|Citrobacter freundii, one member of the family|
Enterobacteriaceae is a large family of Gram-negative bacteria. It was first proposed by Rahn in 1936, and now includes over 30 genera and more than 100 species. Its classification above the level of family is still a subject of debate, but one classification places it in the order Enterobacterales of the class Gammaproteobacteria in the phylum Proteobacteria.
Enterobacteriaceae includes, along with many harmless symbionts, many of the more familiar pathogens, such as Salmonella, Escherichia coli, Klebsiella, and Shigella. Other disease-causing bacteria in this family include Enterobacter and Citrobacter. 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.
Members of the Enterobacteriaceae are bacilli (rod-shaped), and are typically 1-5 ?m in length. They typically appear as medium to large-sized grey colonies on blood agar, although some can express pigments.
Most have many flagella used to move about, but a few genera are nonmotile. Most members of Enterobacteriaceae have peritrichous, type I fimbriae involved in the adhesion of the bacterial cells to their hosts.
They are not spore-forming.
Like other proteobacteria, enterobactericeae 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. Unlike most similar bacteria, enterobacteriaceae generally lack cytochrome c oxidase, although there are exceptions.
Catalase reactions vary among Enterobacteriaceae.
Many members of this family are normal members of the gut microbiota in humans and other animals, while others are found in water or soil, or are parasites on a variety of different animals and plants.
Some enterobacteria are important pathogens, e.g. Salmonella, or Shigella e.g. because they produce endotoxins. Endotoxins reside in the cell wall 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.
The following genera have been validly published, thus they have "Standing in Nomenclature". The year the genus was proposed is listed in parentheses after the genus name.
The following genera have been effectively, but not validly, published, thus they do not have "Standing in Nomenclature". The year the genus was proposed is listed in parentheses after the genus name.
In a clinical setting, three species make up 80 to 95% of all isolates identified. These are Escherichia coli, Klebsiella pneumoniae, and Proteus mirabilis. However, Proteus mirabilis is now considered a part of the Morganellaceae, a sister clade within the Enterobacterales.
Several Enterobacteriaceae strains have been isolated which are resistant to antibiotics including carbapenems, which are often claimed as "the last line of antibiotic defense" against resistant organisms. For instance, some Klebsiella pneumoniae strains are carbapenem resistant.