Enterobacteriaceae

Enterobacteriaceae
Name	Enterobacteriaceae
Taxon	Enterobacteriaceae
Authority	Rahn, 1937
Subdivision ranks	Genera
Subdivision	See text

Enterobacteriaceae. This is a large family of Gram-negative bacteria within the order Enterobacterales. Members are ubiquitous, found in environments ranging from soil and water to the gastrointestinal tracts of humans and animals. While many are harmless commensal organisms, several are significant pathogens responsible for a wide spectrum of diseases, from urinary tract infections to life-threatening sepsis.

Taxonomy and classification

The taxonomic classification has undergone significant revision with the advent of phylogenetic analysis using techniques like 16S ribosomal RNA sequencing. Historically defined by phenotypic traits, the family was recently redefined by the International Committee on Systematics of Prokaryotes. Many former members have been reassigned to new families within the order Enterobacterales, such as Yersiniaceae and Hafniaceae. Key defining criteria for remaining genera include the presence of peritrichous flagella for motility and the ability to reduce nitrate to nitrite. The type genus for the family is Escherichia.

Characteristics and morphology

These are non-spore-forming, facultative anaerobes, typically rod-shaped. They ferment glucose with the production of acid and gas, a key metabolic trait tested in the methyl red and Voges–Proskauer test. Most possess fimbriae for adhesion and produce catalase but not oxidase. A defining structural component is the presence of lipopolysaccharide in their outer membrane, which acts as a potent endotoxin. Many species are motile via flagella, and some, like Salmonella, produce characteristic H antigens.

Pathogenesis and clinical significance

Pathogenic species employ various virulence factors to cause disease. These include enterotoxins, as seen in enterotoxigenic Escherichia coli, and potent cytotoxins like the Shiga toxin produced by Shigella dysenteriae and Escherichia coli O157:H7. They are leading causes of nosocomial infections, particularly in settings like the intensive care unit. Major disease syndromes include gastroenteritis, pneumonia, meningitis in neonates, and hemolytic uremic syndrome. The endotoxin can trigger a severe systemic inflammatory response, leading to septic shock.

Common genera and species

The family includes numerous medically important genera. Escherichia, particularly Escherichia coli, is a common commensal but also a frequent pathogen. Salmonella species, such as Salmonella enterica, are causes of typhoid fever and salmonellosis. Klebsiella, including Klebsiella pneumoniae, is notorious for hospital-acquired infections. Other notable pathogens include Shigella, the agent of bacillary dysentery; Yersinia pestis, the cause of the Black Death; and Proteus mirabilis, often associated with complicated urinary tract infections.

Antibiotic resistance

Resistance is a critical global health threat, heavily driven by the acquisition of mobile genetic elements like plasmids and transposons. Of particular concern are strains producing extended-spectrum beta-lactamase enzymes, which hydrolyze antibiotics including penicillins and cephalosporins. The emergence of carbapenem-resistant Enterobacteriaceae, often mediated by enzymes like Klebsiella pneumoniae carbapenemase, leaves few therapeutic options. Resistance mechanisms also include efflux pumps and modifications of penicillin-binding proteins. Surveillance by agencies like the Centers for Disease Control and Prevention and the World Health Organization tracks the spread of multidrug-resistant clones.

Laboratory identification

Initial isolation typically uses selective and differential media such as MacConkey agar, which exploits the ability to ferment lactose. Key biochemical tests include the indole test, the citrate test, and the triple sugar iron agar test. Automated systems like the VITEK and BD Phoenix are widely used in clinical laboratories. Definitive identification often involves matrix-assisted laser desorption/ionization for rapid profiling. Serological typing, identifying specific O antigen and H antigen structures, remains important for epidemiological investigations of outbreaks.

Category:Enterobacteriaceae Category:Gram-negative bacteria