Enterobacteriaceae
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 Pseudomonadota. In 2016, the description and members of this family were emended based on comparative genomic analyses by Adeolu et al. 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". Morphology. Members of the Enterobacteriaceae are bacilli (rod-shaped), and are typically 1–5 μm 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. Metabolism. Like other Pseudomonadota, 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, there are exceptions. Catalase reactions vary among Enterobacteriaceae. Ecology. 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. Model organisms and medical relevance. "Escherichia coli" is one of the most important model organisms, and its genetics and biochemistry have been closely studied. 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. Historical systematics and taxonomy. "Enterobacteriaceae" was originally the sole family under the order 'Enterobacteriales'. The family contained a large array of biochemically distinct species with different ecological niches, which made biochemical descriptions difficult. The original classification of species to this family and order was largely based on 16S rRNA genome sequence analyses, which is known to have low discriminatory power and the results of which changes depends on the algorithm and organism information used. Despite this, the analyses still exhibited  polyphyletic branching, indicating the presence of distinct subgroups within the family. In 2016, the order 'Enterobacteriales' was renamed to Enterobacterales, and divided into 7 new families, including the emended "Enterobacteriaceae" family. This emendation restricted the family to include only those genera directly related to the type genus, which included most of the enteric species under the order. This classification was proposed based on the construction of several robust phylogenetic trees using conserved genome sequences, 16S rRNA sequences and multilocus sequence analyses. Molecular markers, specifically conserved signature indels, specific to this family were identified as evidence supporting the division independent of  phylogenetic trees. In 2017, a subsequent study using comparative phylogenomic analyses identified the presence of 6 subfamily level clades within the family "Enterobacteriaceae", namely the “Escherichia clade”, “Klebsiella clade”, “Enterobacter clade”, “Kosakonia clade”, “Cronobacter clade”, “Cedecea clade” and a “Enterobacteriaceae incertae sedis clade” containing species whose taxonomic placement within the family is unclear. However, this division was not officially proposed as the subfamily rank is generally not used. Molecular signatures. Analyses of genome sequences from "Enterobacteriaceae" species identified 21 conserved signature indels (CSIs) that are uniquely present in this family in the proteins (subunit M), twitching motility protein PilT, 2,3 -dihydroxybenzoate-AMP ligase, ATP/GTP-binding protein, multifunctional fatty acid oxidation complex (subunit alpha), S-formylglutathione hydrolase, aspartate-semialdehyde dehydrogenase, epimerase, membrane protein, formate dehydrogenylase (subunit 7), glutathione S-transferase, major facilitator superfamily transporter, phosphoglucosamine mutase, glycosyl hydrolase 1 family protein, 23S rrna [uracil(1939)-C(5)]-methyltransferase, co-chaperone HscB, N-acetylmuramoyl-L-alanine amidase, sulfate ABC transporter ATP-binding protein CysA, and LPS assembly protein LptD. These CSIs provide a molecular means of distinguishing "Enterobacteriaceae" from other families within the order "Enterobacterales" and other bacteria. Genera. Validly published genera. 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. Proposed genera. 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. Identification. To identify different genera of Enterobacteriaceae, a microbiologist may run a series of tests in the lab. These include: 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. Antibiotic resistance. 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. Various carbapenemases genes (blaOXA-48, blaKPC and blaNDM-1, blaVIM and blaIMP) have been identified in carbapenem resistant Enterobacteriaceae including "Escherichia coli" and "Klebsiella pneumoniae".