Shigella

Lore & Background

Shigella is a genus of rod-shaped bacteria that do not form spores, cannot move on their own, and require oxygen only facultatively. They stain Gram-negative and are genetically considered a subgroup of *Escherichia coli*. The genus was discovered in 1897 by Kiyoshi Shiga, a Japanese physician who investigated a severe dysentery outbreak affecting over 91,000 people with a mortality rate exceeding twenty percent. Shiga isolated the bacterium from thirty-two patients using Koch’s postulates and later characterized its toxin production while attempting to develop a vaccine. Shigella naturally infects only primates, specifically humans and gorillas, and is a leading global cause of bacterial diarrhea, with an estimated 80 to 165 million annual cases and up to 600,000 deaths. It ranks among the top four pathogens causing moderate-to-severe diarrhea in children in Africa and South Asia. The genus is divided into four groups: *S. dysenteriae* (fifteen serotypes), *S. flexneri* (nine serotypes), *S. boydii* (nineteen serotypes), and *S. sonnei* (one serotype). *S. flexneri* accounts for sixty percent of cases in developing regions, while *S. sonnei* causes seventy-seven percent of cases in developed countries. *S. dysenteriae* often triggers epidemic dysentery in crowded settings like refugee camps. All Shigella carry a virulence plasmid encoding key infection factors, and their chromosomes share most genes with *E. coli*. Infection typically occurs through ingestion, and fewer than one hundred bacteria can cause illness. The bacteria invade the colon’s epithelial lining via M-cells in the small intestine, using a type-III secretion system to inject effector proteins that alter host cell metabolism, disrupt vacuolar membranes, and reorganize actin for intracellular movement and cell-to-cell spread. Some strains produce toxins: *S. flexneri* makes ShET1 and ShET2, while *S. dysenteriae* produces Shiga toxin, which can cause hemolytic-uremic syndrome. Shigella also secretes OspC1 and OspC3 proteins that suppress interferon signaling by mimicking calmodulin-binding proteins, blocking STAT phosphorylation and reducing immune gene expression. Symptoms include diarrhea, fever, nausea, stomach cramps, and flatulence; stool may contain blood, mucus, or pus. Symptoms typically appear two to four days after ingestion and last several days, though they can persist for weeks.

Reader's Guide

Shigella is a significant pathogen because it is a leading cause of bacterial diarrhea worldwide, particularly affecting children in developing regions. The genus is classified into four serogroups: S. dysenteriae, S. flexneri, S. boydii, and S. sonnei. S. flexneri is the most frequently isolated species worldwide, accounting for 60% of cases in the developing world, while S. sonnei causes 77% of cases in the developed world. S. dysenteriae is usually the cause of epidemics in confined populations such as refugee camps. The bacteria invade the epithelial lining of the colon, causing severe inflammation and dysentery. Some strains produce toxins: S. flexneri produces ShET1 and ShET2, while S. dysenteriae produces Shiga toxin, which is associated with potentially fatal hemolytic-uremic syndrome. Shigella uses a type-III secretion system to inject effector proteins into host cells, suppressing interferon signaling and promoting intracellular spread. The genus is phylogenetically considered a subgroup of Escherichia coli.

Did You Know?

• Shigella is genetically nested within Escherichia coli.
• S. dysenteriae produces Shiga toxin, similar to verotoxin from enterohemorrhagic E. coli.
• Shigella is only naturally found in humans and gorillas.

The 1897 Outbreak and the Man Behind the Name

Such epidemics were a recurring and deadly threat to the Japanese public throughout the late nineteenth century. Shiga turned his full attention to this crisis, examining 32 dysentery patients and applying Koch's postulates to successfully isolate and identify the responsible bacterium. He then went on to characterize the organism's methods of toxin production and worked toward developing a vaccine. The bacterium was ultimately named in his honor, cementing his place in the history of microbiology.

Molecular Syringes and Cellular Hijacking

Shigella does not attack the apical surface of intestinal epithelial cells. Instead, it targets the basolateral side, entering the host through M-cells scattered within the small-intestine epithelium. Once inside, the bacterium deploys a type-III secretion system—essentially a molecular syringe—that injects toxic effector proteins directly into the human cell. These effectors can dismantle vacuolar membranes or reorganize actin polymerization, enabling the bacterium to move through the host cell's interior. A key player, the IcsA autotransporter, recruits N-WASP and Arp2/3 complexes to drive actin reorganization, which powers cell-to-cell spread. Additionally, Shigella secretes OspC1 and OspC3 proteins that suppress the host's interferon signaling pathway. These proteins bind calmodulin through their N-terminal alpha-helix, mimicking the interaction normally used by CaMKII, thereby blocking STAT phosphorylation and preventing expression of interferon-stimulated genes. This multi-pronged strategy allows Shigella to multiply intracellularly, destroy tissue, and produce the hallmark dysentery of shigellosis.

Global Burden and the Four Serogroups

It ranks among the top four pathogens causing moderate-to-severe diarrhea in children across Africa and South Asia. The genus comprises four serogroups: A (S. dysenteriae, 15 serotypes), B (S. flexneri, 9 serotypes), C (S. boydii, 19 serotypes), and D (S. sonnei, a single serotype). Geographic patterns are striking: S. flexneri accounts for 60 percent of cases in the developing world, while S. sonnei dominates in developed nations at 77 percent of cases but only 15 percent in developing regions. S. dysenteriae is particularly associated with epidemic dysentery in confined populations such as refugee camps. Phylogenetic evidence suggests Shigella is more properly considered a subgroup of Escherichia coli, and its chromosomes share most genes with the well-studied E. coli K12 strain MG1655.

From Ingestion to Dysentery: The Clinical Picture

Symptoms—diarrhea, fever, nausea, vomiting, stomach cramps, flatulence, and painful bowel movements—usually appear two to four days after exposure, though onset can take up to a week. The stool may contain blood, mucus, or pus, reflecting the severe inflammation and cell death in the colonic epithelium. Some strains produce additional toxins that worsen the clinical picture: S. flexneri generates ShET1 and ShET2, which may contribute to diarrhea, while S. dysenteriae produces the hemolytic Shiga toxin, closely related to the verotoxin of enterohemorrhagic E. coli. Both toxins are linked to hemolytic-uremic syndrome, a potentially fatal complication. In rare cases, young children experience seizures. The illness generally lasts several days but can persist for weeks, and Shigella is also implicated as a pathogenic trigger of reactive arthritis worldwide.