Shigella medical comic

This comic follows Shigella, the cause of bacillary dysentery, from its place in the Enterobacteriaceae family to the bowel wall it destroys. It covers the four species, why so few organisms cause disease, how the bacterium invades and spreads from cell to cell, the toxins behind bloody diarrhoea, the lab work-up and treatment.

For MBBS, NEET-PG, FMGE, NEXT and USMLE revision.

Shigella: what the comic covers

High-yield points

Enterobacteriaceae are Gram-negative bacilli that ferment glucose, reduce nitrate and are oxidase-negative; Shigella is a non-lactose fermenter (S. sonnei ferments late).
There are four species by O antigen; S. dysenteriae type 1 is catalase-negative and produces Shiga toxin.
Shigella resists stomach acid, so as few as 10–100 organisms can cause infection — which makes it highly communicable, especially in children under five.
Invasion starts at intestinal M cells using a plasmid-coded type III secretion system (ipa genes).
Inside cells it moves by polymerising host actin (IcsA) and spreads directly from cell to cell.
Illness often starts with watery diarrhoea and progresses to dysentery with blood and mucus.
Send fresh stool, not rectal swabs; transport in Sachs' buffered glycerol saline. Wet mounts show many pus cells and red cells.
Colonies are pale on MacConkey agar; Shigella is non-motile and produces no H2S, which separates it from Salmonella. Treatment is rehydration plus antibiotics such as ciprofloxacin, guided by local resistance.

Mnemonics from the comic

Infectious dose: It only takes about 100 organisms to start an infection.
Lab report: Pale and stale: pale colonies, non-motile, no H2S.
Prevention: Lockdown: wash your hands, bleach the rest.

Quick questions

Why is Shigella so contagious?

It survives stomach acid, so only 10–100 organisms are needed to cause disease. That is why it spreads easily from person to person, especially among young children.

How do you tell Shigella from Salmonella in the lab?

Both are non-lactose fermenters giving pale colonies on MacConkey agar, but Shigella is non-motile and produces no H2S, whereas most Salmonella are motile and produce H2S.

What is the best specimen for diagnosing shigellosis?

Fresh faeces, ideally the blood- and mucus-stained part, carried in Sachs' buffered glycerol saline if there will be a delay. Rectal swabs are less satisfactory.

Is this comic free to read?

The first pages are free to preview. Members read the full comic, and you can sign up free to read one comic in full.

From the blog

The DCA duel: Salmonella vs Shigella

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