Enterobacterales

Enterobacterales
Name	Enterobacterales
Taxon	Order
Authority	Adeolu et al., 2016
Subdivision ranks	Families
Subdivision	See text.

Enterobacterales. The Enterobacterales are a large and diverse order of Gram-negative bacteria within the class Gammaproteobacteria. This order encompasses many familiar genera of significant medical, environmental, and industrial importance, including notorious pathogens, common commensal organisms, and free-living species. The reclassification and expansion of this group, formerly known as "enteric bacteria," was formally proposed following extensive phylogenetic analysis of 16S ribosomal RNA and multilocus sequence analysis.

Taxonomy and classification

The order Enterobacterales was circumscribed in 2016 by a team led by Mobolaji Adeolu based on comprehensive genomic studies, consolidating many families previously placed in the now-obsolete order "Enterobacteriales." Key families within the order include Enterobacteriaceae, which houses many clinically relevant genera like Escherichia, Klebsiella, Salmonella, and Shigella. Other significant families are Yersiniaceae, containing the plague agent Yersinia pestis, Hafniaceae, Morganellaceae, and Pectobacteriaceae. This reorganization, supported by institutions like the List of Prokaryotic names with Standing in Nomenclature, reflects evolutionary relationships more accurately than previous phenotype-based systems.

Characteristics and morphology

Members of the Enterobacterales are typically rod-shaped (bacilli), non-spore-forming, and facultatively anaerobic. They are chemoorganotrophs with both respiratory and fermentative metabolisms. A nearly universal characteristic is the ability to ferment glucose, often with acid and gas production. Most are motile via peritrichous flagella, though notable non-motile exceptions exist, such as Klebsiella pneumoniae. They possess a characteristic cell envelope structure common to Gram-negative bacteria, including an outer membrane containing lipopolysaccharide, which acts as an endotoxin.

Pathogenesis and clinical significance

Many Enterobacterales are major human pathogens. Escherichia coli includes strains causing urinary tract infections, gastroenteritis, and hemolytic-uremic syndrome. Salmonella enterica serovars are agents of typhoid fever and salmonellosis, while Shigella species cause bacillary dysentery. Yersinia pestis is the causative agent of bubonic plague, a historically devastating disease. Members of the Klebsiella-Enterobacter-Serratia group are frequent causes of nosocomial infections, including pneumonia, bacteremia, and infections in intensive care unit patients.

Epidemiology and transmission

Transmission routes vary by pathogen but often involve the fecal-oral route for enteric diseases. Contaminated food and water are common vehicles, as seen in outbreaks linked to Salmonella in poultry or E. coli O157:H7 in produce. Yersinia pestis is primarily a zoonosis, transmitted by fleas from rodents to humans. In healthcare settings, organisms like Klebsiella and Enterobacter spread via the hands of personnel, contaminated equipment, or environmental surfaces, leading to outbreaks in vulnerable populations, as documented by the Centers for Disease Control and Prevention.

Treatment and antimicrobial resistance

Treatment historically relied on beta-lactam antibiotics like ampicillin and cephalosporins. However, antimicrobial resistance is a critical global threat. Many Enterobacterales now produce extended-spectrum beta-lactamase enzymes, rendering penicillins and cephalosporins ineffective. Of grave concern is the rapid spread of carbapenem-resistant Enterobacterales, which harbor enzymes like Klebsiella pneumoniae carbapenemase and New Delhi metallo-beta-lactamase, leaving few therapeutic options such as polymyxins or novel agents like ceftazidime/avibactam. This crisis is monitored by the World Health Organization and the European Centre for Disease Prevention and Control.

Laboratory identification

Initial isolation often uses selective and differential media like MacConkey agar, which exploits lactose fermentation. Biochemical test panels, such as the API 20E system, are standard for genus and species identification. Automated systems like VITEK or BD Phoenix are widely used in clinical laboratories. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry has revolutionized rapid identification. For epidemiological tracking, methods like pulsed-field gel electrophoresis and whole-genome sequencing are employed by reference laboratories, including those at the National Institutes of Health.

Category:Enterobacterales Category:Bacterial orders