Shigella

Shigella is a Gram-negative bacterium that belongs to the Enterobacteriaceae family.

There are 4 species of Shigella:

S. dysenteriae, S. flexneri; S. boydii; and S. sonnei, and each has its own serotypes.

In humans these species can all cause Shigellosis, which is a contagious infection of the intestines, particularly the colon.

And shigellosis can progress to dysentery, which is when the infection causes inflammation of the colon, resulting in severe abdominal pain and diarrhea.

Now, Shigella is a gram-negative, rod shaped bacterium, meaning it looks like a little red or pink stick on a gram stain.

And it’s a facultative anaerobe, so it can survive with or without oxygen in the environment.

It has no flagellum, making it nonmotile; and it doesn’t form spores.

It’s also a non-lactose fermenter, so it doesn’t ferment lactose; and it’s urease and oxidase-negative, meaning it doesn’t produce these enzymes.

Finally, it doesn’t produce hydrogen sulfide gas either, and this can be used to selectively identify Shigella on special culture mediums like MacConkey agar.

On this medium, Shigella mostly forms white, non-lactose fermenting, non-hydrogen sulfide-producing colonies

Now, once Shigella is ingested, it multiplies in the small intestine, and then passes into the colon.

There, it targets the epithelial layer of the mucosal lining where it infects colonic enterocytes and microfold cells, or M-cells.

And these M-cells phagocytose, which means they eat the bacteria from the intestinal lumen, and then spit it out into the underlying mucosa-associated lymphoid tissues, or MALTs.

MALTs are a type of mucosal immune tissue that extends into the submucosa, and contains plenty of immune cells like macrophages.

The macrophages gobble up Shigella to neutralize the pathogen, but the bacterium induces apoptosis, or programmed cell death, in the macrophage.

Now, when a macrophage dies, it releases a variety of cytokines, including IL-1β, which activates an intense inflammatory process that recruits additional immune cells to wage war at the site of infection.

And this results in the death of epithelial cells, which damages the colonic mucosa, resulting in ulcerations and abscesses.