Microbiology
Summary
Shigella is a gram-negative enteric bacterium that causes gastroenteritis, specifically leading to bloody diarrhea. The most common type in the US is Shigella sonnei. Shigella forms green colonies when plated on Hektoen agar, which differentiates it from Salmonella that forms black colonies. Shigella is non-motile and acid-stable, which means it requires fewer microorganisms to cause infection.
The bacterium invades the human intestinal epithelium, particularly M cells in Peyer's patches, where it uses the host cells' actin cytoskeleton to propel itself from one cell to another. Shigella is a facultative intracellular bacterium as it can survive both on its own and inside another cell. It causes bloody diarrhea due to tissue damage and the release of cytokines, resulting in an underlying inflammatory diarrhea. Shigellosis caused by Shigella dysenteriae can also lead to hemolytic uremic syndrome (HUS). This is due to endothelial damage in the kidneys and hemolysis of red blood cells. Shiga toxin, produced by Shigella, binds to the 60S subunit of ribosomes and inhibits translation. Like Salmonella, Shigella utilizes a type III secretion system to secrete inflammatory cytokines.
Lesson Outline
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FAQs
Shigella infection typically presents as an acute form of gastroenteritis, characterized by bloody diarrhea, abdominal pain, and fever. This is in contrast to Salmonella infection, which can cause gastroenteritis, enteric fever (typhoid fever), or septicemia depending on the strain. While both can cause gastroenteritis, bloody diarrhea is more commonly associated with Shigella infections.
Shigella spp. are able to invade the epithelial lining of the colon and rectum by initially targeting M cells in the lymphoid follicles. Upon invasion, Shigella can cause extensive damage, including the invasion of epithelial cells and the activation of an immune response. This results in the characteristic symptoms of acute gastroenteritis such as bloody diarrhea. The ability of Shigella spp. to invade M cells is a crucial factor in its pathogenicity.
Hektoen agar is a selective and differential medium used to isolate and identify enteric pathogens, including Shigella spp. and Salmonella spp. Shigella spp. will produce non-lactose-fermenting colonies on Hektoen agar, which are greenish-blue in color with or without black centers. This can aid in the identification of Shigella spp. among other enteric pathogens.
Shigella infection triggers both innate and adaptive immune responses in the host. The innate immune response involves the recruitment of neutrophils and macrophages to the site of infection, which contributes to the inflammation and tissue damage seen during the infection. In the adaptive immune response, B- and T-lymphocytes are activated to produce protective antibodies and enhance the clearance of the pathogen. The local inflammation, tissue damage, and immune cell infiltration contribute to the characteristic symptoms of gastroenteritis such as bloody diarrhea, abdominal pain, and fever.
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