Serratia marcescens

Classification

Higher order Taxa

Bacteria(Domain); Proteobacteria(Phylum); Gamma Proteobacteria(Class); Enterobacteriales(Order); Enterobacteriaceae(Family); Serratia(Genus).

Species

Serratia marcescens

Description and Significance

Serratia marcescens bacteria are short rod-shaped, Gram-negative, facultive anaerobes. It was discovered in 1819 by Bartolomeo Bizio in Padua, Italy. Bizio attributed the red or bloody discoloration in cornmeal and breads. Bizio chose the name Serratia to honor his friend named Serratia and secondly marcescens, which is Latin for decay [3]. Serratia marcescens was first thought to be harmless (non-pathogenic). Due to its ability to produce red pigmentation, it was used in 1906 as a marker in order to trace bacterial activity or transmission [4]. It was not until later in the 1950’s, when US government experimented with the Serratia marcescens, that the harmful affects that the bacteria causes. A study using Serratia marcescens was carried out to determine the possibility of biological weapons being transmitted by wind current. It was soon after that there was an increase in the number of pneumonia and urinary tract infections [1]. Although the Serratia marcescens bacterium was classified as a human pathogen in the 1960s, scientist still used it as a bacterial tracer well into the 1970s [10].

Although Serratia marcescens can be found in soil, water and some foods, it is an opportunistic pathogen and can therefore more often than not be seen in people with a compromised immune system. They are mobile bacteria. Optimally, Serratia marcescens grow at 37°C (a human body’s temperature), but it can grow in temperatures that range from 5–40°C. They grow in pH levels that range from 5 to 9 [14]. Serratia marcescens is well known for the red pigmentation it produces called prodigiosin. Prodigiosin has a skeleton that is made up of three pyrrole rings [15] and is not produced at 37°C, but at a lower temperature such as at 30°C [20]. In 1263 a priest with doubts of Christ’s presence in the consecrated Host presided over a mass in the Basilica of Bolsena. After speaking the words of consecration, blood began to drip from the consecrated Host onto his hands and the altar [1]. This event is now known as the Miracle of Bolsena and it was depicted by Raphael on the walls of the Vatican [19]. The red pigmented Serratia marcescens is not present in all strains but in those that it is present, it can resemble blood. Some scientists have tried to explain the miracle of “blood” in the consecrated Host by referring to the prodigiosin in Serratia marcescens [10].

Cell Structure and Metabolism

Serratia marcescens is short and rod shaped. It is a facultative anaerobe, meaning that it can grow in either the presence of oxygen (aerobic) or in the absence of oxygen (anaerobic). Primarily they use fermentation as their means of gathering energy and have enzymes (superoxide dismutase, catalase or peroxides) that protect them from reactive oxygen species, allowing them to live in oxygen environments. Serratia marcescens is a gram negative bacterium. Gram negative bacteria have a thin cell wall made of a single layer of peptidoglycan that is enclosed by an outer membrane. The outer membrane has lipopolysaccharides (LPS), which are special kind of phospholipids composed of fatty acids are attached to a glucosamine phosphate dimer. One glucosamine is then attached to a core polysaccharide and extends to O polysaccharides[18]. The outer membrane also serves as a means to regulate the uptake of nutrients and the exclusion of toxins. The protein pores and transporters found in the envelope layers vary in selectivity.

Serratia marcescens is mobile and travels by several different means. A single Serratia marcescens bacterium can swim with the use of its flagellum [17]. A flagellum is attached to both the inner and outer membrane of the cell. They are helical propellers that drive the cell forward, similar to that of a motor boat [18]. As a group, Serratia marcescens bacteria can swarm together on agar of lower concentrations (0 .5-0.8%) [8]. The swarmer cell length can range from 5-30 µm, are high flagellated and nonseptate. Per swarmer cell Serratia marcescens have about 100 – 1000 flagella [16]. Serratia marcescens can also come together to form a biofilm (complex structure made of secreted mucilaginous to form a protective coating in which they are encased [2]).

