Shigella Overview and Implications

Shigella, a Gram-negative and non-motile bacterium belonging to the Enterobacteriaceae family, consists of four species: S. dysenteriae, S. flexneri, S. boydii, and S. sonnei, designated as serogroups A, B, C, and D, respectively.

Shigellosis, an acute invasive enteric infection, stems from the Shigella genus, clinically marked by diarrhea often accompanied by blood. The condition is widespread in numerous developing countries, occasionally sparking epidemics that result in substantial illness and mortality.

Shigellosis is responsible for around 80 million cases of bloody diarrhea and 700,000 deaths annually, with the majority affecting children under five years old (approximately 70% of cases and 60% of deaths). The onset typically occurs within twelve to fifty hours after consuming contaminated food, leading to symptoms like abdominal pain, cramps, fever, vomiting, and diarrhea.

Factors such as overcrowding and poor sanitation contribute to the prominence of shigellosis in developing countries. Vulnerable groups, including infants, non-breastfed children, those recovering from measles, malnourished children, and adults over 50 years old, face a higher risk of severe illness and death. Transmission primarily happens through the fecal-oral route, person-to-person contact, household flies, contaminated water, and inanimate objects.

Among the Shigella species, Shigella dysenteriae type 1 (Sd1) holds particular significance due to its capacity to induce the most severe disease and potentially lead to extensive regional epidemics.
Shigella Overview and Implications

Shigella dysenteriae

Shigella bacteria cause an infection called shigellosis. Shigella cause an estimated 450,000 infections in the United States each year. Most people with Shigella infection have diarrhea (sometimes bloody), fever, and stomach cramps.

The four species of Shigella are:
*Shigella sonnei (the most common species in the United States)
*Shigella flexneri
*Shigella boydii
*Shigella dysenteriae

Shigella dysentery was the first associated enteric infection to be recognized. S. dysenteriae type 1 (not commonly present in the US, except in travelers returning from endemic areas) produces Shiga toxin, which causes marked watery diarrhea and sometimes hemolytic-uremic syndrome.

S. dysenteriae type 1 is the causative agent of the most severe form of bacillary dysentery, which occurs as epidemics in many developing countries. S. dysenteriae type 1 produces a potent cytotoxin (Shiga toxin) that causes local destruction of colonic epithelium, so disease due to this strain is more severe and prolonged than that produced by other species.

S. dysenteriae is a gram-negative bacterium that causes bacillary dysentery or shigellosis. S. dysenteriae spread by contaminated water and food, causes the most severe disease because of its potent exotoxin.

The disease can be divided into two phases.
*First phase: occurring in the first 1 to 2 days, consists of watery diarrhea and cramping; this phase is mediated by an enterotoxin. Process involves:
- Ingestion
- Noninvasive colonization and cell multiplication
- Production of the enterotoxin by the pathogenic bacteria in the small intestine

*Second phase: is caused by invasion of the organism into the large intestine and produces fever, cramps, bloody diarrhea, and tenesmus.
Shigella dysenteriae

Pathogenesis of Shigella

Shigella are Gram-negative, non-spore forming, facultative anaerobic bacilli closely related biochemically and antigenically to E. coli.

Shigella spp. are transmitted by the fecal-oral route and enter the human body via the ingestion of contaminated food or water. The bacteria are highly infectious, since as few as 10 to 100 microorganisms are sufficient to cause disease.

The organisms invade the cells lining the colonic mucosa and multiply there, killing the cell; this is the cause of the symptoms produced. However, it occasionally invades the bowel beyond the surface lining.

The pathogenesis of Shigella is based on the bacteria’s ability to invade and replicate within the colonic epithelium, which triggers an intense acute inflammatory response with infiltration by polymorphonuclear leukocytes.

An early symptom, diarrhea (possibly elicited by enterotoxins and/or cytotoxin), may occur as the organisms pass through the small intestine.

The pathogenesis of Shigella is a multi-step process which depends on the capacity of the bacteria to cross the colonic mucosa via M cells (microfold cells) associated with Gastrointestinal Associated Lymphoid Tissue (GALT). M cells are specialized epithelial cells (EC), which continuously sample particles from the gut lumen and deliver them to the underlying mucosal lymphoid tissue, where immune responses can be initiated.

The bacteria then invade epithelial cells and have the capacity to reprogram these cells to produce pro-inflammatory mediators, such as interleukin 8, which play a major role in the strong inflammatory response facilitating further bacterial invasion.

