Enterobacterales !
- Bacteria
Gut rods such as E. coli, Salmonella, Shigella, Klebsiella, Proteus and Yersinia: diarrhea, urinary infection and sepsis.
Updated 2026-11-01
What they are
An order of Gram-negative rods that live in the human and animal gut or in the environment. All ferment glucose, are oxidase-negative and reduce nitrate to nitrite, which is what the urine dipstick reads. They are the main cause of urinary tract infection and a leading cause of sepsis and of carbapenem resistance in hospitals.
What they share
- Gram-negative rods, facultative anaerobes
- Oxidase-negative, glucose fermenters, nitrate to nitrite
- LPS (endotoxin) in the outer membrane
- MacConkey agar: lactose fermenters pink (E. coli, Klebsiella), non-fermenters colorless (Salmonella, Shigella, Proteus)
Microorganisms in this group
- Escherichia coliA normal gut bacterium. Some strains cause most urinary infections, plus diarrhea, newborn meningitis and sepsis.Exam priority: high
- Salmonella TyphiA Salmonella that infects only humans and causes typhoid fever: a long high fever from contaminated water or food.Exam priority: high
- Non-typhoidal SalmonellaSalmonella from eggs, chicken and reptiles that causes diarrhea with fever; in infants, the elderly and the immunosuppressed it can enter the blood.Exam priority: medium
- ShigellaA bacterium that invades the colon and causes dysentery: bloody diarrhea, fever and straining. A tiny dose is enough to get sick.Exam priority: medium
- Klebsiella pneumoniaeA gut bacterium with a thick capsule. It causes pneumonia, urinary infection and sepsis in hospitals; many strains resist key antibiotics.Exam priority: medium
- Proteus mirabilisA bacterium that causes urinary infections, especially with catheters, and builds large stones by making urine alkaline.Exam priority: medium
- YersiniaA genus with Y. pestis, which causes plague through rodent flea bites, and Y. enterocolitica, which causes diarrhea from pork.For completeness
Test yourself
Think of the answer, then tap the question to see it.
What color do lactose-fermenting Enterobacterales turn on MacConkey agar?
Pink or red; non-fermenters such as Salmonella and Shigella give colorless colonies.
Which two Enterobacterales make H2S and give black colonies?
Salmonella and Proteus.
Which test separates Enterobacterales from Pseudomonas?
Oxidase: Enterobacterales are oxidase-negative, Pseudomonas is oxidase-positive.
Which Gram-negative cell wall component causes fever and septic shock?
Lipopolysaccharide (LPS), whose lipid A is the endotoxin.
Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Escherichia coli Infections (MSD Manual Professional Edition, 2026); About Klebsiella (CDC, 2025); Typhoid (WHO, 2023).
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Representative model, for educational purposes only.