Hydrolysis of casein is not a common trait and is, therefore, useful in the differentiation of Serratia marcescens from the 438 strains of Enterobacteriaceae and Pseudomonadaceae families [12]. Serratia marcescens has the reproducible capability to break down casein in milk agar. Casein is a protein precipitated from milk that forms the basis of cheese and certain plastics [5]. Serratia marcescens uses extracellular enzymes called proteases to break down the peptide bond (CO-NH) that links together casein [4]. The hydrolysis of casein produces clearing on milk agar plates. Similarly, an extracellular enzyme called gelatinase breaks down gelatin. Gelatin is an incomplete protein that lacks tryptophan. Gelatin hydrolysis transforms the protein to amino acids and causes it to liquefy in cold conditions (under 25 °C) when it would otherwise be solid [4].

There are other biochemical tests that help to identify Serratia marcescens in the lab. Serratia marcescens will give a negative methyl red test due to their production of 2, 3 - butanediol and ethanol. Confirmation of a methyl red test can be done so by testing for acetoin (a precursor to 2, 3 - butanediol production) using the Voges-Proskauer test [18]. The Voges-Proskauer test, which shows an organism’s ability to convert pyruvate to acetoin, will be positive [4]. Serratia marcescens is negative for acid production on lactose, but glucose and sucrose (with gas production) to produce pyruvate. Nitrate tests are positive since nitrate is used as the final electron acceptors rather than oxygen [4]. Citrate (positive test) is used by Serratia marcescens to produce pyruvic acid. It is positive for decarboxylase, which is the removal of a carboxyl group from an amino acid, producing an amine and carbon dioxide. The red pigmentation (prodigiosin) that Serratia marcescens is known for is only present in some of the strains. It is known exactly why this is but it is hypothesized that the bacteria has a mechanism that control the genes synthesizing the proteins needed to make prodigiosin [3]. The prodigiosin can trigger a body’s immune system (antibodies and T cells). It is thought that as a result, Serratia marcescens bacteria living in a human body will limit the production of prodigiosin synthesis. Many strains appear to have lost the ability to produce it at all. [3]

References

1. amh10. “Serratia Marcescens.” MicrobLog.com. 4 August 2006. 7 Nov. 2008. © 2008 <http://microblog.me.uk/89>

2. "biofilm." The American Heritage® Science Dictionary. Houghton Mifflin Company. 02 Dec. 2008. <Dictionary.com http://dictionary.reference.com/browse/biofilm>.

3. Bry, Lynn. “Re: How and why did Serratia marcescens produce prodigiosin?” 5 June 2005. 2 Dec. 2008. <http://www.madsci.org/posts/archives/2005-06/1117999360.Mi.r.html>

4. “Excerpt from Serratia.” Emedicine.com. 19 Nov. 2008. © 1996-2006 by WebMD <http://www.emedicine.com/med/byname/serratia.htm>

5. Harshey Rasika M. “Bees aren't the only ones : swarming in Gram-negative bacteria.” Molecular Microbiology (1994) 13(3), 389-394

6. “Serratia Marcescens Bacteria.” serratia-marcescens.org. 9 Nov. 2008. <http://www.serratia-marcescens.org/>

7. "r factor." Dictionary.com Unabridged (v 1.1). Random House, Inc. 07 Dec. 2008. <Dictionary.com http://dictionary.reference.com/browse/r factor>.

8. "Serratia marcescens." WordNet® 3.0. Princeton University. 6 Nov. 2008. <Dictionary.com http://dictionary.reference.com/browse/serratia marcescens>.

9. “Serratia marcescens.” Wikipedia.org. 24 Oct. 2008. 19 Nov. 2008. <http://en.wikipedia.org/wiki/Serratia_marcescens>

10. Schlegel, Hans. General Microbiology. © Georg Thieme Verlag, Stuttgart 1992. pp. 88.

11. Simurda, Maryanne. “Department of Biology Faculty and Research.” 9 Nov. 2008. <http://biology.wlu.edu/simurda.htm>

12. Slonczewski, Joan and John Foster. Microbiology: An Evolving Science. © 2009 W.W. Norton & Company, Inc. pp. 91,

13. “The Miracle Microbe: Serratia marcescens.” 18 Nov. 2008. © 1999 Comm Tech Lab, Michigan State Univeristy. < http://commtechlab.msu.edu/sites/dlc-me/zoo/microbes/serratia.html>

14. Yuko Tanaka, Junko Yuasa, Masahiro Baba, Taichiro Tanikawa, Yoji Nakagawa and Tohey Matsuyama. “Temperature-Dependent Bacteriostatic Activity of Serratia marcescens”. Microb. Environ.. Vol. 19: 236-240 (2004) .