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[Microbiology](https://durgical.com/en/microbiology/) · [Bacteria](https://durgical.com/en/microbiology/#d-bacteria)

# 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)

## Key points

- Lactose fermenters: E. coli, Klebsiella, Enterobacter. Non-fermenters: Salmonella, Shigella, Proteus, Yersinia.
- H2S producers (black colonies on XLD or TSI): Salmonella and Proteus; Shigella is not.
- Extended-spectrum beta-lactamases (ESBL) and carbapenemases (KPC) move between species on plasmids.
- LPS triggers Toll-like receptor 4 (TLR4) and can lead to septic shock.

## Microorganisms in this group

- [**Escherichia coli** A normal gut bacterium. Some strains cause most urinary infections, plus diarrhea, newborn meningitis and sepsis. Exam priority: high](https://durgical.com/en/microbiology/escherichia-coli/)
- [**Salmonella Typhi** A Salmonella that infects only humans and causes typhoid fever: a long high fever from contaminated water or food. Exam priority: high](https://durgical.com/en/microbiology/salmonella-typhi/)
- [**Non-typhoidal Salmonella** Salmonella 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](https://durgical.com/en/microbiology/non-typhoidal-salmonella/)
- [**Shigella** A bacterium that invades the colon and causes dysentery: bloody diarrhea, fever and straining. A tiny dose is enough to get sick. Exam priority: medium](https://durgical.com/en/microbiology/shigella/)
- [**Klebsiella pneumoniae** A gut bacterium with a thick capsule. It causes pneumonia, urinary infection and sepsis in hospitals; many strains resist key antibiotics. Exam priority: medium](https://durgical.com/en/microbiology/klebsiella-pneumoniae/)
- [**Proteus mirabilis** A bacterium that causes urinary infections, especially with catheters, and builds large stones by making urine alkaline. Exam priority: medium](https://durgical.com/en/microbiology/proteus-mirabilis/)
- [**Yersinia** A genus with Y. pestis, which causes plague through rodent flea bites, and Y. enterocolitica, which causes diarrhea from pork. For completeness](https://durgical.com/en/microbiology/yersinia/)

## 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.

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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](https://www.msdmanuals.com/professional/infectious-diseases/gram-negative-bacilli/escherichia-coli-infections) (MSD Manual Professional Edition, 2026); [About Klebsiella](https://www.cdc.gov/klebsiella/about/index.html) (CDC, 2025); [Typhoid](https://www.who.int/news-room/fact-sheets/detail/typhoid) (WHO, 2023).

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Representative model, for educational purposes only.

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