Shigella

Shigella is a genus of rod-shaped bacteria that causes a disease called shigellosis. These bacteria are Gram-negative and facultatively anaerobic, meaning they can live with or without oxygen. They are non-spore-forming and nonmotile, so they do not move on their own. Scientists have found that Shigella is genetically nested within the Escherichia coli family. Because of this close relationship, many phylogenetic studies suggest it is actually a subgroup of E. coli. Shigella is a major global health concern because it is a leading cause of bacterial diarrhea. It is specifically one of the top four pathogens causing moderate-to-severe diarrhea in children across Africa and South Asia.

The infection process usually begins when a person ingests the bacteria. It does not take many bacteria to make someone sick. Depending on the host's health, fewer than 100 bacterial cells may cause an infection. Once inside the body, Shigella targets the epithelial lining of the colon. It enters the host by invading M-cells, which are cells interspersed within the epithelia of the small intestine. The bacteria prefer the basolateral side of these cells rather than the apical surface. Once they have invaded, the bacteria multiply inside the cells and spread to neighboring cells. This process causes severe inflammation and the death of the cells lining the colon.

To survive and spread, Shigella uses a sophisticated mechanism called a type-III secretion system. You can think of this system as a biological syringe. It allows the bacteria to translocate toxic effector proteins directly into the host cell. These proteins can change how the target cell works. For example, they can cause the lysis of vacuolar membranes. They can also reorganize actin polymerization, which helps the bacteria move inside the host cell. One specific protein, called IcsA, triggers actin reorganization by recruiting N-WASP and Arp2/3 complexes. This helps the bacteria spread from one cell to another.

Shigella also has ways to trick the body's immune system. The bacteria use their secretion system to release OspC1 and OspC3 proteins. These proteins work to suppress the interferon signaling pathway. This pathway is a key part of the host's defense against infection. Specifically, these proteins target the JAK/STAT signaling pathway to prevent the expression of interferon-stimulated genes. They do this by binding to calmodulin, a protein required for the phosphorylation of STAT. The bacterial proteins mimic a protein called CaMKII, which causes calmodulin to make a mistake. This prevents the normal immune response from working correctly.

Scientists classify Shigella into different groups based on their characteristics. There are three main serogroups: Serogroup A, B, and C, plus Serogroup D. Serogroup A is S. dysenteriae, which contains 15 serotypes. Serogroup B is S. flexneri, which has 9 serotypes. Serogroup C is S. boydii, which includes 19 serotypes. Serogroup D is S. sonnei, which has only one serotype. While groups A through C are physiologically similar, S. sonnei can be told apart using biochemical metabolism assays. Different species cause different patterns of disease depending on where you live.

These different species have very different impacts on the world. S. flexneri is the most common species found worldwide. It accounts for 60% of Shigella cases in the developing world. In contrast, S. sonnei is the most common in the developed world, causing 77% of cases there. However, S. sonnei only accounts for 15% of cases in the developing world. S. dysenteriae is often responsible for large epidemics of dysentery. These outbreaks frequently happen in confined populations, such as those in refugee camps. Some strains also produce dangerous toxins. S. flexneri produces ShET1 and ShET2, while S. dysenteriae produces the hemolytic Shiga toxin.

The history of this discovery is tied to the work of Kiyoshi Shiga. He was a Japanese physician who studied at the Tokyo Imperial University School of Medicine. After graduating, he worked as a research assistant for Shibasaburo Kitasato. In 1897, Shiga studied a massive outbreak of a disease known in Japan as sekiri. This epidemic affected more than 91,000 people, and the mortality rate was over 20%. By studying 32 patients and using Koch's postulates, Shiga successfully isolated the bacterium. He also identified how the Shiga toxin is produced and worked to create a vaccine.

The impact of Shigella on human health is massive. Every year, there are an estimated 80 to 165 million cases of shigellosis. These infections lead to between 74,000 and 600,000 deaths annually. Symptoms can appear two to four days after ingestion, though they can sometimes take a week. Common symptoms include fever, nausea, vomiting, stomach cramps, and flatulence. The infection often causes large, painful bowel movements. Because the stool may contain blood, mucus, or pus, the condition is often called dysentery. In rare instances, young children may even experience seizures.