Enterobacteriaceae

Enterobacteriaceae is a large family of Gram-negative, oxidase-negative, non-spore-forming bacilli that are predominantly facultative anaerobes. They ferment glucose and often produce gas; most are motile with peritrichous flagella, though some are non-motile. Members primarily inhabit the intestinal tracts of humans and animals, but many species also occur in soil, water, and foods.

The family includes genera such as Escherichia, Klebsiella, Enterobacter, Citrobacter, Proteus, Serratia, Salmonella, Shigella, Yersinia, Morganella, and Providencia. Lactose fermentation on MacConkey agar distinguishes groups: lactose-fermenters like E. coli, Klebsiella, and Enterobacter; non-lactose fermenters such as Salmonella and Shigella. They are generally oxidase negative and possess distinct O and H antigens; many also have K capsules.

Pathogenic potential ranges from opportunistic to enteric pathogens. Clinically important problems include urinary tract infections, pneumonia, bacteremia, intra-abdominal infections, and gastroenteritis. Notable pathotypes of E. coli include enterotoxigenic, enteropathogenic, enterohemorrhagic, enteroinvasive, and uropathogenic strains. Salmonella and Shigella commonly cause gastroenteritis; Yersinia includes enteropathogenic species and Y. pestis as a rare but historically significant pathogen.

Laboratory identification relies on Gram staining and biochemical testing: Gram-negative rods that are oxidase negative, catalase positive, with various lactose-fermentation and H2S patterns. Virulence factors include fimbriae, capsules, toxins, and virulence plasmids. Antimicrobial resistance, including beta-lactamases and carbapenemases, poses treatment challenges.

Ecology and transmission: these bacteria are part of the gut microbiota but can contaminate food and water, causing outbreaks. They are a central focus of clinical microbiology due to their diversity and public health importance.