largest, most heterogeneous collection of medically important gram negative rod
More than 50 genera and hundreds of species
and subspecies have been described and these classifed based on biochemical properties,
antigenic structure, and molecular analysis of their genomes by gene sequencing and protein composition by mass spectrometry.
are ubiquitous organisms found
worldwide in soil, water, and vegetation and are part of the normal intestinal flora of most animals including humans
these bacteria cause a variety of human diseases, including one quarter to one third of all bacteremias, more than 70% of urinary tract infections (UTIs), and many intestinal infections
Some organisms (Salmonella serotype Typhi, Shigella species, Yersinia pestis) are always associated with human disease when present in clinical specimens
( Escherichia coli, Klebsiella pneumoniae, Proteus
mirabilis) are members of the normal commensal fora that can cause opportunistic infections.
those normally commensal organisms that become pathogenic when they acquire virulence genes present on plasmids, bacteriophages, or pathogenicity islands ( E. coli).
All members can grow
rapidly, aerobically and anaerobically (facultative anaerobes), on a variety of nonselective ( blood agar) and selective ( MacConkey agar) media
ferment glucose reduce nitrate, and are catalase positive and oxidase negative
appearance of the bacteria on culture media has been used to differentiate common members of the Enterobacteriaceae. For example, fermentation of lactose detected by color changes in lactose-containing media such as the commonly used MacConkey agar
Salmonella, Shigella, and Yersinia spp.—colorless
Escherichia, Klebsiella, Enterobacter, Citrobacter,
and Serratia—pink-purple colonies
Most Enterobacteriaceae are motile, with the exception of some common genera
Klebsiella,Shigella, Yersinia