Enterobacterales

Enterobacterales is an order of Gram-negative, non-spore-forming bacteria within the class Gammaproteobacteria of the phylum Proteobacteria (also called Pseudomonadota in some classifications). The order comprises several families, the largest being Enterobacteriaceae, and includes others such as Erwiniaceae, Hafniaceae, Morganellaceae, Plesiomonadaceae, and Yersiniaceae. Members inhabit a wide range of environments, from the intestinal tracts of humans and animals to soil and water, and many are important in clinical microbiology.

Most Enterobacterales are facultatively anaerobic, rod-shaped bacteria that are typically oxidase-negative and catalase-positive. They commonly ferment glucose; lactose fermentation varies among groups, with lactose-fermenting genera (for example, Escherichia, Klebsiella, Enterobacter) forming lactose-positive colonies on MacConkey agar, while non-lactose fermenters (such as Salmonella and Shigella) do not. The organisms are often motile, though some are nonmotile. Ecologically, many are part of the normal gut microbiota, but numerous species are opportunistic or primary pathogens.

Clinically important members include Escherichia coli, Salmonella, Shigella, Klebsiella, Enterobacter, Serratia, Proteus, and Yersinia. Collectively, Enterobacterales cause a broad spectrum of diseases, including gastroenteritis, urinary tract infections, pneumonia, biliary tract infections, and septicemia. Antibiotic resistance within this order is a major concern, with notable mechanisms such as extended-spectrum beta-lactamases (ESBLs) and carbapenemases limiting treatment options in healthcare settings. Taxonomy within Enterobacterales has undergone revisions, with multiple families recognized beyond Enterobacteriaceae, reflecting evolving understanding of evolutionary relationships.