Classification

Higher order Taxa

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

Species

Serratia marcescens

Serratia marcescens on an XLD agar plate. From the Wikipedia

Description and Significance

Serratia marcescens bacteria are motile,short rod-shaped, Gram-negative, facultative anaerobes classified as opportunistic pathogens. It was discovered in 1819 by Bartolomeo Bizio in Padua, Italy. Bizio chose the name Serratia to honor his friend named Serratia and marcescens comes from the Latin word 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 that the US government experimented with the Serratia marcescens and the harmful affects that the bacteria causes were revealed. A study using Serratia marcescens was carried out to determine the possibility of biological weapons being transmitted by wind current. It was soon found 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].

Optimally, Serratia marcescens grows at 37°C, 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 lower temperatures of 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].

Genome Structure

The genome of the Serratia marcescens strain Db11 was sequenced by the Sanger Institute with the collaboaration of Dr.Jonathan Ewbank of the Centre d'Immunologie de Marseille Luminy. The completed genome consists of a single circular chromosome of 5,113,802 bp containing a G+C content of 59.51% (reference #1)

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 and 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 positive 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 not exactly knownwhy 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]

Ecology

Serratia marcescens is ubiquitous. It is commonly found in soil, water, plants and animals. It is widely present in non potable water in underdeveloped countries due to poor chlorination. This microorganism is responsible for contamination to Petri plates in laboratories, and is also found to grow on bread and communion wafers that have been stored in damp regions. Although S. marcescens is a pathogenic microorganism, it is only so with immunocompromised individuals such as those found in hospitals where many of the infections take place. The mode of transmission of this microorganism is by either direct contact, catheters, droplets, saline irrigation solutions, and other believed to be sterile solutions (reference 9).

Pathology

Serratia marcescens is an opportunistic pathogen that cause nosocomial infections. It is resistant to many antibiotics traditionally used to treat bacterial infections, such as penicillin and ampicillin [8]. This is due to all of Serratia marcescens’ characteristics; unique membrane (LPS) as a Gram-negative bacteria, the ability to survive in aerobic and anaerobic conditions, and its motility. Most strains are resistant to several antibiotics because of the presence of R-factors on plasmids [1] There are many diseases that are associated with Serratia marcescens: sepsis, bacteremia, meningitis and cerebral abscesses, urinary tract infections, osteomyelitis, ocular infections, and endocarditis [8]. Due to the wide range of diseases Serratia marcescens causes, there is not one determining symptom or source of origin. The biofilms produced are generally pathogenic in the body [2].

Also, as mentioned in the cell structure, LPS is attached to the outer membrane of the Gram negative bateria. LPS acts as an endotoxin (a cell component that is harmless as long as the pathogen remains intact). The release of LPS would over-stimulate the host defenses and cause them to undergo lethal endotoxic shock [16]. The presence of LPS therefore makes it difficult to kill the Serratia marcescens bacteria without causing the death of the host’s cells.

Some of the antibiotics been have proven effective against Serratia marcescens are the antipseudomonal beta-lactam antibiotics. They kill bacteria by inhibiting bacteria cellular wall synthesis. Though they have been developed and used to kill pseudomonas, they have also proven effective against Serratia marcescens [10]. In an article titled “Organisms Found On Contact Lenses Can Provide Clues To Cause Of Corneal Eye Infection,” it stated that Serratia marcescens was the most common organism that was found in both corneal scrapings and contact lenses [9]. It has been found, however, that polyquaternium-1 (a biocide used commercially in a contact lens disinfecting solution) is active against the cytoplasmic membrane of Serratia marcescens [6].

R-factors

S.marcescens contain these R-factors which are a specific types of plasmids that may carry one or more genes that ultimatley results in the resistance to different types of antimicrobial agents.The contribution of R-factors to the resistance of Serratia to various drugs was studied by Medeiros & O'Brien in 1969. Here they found that out of 22 multiple resistant strians, 21 were able to transfer some form of their resistance to others. Therefore, it was concluded that drug resistance was far more prevalent in Serratia than in any other commonly isolated member of the Enterobacteriaceae. It was also found that not only did the R-factors mediate drug resistance to the strains that were once susceptible to certain drugs, but it further conferred additional resistance to drugs which the strains were already previously resistant to. Since then, other experiments have concluded that the transfer system of R-factors in Serratia marcescens may be temperature sensitive and more likely to occur between those organisms that are found to be more closely related phylogenetically[reference 6].

Efflux Pumps

Not only does S. marcescens have R-factors which encode genes for particular drug resistance, but it also contains sophisticated efflux pumps which further remove toxins that may be fatal to the microorganism. Specifically, SdeXY was the first multidrug efflux pump belonging to the RND (Resistance-Nodulation-Cell Division)family found in S.marcescens. The SdeY gene is found to be a member of the RND family, while the SdeX is a member of the membrane fusion proteins. When found working properly (unmutated), they reduce the susceptibility to erythromycin, tetracycline, norfloxacin, benzalkonium chloride, ethidium bromide, acriflavine, and rhodamine 6G (reference 4). Other efflux pumps have also been categorized such as the SdeAB RND pump and the SdeCDE RND pump. The former functions with a broad substrate specificity and the latter consists of a membrane fusion proteins (MFP)and two different RND transporters (SdeD and SdeE)[reference 3].

Another type of multidrug efflux pump found in this microorganism was an ABC-type efflux pump called SmdAB. Both SmdA and SmdB genes must be present and are necessary for some type of resistance to be revealed by the organism [reference 5].

Current Research

One recent study suggests that Serratia marcescensost3 produces a novel prodigiosin called MAMPDM ((2,2'-[3-methoxy-1'amyl-5'-methyl-4-(1"-pyrryl)]dipyrrylmethene))that may have an important impact on cancer treatment. This red pigment demonstrated a selective cytotoxic activity in cancer cell lines, but in contrast it revealed a reduced toxicity to non malignant cells. They came to the conclusion that Serratia marcescensost3 may in one time be used as a source to develop an anti-cancer compound [reference 10].

Another study suggests that Serratia marcescens strain NCTC 10211 may serve as a probiotic in inhibiting the growth of H.pylori. Upon examining several different species and bacteria strains, Serratia marcescens strain NCTC 10211 revealed striking inhibition zones when inoculated with different Hp strains. It is still not well understood what are the active components and the mechansims by which various strains such as NCTC 10211 inhibit the Hp proliferation. Further research is needed to isolate and characterize the inhibitory substances so that they may be used for Hp therapy. It is largely unclear whether some organisms have bactericidial activity or that they influence the physiology and morphology of Hp [reference 11].

References

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>

5. "casein." Dictionary.com Unabridged (v 1.1). Random House, Inc. 03 Dec. 2008. <Dictionary.com http://dictionary.reference.com/browse/casein>.

6. Codling Caroline, Brian V. Jones, Eshwar Mahenthiralingam, A. Denver Russell and Jean-Yves Maillard. “Identification of genes involved in the susceptibility of Serratia marcescens to polyquaternium-1.” Journal of Antimicrobial Chemotherapy (2004) 54, 370–375

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

9. JAMA and Archives Journals. "Organisms Found On Contact Lenses Can Provide Clues To Cause Of Corneal Eye Infection." ScienceDaily 13 September 2007. 10 December 2008 <http://www.sciencedaily.com¬ /releases/2007/09/070910160823.htm>.

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

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

12. Salisbury, Willaiam and Joseph Likos. Hydrolysis of casein: a differential aid for the identification of Serratia marcescens. J. clin. Path., 1972, 25, 1083-1085

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

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

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

20. 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) .

21.

Serratia marcescens

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