Enterobacteriaceae: E. coli, Salmonella, Shigella & Vibrio

Enterobacteriaceae are the large family of gram-negative rods that live in the human gut: Escherichia, Klebsiella, Salmonella, Shigella, Proteus and Yersinia. Most are harmless residents, but the pathogenic members cause the commonest community infections in India: urinary tract infections, typhoid fever, bacillary dysentery and cholera (from the closely related Vibrio). They are defined in the laboratory by two traits: they ferment glucose and they are oxidase-negative.

This matters because a doctor in India meets these organisms daily: E. coli in nearly every uncomplicated UTI, Salmonella typhi in prolonged fever, Shigella in bloody diarrhoea, and Vibrio cholerae wherever clean water fails.

Key Concepts

• Family traits: gram-negative rods, glucose fermenters, oxidase-negative, most reduce nitrates; grown on MacConkey agar where lactose fermenters turn pink.
• E. coli: the commonest cause of community urinary infection; most strains harmless, but O157:H7 produces Shiga toxin causing bloody diarrhoea and haemolytic uraemic syndrome.
• Klebsiella pneumoniae: encapsulated, causes UTI and a severe currant-jelly pneumonia; Klebsiella ozaenae and rhinoscleromatis cause chronic nasal disease.
• Salmonella typhi: causes typhoid: stepwise rising fever, rose spots, relative bradycardia; blood culture is positive in the first week; the Widal test is supportive, not definitive.
• Shigella: causes bacillary dysentery with blood and mucus; its infective dose is tiny (10 to 100 organisms) because it survives stomach acid.
• Vibrio cholerae: comma-shaped, oxidase-positive (the family exception), with a potent enterotoxin; causes litres of rice-water diarrhoea.

How It Works

1. Vibrio cholerae is swallowed in contaminated water and colonises the small intestine.
2. Its B subunit binds the GM1 receptor on gut cells and delivers the A subunit inside.
3. The A subunit ADP-ribosylates the Gs protein, locking adenylate cyclase permanently on.
4. Cyclic AMP floods the cell, forcing chloride channels open; water follows by osmosis.
5. Litres of isotonic fluid pour into the gut lumen as painless rice-water stools, and the patient dies of dehydration and shock, not of tissue damage, which is why rapid rehydration alone saves lives.

Worked Example

A 60 kg man with cholera is severely dehydrated (about 10 percent of body weight lost as fluid). Step 1: fluid deficit = 10 percent of 60 kg = 6 litres. Step 2: WHO plan C prescribes 100 mL per kg of Ringer's lactate IV: 100 times 60 = 6,000 mL total, matching the deficit. Step 3: give the first 30 mL per kg rapidly: 30 times 60 = 1,800 mL in the first 30 minutes. Step 4: give the remaining 70 mL per kg: 70 times 60 = 4,200 mL over the next 2.5 hours. Step 5: 1,800 plus 4,200 = 6,000 mL; then switch to ORS, 200 to 400 mL after each loose stool, to match ongoing losses.

Common Misconceptions

• "A positive Widal test proves typhoid." Widal cross-reacts and stays positive after old infection or vaccination; blood culture in the first week is the gold standard, and a single Widal titre is never enough.
• "All E. coli are dangerous." E. coli is a normal gut resident; only specific strains (enterotoxigenic, enterohaemorrhagic O157:H7, uropathogenic) cause disease.
• "Antibiotics are the main treatment for cholera." Rehydration decides survival

Real-World Applications

• UTI management: empirical nitrofurantoin or fosfomycin guided by urine culture.
• Typhoid control: blood cultures, ceftriaxone or azithromycin for resistant strains, and typhoid conjugate vaccination.
• Dysentery outbreaks: ciprofloxacin or azithromycin for Shigella, with handwashing breaking transmission.
• Cholera response: ORS corners, IV Ringer's lactate, and oral cholera vaccine in endemic pockets.

Enterobacteriaceae are glucose-fermenting, oxidase-negative gut rods whose pathogenic members cause UTI, typhoid, dysentery and cholera. Lactose fermentation and simple biochemical tests name them, blood culture beats Widal for typhoid, and rehydration math beats antibiotics for cholera.