Shigella Species

The Pathogen, Virulence Factors, and Pathogenesis

Shigella are human-adapted Escherichia coli that have acquired the ability to invade human intestinal mucosa and induce disease. They are nonmotile, nonencapsulated, gram-negative bacilli in the family Enterobacteriaceae. Shigella do not ferment lactose and lack urease activity. About 50 serotypes, belonging to four species (or serogroups), have been identified: group A (S. dysenteriae), group B (S. flexneri), group C (S. boydii), and group D (S. sonnei). ^,

The central pathogenesis is the invasion of colonic mucosa by transcytosis through M cells that cover the lymphoid follicles of the intestine. ^, At the dome area of the lymphoid follicle, the bacteria are phagocytosed by macrophages, which the bacteria kill by inducing apoptosis, dampening the innate immune recognition and evading inflammasome activation. With this strategy, shigellae reach the subepithelial tissue and invade colonic epithelial cells through their basolateral surface. During the initial step of cellular entry, shigellae induce actin polymerization in epithelial cells with formation of filopodes. ^,

After entry, the bacteria free themselves from membrane-bound vacuoles, proliferate in the cytoplasm, and spread by actin-based motility, which enables infection of neighboring cells by formation of finger-like protrusions. ^, Intracellular shigellae also reprogram host cells to express proinflammatory cytokines, such as interleukin-1 (IL-1), IL-6, and IL-8, which augment local inflammation and attract neutrophils. Leukocytes transmigrate the epithelium into the lumen of the colon. Shigella kills neutrophils by inducing necrosis, and the release of granular proteins contributes to further epithelial disruption. ^{, ,}

Specific chromosomal genes-encoded factors enhance replication within tissues (siderophore aerobactin), increase bacterial resistance to nonspecific defense mechanisms (somatic antigens), and prevent killing by phagocytic cells (superoxide dismutase). S. dysenteriae serotype 1 organisms increase the severity of illness by producing Shiga toxin, which contains an enzymatically active (A) subunit and five cell-binding (B) subunits. ^, The A subunit irreversibly inhibits mammalian protein synthesis by cleaving an N -glycosidic bond at adenine 4324 in the 28S ribosomal RNA, blocking elongation factor 1-dependent binding of aminoacyl transfer RNA to the ribosome. Five identical copies of the subunit B bind to globotriaosylceramide, the glycolipid cell receptor.

Chromosomal genes also act by regulating the expression of genes on the Shigella virulence plasmid. The malA gene product functions in the inner membrane as a sensor for the high osmolarity of the colonic contents in the primate intestine, inducing the expression of plasmid invasion genes. The chromosomal keratoconjunctivitis provocation gene (kcpA) is responsible for movement of Shigella in the cytoplasm. The virR regulatory loop gene responds to conditions outside the bacteria by inhibiting expression of the invasive phenotype at 30°C.

All virulent Shigella species contain a 120–140 megadalton (200–220 kilobase) plasmid. Plasmid genes are involved in regulation, invasion, and intercellular spread; virF governs the plasmid virulence regulon by directly activating transcription of two other gene loci, virG and virB. The latter locus then activates invasion plasmid antigens ipaABCD . Intracellular and intercellular bacterial spread is encoded by virG, which has a sequence of repeating motifs in its amino terminus involved in polymerization of actin monomers to provide the force to propel Shigella within the cytoplasm and into adjacent cells. ^,

This model explains the histologic observations of early neutrophil efflux in colonic crypts and bacterial invasion. The desquamation, ulceration, and formation of microabscesses in the colonic mucosa inhibit the absorption of water, producing frequent and scanty stools that contain blood, inflammatory cells, and mucus. Invasion beyond the mucosa to the bloodstream is uncommon.

Although mucosal secretory immunoglobulin (Ig) A antibodies develop against the virulence plasmid invasion antigens (ipaABCD) after natural infection, immunity is serotype-specific and the presence of LPS antibodies correlate with protection. ^{, ,} Specific host microRNAs that are produced impair Shigella actin-based motility.

