Enterobacteriaceae

The family Enterobacteriaceae is a very large family of Gram-negative bacteria that has been studied and characterized for almost a hundred years. It was first proposed by Rahn in 1936 and now included over 50 genera and over 200 species. Its classification above the level of family is a subject of debate, but one classification has it in the order Enterobacteriales 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, Yersinia pestis, Klebsiella, and Shigella. Other disease-causing bacteria in this family include Proteus, Enterobacter, Serratia, and Citrobacter. 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 members of Rickettsia. 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 μm in length. They typically appear as medium to large-sized grey colonies on blood agar, although some can express pigments (such as Serratia marcescens). 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 (e.g. Photorhabdus). Unlike most similar bacteria, enterobacteriaceae generally lack cytochrome C oxidase, although there are exceptions (e.g. Plesiomonas shigelloides).