Colitis in the rectosigmoid mucosa, with concomitant malabsorption, results in the characteristic sign of bacillary dysentery: scanty, unformed stools tinged with blood and mucus.

The severity of Shigella infections is dependent upon many factors such as previous exposure and infecting dose. These infections are usually self-limiting, but dysentery can be a life-threatening illness in infants and in the elderly or malnourished.
Pathogenesis of Shigella

Shigella infection

Shigella is a Gram-negative, non-motile bacillus belonging to the Enterobacteriaceae family. There are four species of Shigellae: S. dysenteriae, S. flexneri, S. boydii and S. sonnei.

It is an invasive disease of the colon caused mainly by Shigella sonnei and Shigella flexneri. Shigella is related to the acute inflammations. The essential steps of the colonic infections by the bacterium Shigella are including the adhesion, invasion, intracellular replication, and cell-to-cell spreading.

People who are sick with Shigella usually start experiencing symptoms 1 to 2 days after putting something in their mouth or swallowing something that has come into contact with the bacteria. Symptoms of shigellosis may include:
• Diarrhea (sometimes bloody)
• Fever
• Stomach pain
• Feeling the need to pass stool even when the bowels are empty

Yearly, about 165 million cases of Shigella diseases are recorded in the world, which lead to 1 million states of death, especially in the developing countries.

Shigellosis occurs predominantly in developing countries due to overcrowding and poor sanitation. Infants, non-breast-fed children, children recovering from measles, malnourished children, and adults older than 50 years have a more severe illness and a greater risk of death. The mode of transmission occurs via the faecal-oral route, person-to-person contact, household flies, infected water, and inanimate objects.

People who have been infected with a specific type of Shigella are not likely to get infected with that specific type again for at least several years. However, they can still get infected with other types of Shigella.
Shigella infection

Prevalence of shigellosis

In developing countries, diarrheal diseases are important causes or morbidity and mortality. Together with plaque, cholera and influenza, bacillary dysentery (shigellosis) has been one of the world’s great scourges.

In general, Shigella is one of the most communicable and severe forms of the bacterial induced diarrhea. No group of individuals is immune to shigellosis, but certain individuals are at increased risk.

Fecal pollution of water supply, contamination of food by infected food handlers or transfer of the bacilli by flies are the principle means of transmission.

This disease is common in developing countries, where sanitary conditions are sub-optimal. The disease also associated with poverty, crowding, water resources inadequate in quantity and quality and malnutrition.

The global burden of shigellosis has been estimated at 165 million cases per year, of which 163 million are in developing countries. More than one million deaths occur in developing countries yearly due to Shigella infection.
Prevalence of shigellosis

Shigellosis: characteristics and causes

Also known as bacillary dysentery, shigellosis, an invasive disease of the human colon is present worldwide. Shigellosis is a significant cause of foodborne illness in developed as well as underdeveloped countries. It is a bacterial infection cause by organsism in the Shigella genus, with four species that include S. dysenteriae, S. flexneri, S. boydii and S. sonnei.

Food microbiologists should be aware that shigella species can cause a rather severe form of foodborne illness and relatively low numbers of the organisms can cause disease.

Shigellosis is characterised by fever abdominal cramps, tenesmus, and dysenteric stools containing mucus blood and pus. These symptoms reflect invasion of the colonic mucosa by Shigella, the critical stage in the pathogenc process.

Product suspected of causing food poisoning should be analysed for shigella. Shigella sp. cause a diarrhea. The diarrhea may be bloody or nonbloody. Humans and other large primates are the only natural reservoirs of Shigella spp.

Transmission of from person to person by ingestion of fecally contaminated food or water. Disease onset typically occurs between 12 hours to 2 days after exposure. The infectious dose is quite small, as few as 10-200 organisms.
Shigellosis: characteristics and causes

Enterotoxin of Shigella

Shigella grows optimally at pH 4.5, but survives in a pH range of 2 to 5, permitting easy transit of these bacteria through the stomach.

Shigella strains produce at least two enterotoxins including a chromosome encoded Shigella enterotoxin 1 and a large plasmid associate Shigella enterotoxin 2.

Enterotoxins are bacterial products that act on the mucosal epithelium of the intestine, causing fluid secretion and profuse watery diarrhea. Normally they are heat stable and a low of molecular weight and water soluble.

Shigella enterotoxin 1 is chromosomally encoded, iron dependent toxin of 55 kDa that is primarily expressed by S. flexnery while encoded Shigella enterotoxin 2 is a plasmid-encoded protein of 63 kDa.

