Picture of Serratia marcescens: 
Description and Significance
The microbe Serratia marcescens was discovered in 1819 by the Italian pharmacist Bartolomeo Bizio after he found a bloody discoloration on his polenta. Once he determined this microbe was the cause of the infection, Bizio determined the identity of it by naming Serratia after an Italian physicist, Serrafino Serrati, and marcescens after the Latin word for decaying. These microorganisms are a motile gram-negative bacillus bacteria that are capable of growing in temperatures from 5°C to 40°C and in pH levels between 5 and 9. These bacteria are facultative anaerobes, meaning they are able to grow in a location with or without oxygen. Because of this, they can perform nitrate reductions when in an anaerobic environment. Before the 1960s, S. marcescens were thought to be harmless and non-pathogenic; however, news was released about how the United States military used to perform experiments in order to stop the spread of the microorganism which resulted in many being exposed to the newly identified pathogen. It was later discovered that they most commonly infect those with weakened immune systems including hospital patients in the ICU. Serratia marcescens are known for being involved in urinary and respiratory infections, septicemia, endocarditis, eye infections, osteomyelitis, meningitis, and wound infections; they transmit these infections through direct contact with other organisms or through medical equipment that would be considered to be sterile including catheters and saline irrigation solutions. They are environmental organisms, meaning these bacteria have a large host range and can infect invertebrates, vertebrates, and plants. Since these organisms are environmental isolates, some strains are capable of making the red pigment called prodigiosin, which scientists used as markers to track the activity or transmission from bacteria cultures.
The Serratia marcescens strain Db11 has a complete genome that contains a single circular chromosome of 5,113,802 base pairs which consists of a G+C content of 59.51%. S. marcescens has 541 genome assemblies and four sequence reads. The average median total length (Mb) of the genome for S. marcescens is 5.1985 and their median protein count is 4798.
Cell Structure, Metabolism and Life Cycle
Serratia marcescens is a short rod-shaped gram-negative bacterium and has a thin cell wall. This cell wall is made up of a single peptidoglycan layer that is surrounded by an outer membrane which contains lipopolysaccharides, molecules that have both lipids and polysaccharides. The purpose of the outer membrane is that it regulates the uptake of nutrients and prohibits toxins. S. marcescens are also motile and use flagellum to travel. These bacteria are able to move on their own or travel in groups, called swarmer cells, where they can form a biofilm.
The main method that Serratia marcescens collects energy is through fermentation; however, they do contain enzymes protecting them from reactive oxygen species if they were to live in a location with oxygen. They also use the fermentation process to produce the enzyme protease, which they use to destroy peptide bonds in casein, a protein commonly found in milk, resulting in a clearing on a milk agar plate. What separates S. marcescens from other gram-negative bacillus bacteria is that it can undergo casein hydrolysis where they create extracellular metalloproteinases, a protease enzyme that has a catalytic mechanism where it includes a metal. Scientists have discovered that these extracellular metalloproteinases may have a role in working with cell-to-extracellular matrix interactions. This bacterium can also perform degradation of tryptophan that is found in gelatin, and of citrate which is where their source of carbon comes from.
Serratia marcescens live inside and feed on its host’s epithelial cells, for example, cells that make up different organs such as the gut and kidneys. While not much research has been done on how S. marcescens can live and reproduce in its host, the NBCI has conducted a study on how these microbes can inhabit epithelial cells within the ovaries of Chinese hamsters. Through this research they were able to determine the steps that these bacteria take in order to live within its host’s cells. The scientists discovered that once the bacteria get inside the host, they use their flagellum to attach to epithelial cells. Then, they make their way inside the cell and most are placed inside compartments that are autophagic-like non-degradative and non-acidic with a few being stored in acidic and degradative compartments. Next, the autophagy system within the body targets the vacuoles that contain Serratia marcescens, which causes acidification to occur. The microbes respond to this attack by quickly neutralizing the acidification and develop a non-canonical autophagic pathway. Finally, the epithelial cells begin go through the autophagic process and consumes its own tissues because the Serratia marcescens bacterium is feeding on the cell.
