By: Heather Williams
The human body has amazing mechanisms to fight disease. From mechanical barriers such as skin and membranes, antibodies that attack foreign cells, and even pH to kill harmful organisms that invade the body. We have intricate systems in place to protect ourselves. Yet some of the craftier bacteria find ways to make us sick. Some are designed to get past those road blocks our bodies put up. One particular bacteria even uses our own bodies against us to make their way from the mouth to the colon. This particular bacteria is known as Shigella and causes the disease Shigellosis.
Shigellosis is a bacterial illness primarily spread by fecal-oral transmission with humans being the primary mode of transmission. Generally, an infected individual who is shedding the bacteria does not use proper sanitation procedures when preparing foods or leaving their microscopic waste in areas others will have contact with. Other ways of disease transmission include ingestion of contaminated water such as untreated wading pools or shared water fountains. In some cases, Shigella can be transmitted sexually. Shigellosis is highly contagious, with a very small amount of the bacteria that can make a person sick. The infective dose is as few as 10 individual bacteria for S dysenteriae (type A species of Shigella) and 100-200 individual bacteria for S flexneri and S sonnei (type B and D species of Shigella respectively).
The type A Shigella is known for more severe symptoms, while type D Shigella often has milder symptoms. Regardless of the specific type of Shigella causing the infection, those with compromised immune systems are at risk of sever illness. Symptoms include watery, bloody, or mucoid diarrhea, fever, nausea, and stomach cramps. Some patience may have vomiting or even seizures and even arthritis. Some long-term affects may include impaired physical and cognitive development, poor gastrointestinal health, and even kidney damage as a very rare reaction to Shigellosis may occur caused by hemolytic uremic syndrome.
The incubation period for Shigellosis ranges from 12 hours to 7 days, with 2 to 4 days after exposure being the most common time before symptoms appear. The more bacteria consumed, the earlier symptoms will appear. People will continue to shed the virus for up to 4 weeks, with a decreased time period if the patient receives appropriate antibacterial treatment.
Instances of Shigellosis Rising
According to a Mobility and Mortality Weekly Report article published about trends in foodborne disease, the average incidence of shigellosis in the United States in 2013 was 4.82 cases per 100,000 individuals. The instances of children younger than 5 years old are statistically much higher with 27.77 per 100,000. This is higher than 2012 data of 2.5 cases per 100,000 and 0.49 cases per 100,00 in children under 15 years old. World-wide an estimated 164.7 million cases are observed with 163.2 million occurring in developing countries. Cases involving children under 5 years old account for 60% of all illnesses and 61% of all deaths.
How Does It Survive?
Shigella is a non-motile, which means that it doesn’t have a way to transport itself through the body, so it relies on the general movement of the digestive system to move it though the body. It is not spore-forming, so it doesn’t have a protective incubation phase to shield it from the acidic pH of the stomach and intestinal tract. It is even non-encapsulated, which would have provided some form of protection.
Somehow, the Shigella bacteria can survive the low pH of the gastric juices. In studies, it has even been shown to withstand acid treatments of a pH 2.5 for at least 2hrs. Withstanding this pH allows the bacteria to not be affected by the bile produced in the body. Beyond pH, the bacteria have some amazing coping mechanisms to reach the colon. To do this, the bacteria use bile salts produced by the gastrointestinal tract to protect itself in absence of the ability to spore or encapsulate.
“For the first time, we have identified how Shigella not only resists bile but also use this alkaline fluid produced by the liver to its advantage,” says Christina S. Faherty, PhD, of the Mucosal Immunology and Biology Research Center (MIBRC) at Massachusetts General Hospital, senior author of a paper published in the June issue of Infection and Immunity. “We analyzed how the pathogen’s gene expression changes in response to bile salts exposure. The changes we identified pointed to the use of antibiotic resistance mechanisms to resist bile, to the development of a more infectious organism through increased virulence gene expression, and to one better able to survive the colonic environment due to additional gene expression changes.” The Shigella bacteria have found a way to use the bile salts to clump together to form a biofilm or community of bacteria that in essence creates a protective coating that protects the bacteria from the harsh environmental conditions of the gastrointestinal tract. According to research, Faherty’s team also found that the reabsorption of bile salts that normally takes place in the lower small intestine causes the biofilm to disperse, releasing the hyper-virulent bacteria to infect tissues in the colon.
Treatment and Prevention for Shigellosis
No vaccine is currently available for Shigella. The best way to avoid Shigella infection is to wash hands frequently, especially before eating or activities that would lead to putting your hands in your mouth. Additionally, strict adherence to food and water safety precautions when traveling to developing countries or in areas more high risk for disease transmission can reduce the risk of infection.
When presenting symptoms of Shigellosis, diagnostic tests can be performed. Diagnosis if often determined by rapid diagnostic tests, but should be confirmed through culture of a stool specimen or rectal swab. This culture option will allow the doctor to identify any antibiotic resistance so the appropriate medication can be prescribed.
Dr. Faherty is excited and hopeful with the research being done at Massachusetts General Hospital on Shigella survival mechanisms. “Researchers have been trying to find a successful candidate vaccine to fight Shigella for more than 50 years,” she says. “By identifying some of the early mechanism of how Shigella navigates the intestine and demonstrating how the bacteria use bile as a signal to prepare for infection in the colon, we now have a greater understanding for developing potential new therapies.”