Enterobacteriaceae

Not to be confused with Enteric bacteria.

Enterobacteriaceae
Citrobacter freundii, one member of the family
Scientific classification
Domain:	Bacteria
Phylum:	Proteobacteria
Class:	Gammaproteobacteria
Order:	Enterobacteriales
Family:	Enterobacteriaceae Rahn, 1937

The Enterobacteriaceae are a large family of Gram-negative bacteria that includes, along with many harmless symbionts, many of the more familiar pathogens, such as Salmonella, Escherichia coli, Yersinia pestis, Klebsiella and Shigella. Other disease-causing bacteria in this family include Proteus, Enterobacter, Serratia, and Citrobacter. This family is the only representative in the order Enterobacteriales of the class Gammaproteobacteria in the phylum Proteobacteria. Phylogenetically, in the Enterobacteriales, several peptidoglycan-less insect endosymbionts form a sister clade to the Enterobacteriaceae, but as they are not validly described, this group is not officially a taxon; examples of these species are Sodalis, Buchnera, Wigglesworthia, Baumannia cicadellinicola and Blochmannia, but not former Rickettsias. Members of the Enterobacteriaceae can be trivially referred to as enterobacteria or "enteric bacteria", as several members live in the intestines of animals. In fact, the etymology of the family is enterobacterium with the suffix to designate a family (aceae) — not after the genus Enterobacter (which would be "Enterobacteraceae")— and the type genus is Escherichia.

Characteristics

Members of the Enterobacteriaceae are rod-shaped, and are typically 1-5 μm in length. They appear as small grey colonies on blood agar. Like other proteobacteria, enterobacteria have Gram-negative stains, and they are facultative anaerobes, fermenting sugars to produce lactic acid and various other end products. Most also reduce nitrate to nitrite, although exceptions exist (e.g. Photorhabdus). Unlike most similar bacteria, enterobacteria generally lack cytochrome C oxidase, although there are exceptions (e.g. Plesiomonas shigelloides). Most have many flagella used to move about, but a few genera are nonmotile. They are not spore-forming. Catalase reactions vary among Enterobacteriaceae.

Many members of this family are a normal part of the gut flora found in the intestines of humans and other animals, while others are found in water or soil, or are parasites on a variety of different animals and plants. Escherichia coli is one of the most important model organisms, and its genetics and biochemistry have been closely studied.

Most members of Enterobacteriaceae have peritrichous, type I fimbriae involved in the adhesion of the bacterial cells to their hosts. Some enterobacteria produce endotoxins. Endotoxins reside in the cell cytoplasm and are released when the cell dies and the cell wall disintegrates. Some members of the Enterobacteriaceae produce endotoxins that, when released into the bloodstream following cell lysis, cause a systemic inflammatory and vasodilatory response. The most severe form of this is known as endotoxic shock, which can be rapidly fatal.

Identification

To identify different genera of Enterobacteriaceae, a microbiologist may run a series of tests in the lab. These include:

• Phenol red
• Tryptone broth
• Phenylalanine agar for detection of production of deaminase, which converts phenylalanine to phenylpyruvic acid
• Methyl red or Voges-Proskauer tests depend on the digestion of glucose. The methyl red tests for acid endproducts. The Voges Proskauer tests for the production of acetylmethylcarbinol.
• Catalase test on nutrient agar tests for the production of catalase enzyme, which splits hydrogen peroxide and releases oxygen gas.
• Oxidase test on nutrient agar tests for the production of the enzyme oxidase, which reacts with an aromatic amine to produce a purple color.
• Nutrient gelatin tests to detect activity of the enzyme gelatinase.

In a clinical setting, three species make up 80 to 95% of all isolates identified. These are Escherichia coli, Klebsiella pneumoniae and Proteus mirabilis.

Antibiotic resistance

Main article: Carbapenem resistant enterobacteriaceae

Several Enterobacteriaceae strains have been isolated which are resistant to antibiotics including carbapenem, which are often claimed as "the last line of antibiotic defense" against resistant organisms. For instance, some Klebsiella pneumoniae strains are carbapenem resistant.

Examples/classification

The following inexhaustive list details bacterial genera classified as members of Enterobacteriaceae.

Genera

• Citrobacter
• Cronobacter
• Enterobacter
• Escherichia, e.g. Escherichia coli
• Hafnia (bacterium)
• Klebsiella, e.g. Klebsiella pneumoniae
• Pantoea
• Plesiomonas
• Proteus, e.g. Proteus vulgaris, proteus mirabilis
• Raoultella
• Salmonella
• Serratia, e.g. Serratia marcescens
• Shigella

External links

• Enterobacteriaceae genomes and related information at PATRIC, a Bioinformatics Resource Center funded by NIAID
• Evaluation of new computer-enhanced identification program for microorganisms: adaptation of BioBASE for identification of members of the family Enterobacteriaceae [1]
• Brown, A.E. (2009). Benson's microbiological applications: laboratory manual in general microbiology. New York: McGraw- Hill.