, it sometimes occurs as a secondary pathogen in viral flacherie disease.

Historical
In the 1950s S. marcescens was erroneously believed to be non-pathogenic and its reddish coloration was used in school experiments to track infections. It has also been used as a simulant in biological warfare tests by the United States Military. On September 26 and 27, 1950, the United States Navy conducted a secret experiment named "Operation Sea-Spray" in which some S. marcescens was released by bursting balloons of it over urban areas of the San Francisco Bay Area in California. Although the Navy later claimed the bacteria were harmless, beginning on September 29 eleven patients at a local hospital developed very rare, serious urinary tract infections and one of these individuals, Edward J. Nevin, died. Cases of pneumonia in San Francisco also increased after S. marcescens was released. ,

Since 1950, S. marcescens has steadily increased as a cause of human infection, with many strains resistant to multiple antibiotics. The first indications of problems with the influenza vaccine produced by Chiron Corporation in 2004 involved S. marcescens contamination.

Because of its red pigmentation, caused by expression of the pigment prodigiosin, and its ability to grow on bread, S. marcescens has been evoked as a naturalistic explanation of Medieval accounts of the "miraculous" appearance of blood on the Eucharist that led to Pope Urban IV instituting the Feast of Corpus Christi in 1264. This followed celebration of a Mass at Bolsena in 1263, led by a Bohemian priest who had doubts concerning transubstantiation, or the turning of bread and wine into the body and blood of Christ during the Mass. During the Mass, the eucharist appeared to bleed and each time the priest wiped away the blood, more would appear. This event is celebrated in a fresco in the Apostolic Palace in the Vatican City, painted by Raphael.

9. Brown, Alfred E (2007). Benson's Microbiological Applications. 10. http://jb.asm.org/cgi/reprint/114/3/999.pdf