Ecology and Known Roles in Symbiosis
Serratia marcescens are found everywhere but they are most common in water, soil, animals, and plants. This bacterium is known to be found most of the time in damp environments such as contaminated water and bathrooms. These microbes make up the pink slimy discoloration that often forms in shower corners, toilets, and tile grout. These locations provide their food source of materials that either have phosphorus or fatty substances, including shampoo and soap. They also invade bread and petri dishes that are in laboratories. S. marcescens is an opportunistic parasitic endosymbiont that most commonly lives in the gut or other organs made up of epithelia cells of its host. For example, strains of this bacteria have been found to live in the lumen of female adult Anopheles stephensi mosquitoes where it prevents the malaria infection caused by a plasmodium. Serratia marcescens also cause many diseases in organisms such as white pox disease in elkhorn coral and cucurbit yellow vine disease in melon fields.
A unique fact about Serratia marcescens is that when the microbe lives in bread, it appears that the bread is “bleeding”. This idea became influential back in the medieval time period for the Catholic church. A Bohemian priest had trouble believing that the bread and wine changed into the body and blood of Christ, but during a mass the bread looked as if it was bleeding. This “miracle” was a turning point in the Catholic church and was painted as a mural in the house of the pope, Apostolic Palace, in the Vatican City. Scientists have studied this event and believe that Serratia marcescens was the cause of the “bloody” bread. Another interesting fact is that marcescens is the most common species of Serratia that is found in hospitals and it is the only pathogenic species of Serratia. Most strains of this microbe are also resistant to many types of antibiotics such as ampicillin and macrolides, so an aminoglycoside and an antipseudomonal beta-lactam are commonly prescribed.
1. Buckle, J. (2015). Serratia marcescens. Retrieved January 26, 2020, from https://www.sciencedirect.com/topics/medicine-and-dentistry/serratia-marcescens
2. Serratia marcescens (ID 1112). (n.d.). Retrieved January 26, 2020, from https://www.ncbi.nlm.nih.gov/genome/?term=Serratia+marcescens
3. Climaco, A. B., & Prasad, P. J. (2019, November 11). Serratia. Retrieved January 26, 2020, from https://emedicine.medscape.com/article/228495-overview
4. Mahlen S. D. (2011). Serratia infections: from military experiments to current practice. Clinical microbiology reviews, 24(4), 755–791. doi:10.1128/CMR.00017-1, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3194826/
5. Ewbank, J. (n.d.). Serratia marcescens. Retrieved January 26, 2020, from https://www.sanger.ac.uk/resources/downloads/bacteria/serratia-marcescens.html
6. Bai, L., Wang, L., Wang, G., Wang, S., & Vega-Rodriguez, J. (2019, June 25). A Gut Symbiotic Bacterium Serratia marcescens Renders Mosquito Resistance to Plasmodium Infection Through Activation of Mosquito Immune Responses. Retrieved January 26, 2020, from https://www.frontiersin.org/articles/10.3389/fmicb.2019.01580/full
7. Serratia marcescens. (2019, December 26). Retrieved January 26, 2020, from https://en.wikipedia.org/wiki/Serratia_marcescens
8. Serratia marcescens (ID 1112). (n.d.). Retrieved January 26, 2020, from https://www.ncbi.nlm.nih.gov/genome/1112?genome_assembly_id=603823
9. Gould, S. E. (2014, January 19). Swirling and whirling: the movement of spherical bacteria. Retrieved January 26, 2020, from https://blogs.scientificamerican.com/lab-rat/swirling-and-whirling-the-movement-of-spherical-bacteria/
10. Fedrigo, G. V., Campoy, E. M., Di Venanzio, G., Colombo, M.I., & García Véscovi, E. (2011, August 25). Serratia marcescens is able to survive and proliferate in autophagic-like vacuoles inside non-phagocytic cells. Retrieved January 27, 2020, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3162031/
This page was authored by Kiley Robison as part of the 2020 UM Study USA led by Dr. Erik Hom at the University of Mississippi.