Shigellosis primarily affects the colon. Macroscopically, the affected mucosal surface is edematous and erythematous, covered by a layer of gray mucopurulent exudate with various degrees of ulceration and hemorrhage. Microscopic examination typically reveals hyperemia, edema, and neutrophil infiltration of the colonic mucosa. In early stages, the surface epithelium can reduce in height and be infiltrated with neutrophils. As the disease progresses, epithelial cells detach from the edematous lamina propria, causing microulcerations. With advanced disease, large numbers of lymphocytes and plasma cells are seen in the lamina propria. At the base of a deep necrotic, ulcerated mucosa, a thick pseudomembrane forms, composed of neutrophils, erythrocytes, fibrin, and desquamated epithelial cells. Crypt abnormalities include a marked reduction of goblet cells, hyperplasia of epithelial cells, crypt abscesses, and distention due to exudates. Aphthoid ulcers form over lymphoid aggregates.

Pathologic changes including hyperemia, edema, focal hemorrhage, and swelling of endothelial cells in the small blood vessels near the lamina propria. In some cases, thrombosis of blood vessels in the lamina propria and submucosa is associated with widespread necrosis of the colonic mucosa. The pathophysiologic basis for the initial watery diarrheal presentation may be related to enterotoxin production, but it has not been fully elucidated.

Epidemiology

Most shigellosis-related mortality occurs in resource-limited countries, especially in sub-Saharan Africa and south Asia, ^{, ,} but its burden is worldwide. ^{, ,} In 2016, US public health laboratories reported 12,597 cases (3.9/100,000) of culture-confirmed shigellosis to the Centers for Disease Control and Prevention (CDC) compared with 7638 in 2013 (2.7/100,00). As there is substantial underreporting, the actual numbers are estimated as 20 times higher. Highest rates of disease, hospitalizations, and mortality are among children aged 1–4 years, particularly 12–36 months. ^{, , , , ,} For example, the prevalence in the US in 2016 in children 1–4 years was 18.1/100,000, nearly five times higher than that in the general population. Other variables associated with increased rates of shigellosis include childcare center attendance, ^{, ,} no-breast feeding, ^, low socioeconomic status, ^{, , , ,} ethnicity, ^{, ,} travel to endemic areas, ^{, ,} and men who have sex with men (MSM). ^{, , ,}

In the US, the seasonality of shigellosis has changed from a peak in summer to a high incidence in the late summer and fall. In developing countries with tropical climates, Shigella infection is common throughout the year, but higher isolation rates occur during the summer and rainy seasons.

The prevalence of Shigella serotypes varies over time and geography. S. sonnei is the predominant serotype in developed countries (∼80% of Shigella isolates in US) and increasing worldwide . ^{, , , , , ,} In developing countries, S. flexneri is still most common, with occasional outbreaks caused by S. dysenteriae serotype 1. ^{, , , , , ,} Shigella boydii is uncommon. ^{, ,}

Shigella are hardly recovered for up to 6 months from contaminated water samples maintained at room temperature, and can survive for up to 30 days in foods such as milk, whole eggs, oysters, shrimp, and flour. Shigella can be isolated from toilet surfaces in homes of patients with dysentery and from the fingers of patients who use toilet paper. The ability to survive coupled with a low infectious dose (10 organisms cause diarrhea in adult volunteers) explains its high infectivity. ^, Most cases of shigellosis result from person-to-person transmission of a small inoculum via the fecal-oral route. There is a high risk of spread within families, in childcare centers, in custodial care institutions, and in internment camps. ^{, , , ,}

Water and food are responsible for outbreaks of shigellosis in military camps, cruise ships, community gatherings, and restaurants. In the US, 58% of the reported Shigella outbreaks are restaurant-associated, including large outbreaks caused by commercially prepared foods distributed in multiple states. Foods usually consumed raw were implicated in 24% of the outbreaks and infected food handlers in 23%. Water plays a major role in transmission in resource-poor countries, as does overcrowding and unsanitary conditions. ^{, ,}

Healthy people who become infected with Shigella can shed organisms for up to a month; appropriate antibiotic treatment reduces the period of excretion to few days. ^, Malnourished children shed the organism for prolonged periods as a result of their inability to eliminate the organism or to prevent reinfection. In patients with AIDS, prolonged, relapsing, symptomatic shigellosis and lengthy carrier states have been described despite proper treatment.

Clinical Manifestations