Serratia

Serratia
Scientific classification
Kingdom:	Bacteria;
Phylum:	Proteobacteria;
Class:	Gamma Proteobacteria;
Order:	Enterobacteriales;
Family:	Enterobacteriaceae;
Genus:	Serratia; Bizio 1823
Species
	S. entomophila; S. ficaria; S. fonticola; S. grimesii; S. liquefaciens; S. marcescens; S. odoriferae; S. plymuthica; S. proteamaculans; S. quinivorans; S. rubidaea; S. ureilytica;

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This page is about microbiologic aspects of the organism(s). For clinical aspects of the disease, see Serratia infection.

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

Serratia is a short, facultatively anaerobic, Gram-negative, rod-shaped bacteria of the Enterobacteriaceae family. Serratia species are ubiquitous opportunistic pathogens that are frequently present under damp conditions in food, plants, animals, soil, and household items.

Microbiological Characteristics

Serratia is a ubiquitous opportunistic pathogen that is frequently present under damp conditions in food, plants, animals, soil, and household items (e.g. bathroom tiles).
Serratia is a short, facultatively anaerobic, Gram-negative, rod-shaped bacteria of the Enterobacteriaceae family.
It is often associated with hospital-acquired infections.
Serratia contains enzymes that facilitate its survival under oxygenated conditions against reactive oxygen species. Enzymes include superoxide dismutase, catalase, and peroxidase.
Serratia has a unique thin cell wall that contains lipopolysaccharides (LPS) that does not resemble the LPS-containing cell walls of other Gram-negative bacteria.
Out of a total of 12 Serratia species (see Scientific Classification Table), 8 have been reported to be infectious in humans. To view the list of infectiuous Serratia species, click here.
Serratia is notoriously known for its antimicrobial resistance due to the presence of R-factor and efflux pumps.

R-factors are genes on plasmids that code for antibiotic resistance and may be transferred from one strain to another.
Efflux pumps, such as the SdeXY, SdeY, and ABC-type efflux pumps, may eliminate toxins. These pumps may reduce susceptibility to antimicrobial agents, such as erythromycin, tetracycline, norfloxacin, benzalkonium chloride, ethidium bromide, acriflavine, and rhodamine.

Taxonomy

Serratia belongs to the following higher order taxa:

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

Species

The following is the list of Serratia species:

S. entomophila
S. ficaria
S. fonticola
S. grimesii
S. liquefaciens
S. marcescens
S. odoriferae
S. plymuthica
S. proteamaculans
S. quinivorans
S. rubidaea
S. ureilytica

Serratia marcescens

S. marcescens is a thin, motile (flagellated), non-septated, Gram-negative, facultatively anaerobic rod-shaped bacteria that can grow in temperatures ranging from 5–40 °C and in pH levels ranging from 5 to 9.

S. marcescens is able to perform casein hydrolysis, which facilitates the production of extracellular metalloproteases thought to function in cell-to-extracellular matrix interactions.
S. marcescens also exhibits tryptophan- and citrate-degradation. Pyruvic acid, an end-product of tryptophan degradation, and carbon, an end-product of citrate degradation, are then incorporated into metabolic processes.
S. marcescens produces a reddish-orange (bloody) pigment called prodigiosin. Not all strains, however, are able to produce prodigiosin. It is thought that prodigiosin is an antigen, and during an infection, the production of prodigiosin is limited to avoid the triggering of human immune responses.

Identification of the organism is commonly done via the following tests:

Methyl red test, which determines if a microorganism performs mixed-acid fermentation. Typically, S. marcescens results in a negative test due to the production of 2, 3-butanediol and ethanol.
Voges-Proskauer test, which determines the organism's ability to convert pyruvatet to acetonin. Typically, S. marcescens results in a positive test.
Nitrate test, which determines the organism's ability to produce nitrate products. Typically, S. marcescens results in a positive test.

Identification

The following table demonstrates the microbiological characteristics and a comprehensive list of techniques to identify S. marcescens:

Test	Result^[1]
Gram stain	-
Oxidase	-
Indole production	-
Methyl Red	>70% -
Voges-Proskaeur	+
Citrate (Simmons)	+
Hydrogen sulfide production	-
Urea hydrolysis	>70% -
Phenylalanine deaminase	-
Lysine decarboxylase	+
Motility	+
Gelatin hydrolysis, 22°C	+
Acid from lactose	-
Acid from glucose	+
Acid from maltose	+
Acid from mannitol	+
Acid from sucrose	+
Nitrate reduction	+ (to nitrite)
Deoxyribonuclease, 25°C	+
Lipase	+
Pigment	some biovars produce red
Catalase production (24h)	+

Clinical Association

Clinically, Serratia may infect multiple organ systems. It may be responsible for urinary tract infection, pneumonia , osteomyelitis, meningitis, cerebral abscess formation, endocarditis, intra-abdominal infections, eye and tear duct infections (conjunctivitis, keratitis, endophthalmitis), otitis media, and rarely parotitis. To learn more about the clinical infection of Serratia, click here.

Gallery

Blood agar base plate cultivated colonial growth of Gram-negative, rod-shaped and facultatively anaerobic Serratia marcescens bacteria. From Public Health Image Library (PHIL). ^[2]
Antibiogram of Serratia marcescens. From WikiMedia.org. ^[3]
Serratia marcescens on bread. From WikiMedia.org. ^[4]
Serratia marcescens appears as bloody red spots on culture medium. From WikiMedia.org. ^[5]
Serratia marcescens appears as bloody red spots on culture medium. From WikiMedia.org. ^[6]
Serratia marcescens on slates of bread shelves. From WikiMedia.org. ^[6]