Shigellae caused bacillary dysentery. Ingested bacilli may infect villi of large intestine and multiply inside them.

Inflammatory reaction results in epithelial necrotic patch which later on become transverse superficial ulcers. 

Transmission is by fecal-oral route, and stubborn reservoirs include day care centers, urban ghettos and rural village in developing countries.
Enterotoxin of Shigella

Shigella and Yersinia

Shigella and Yersinia
Shigella
Shigella causes little less than 10 percent of all food borne illness in the United States. It is widespread worldwide and is very virulent: as few as ten cells cause infection.

Shigellosis (the disease caused by Shigella) usually strikes between twelve and fifty hours after the contaminated food is consumed.

It can cause abdominal pain, cramps, diarrhea, fever and vomiting.

On rare occasions it can cause Reiter’s syndrome, reactive arthritis and hemolytic uremic syndrome.

It is often found in prepared salads, raw vegetables, milk, other dairy products and poultry.

Yersinia
There are three pathogenic species of Yersinia. Y. pestis causes the plaque and is not transmitted through food. Y. enterocolitica and Y. pseudotuberculosis cause gastrointestinal problems, including abdominal pain, diarrhea and vomiting.

Yersinia infections often mimic appendicitis and can sometimes result in unnecessary surgery.

The bacteria can also cause infectious in wounds, joints and the urinary tract.

Y. pseudotuberculosis is very rare in the United states but occurs more frequently in Japan, Scandinavia and other parts of northern Europe.

Strains of Y. enterocolitica can be found in meats, including beef, pork, lamb, oyster and fish and also in raw milk.

Although most people recover quickly from yersiniois, about 2 to 3 percent develop reactive arthritis.
Shigella and Yersinia

Shigella association with Foods

Shigella association with Foods
The principal foods involved in Shigella outbreaks were a variety of salads and seafood which became contaminated during handling by infected workers.

This was frequently coupled with improper refrigeration. After the initial infection from contaminated food, the disease readily spreads from person to person by the fecal-oral route of transmission.

Shigellas usually are considered to be relatively fragile; i.e., they do not survive well outside the host. Like most other members of the family Enterobacteriaceae, they are readily killed by most heat treatments employed in the processing and preparation of foods, and do not survive well at pH below 4.5.

However, studies on the survival of Shigella suggest that under certain selected conditions, they can survive for extended periods in foods.

For example, Shigella sonnei and Shigella flexneri have been reported to survive at 25 degree C (77 degree F) in flour and in pasteurized whole milk for more than 170 days; in eggs, clams, and shrimp for more than 50 days; and in eggs whites for more than 20 days.

At lower temperatures such as -20 degree and 0.5 degree C survival for longer periods has been reported.

Laboratory data suggest that Shigella may survive for extended periods on foods and may grow under selected conditions. However, in practice, shigellae are rarely isolated from processed products.

Manufacturers do not routinely test their products, raw materials or processing environment for Shigella, and there is no evidence to suggest that routine testing is warranted.

Most outbreaks result from contamination of raw or previously cooked foods during preparations in the homes or in foodservice establishments. Generally, the source of contamination can be traced to a carrier whose personal hygiene is poor.
Shigella association with Foods

Shigella as a Pathogen

Shigella as a Pathogen
Shigellosis, or bacillary dysentery, as it is commonly known, is caused by bacteria of the genus Shigella (S. dysenteriae, S. flexneri, S. boydii and S. sonnei). The normal habitat for Shigellae is the intestinal tract of humans and other primates. They are rarely found in other animals. The main source of Shigellae involved in outbreaks is asymptomatic carriers or persons recovering from disease. Shigellae may persist in the intestinal tract for month.

Symptoms of shigellosis, after an incubation period of 1-7 days (usually less than 4 days), include diarrhea, abdominal pain, fever, and vomiting. The severity of the disease may vary from very mild to severe diarrhea with bloody stools, mucus secretion, and dehydration. Generally foodborne shigellosis is characterized by a high attack rate common source epidemiology, and short incubation periods of 7 – 36 hours. Symptoms usually persist for 3 – 14 days. Frequently, an asymptomatic carrier state may develop during convalescence, lasting from a few days to several months. Human volunteer studies indicate that ingestion of as few as 1 – 100 organisms can induce illness.

The disease is caused by invasion of the intestinal mucosa. An enterotoxin, classically referred to a Shiga toxin, may be produced by S. dysenteriae and possibly by S. flexneri.
Shigella as a Pathogen