Yersinia: Plague, Food Poisoning, and a Potential Cancer Treatment
Harmful and Potentially Helpful Bacteria
Bacteria in the genus Yersinia have a terrible reputation. Yersinia pestis causes plague and was responsible for the frightening episode in history known as the Black Death. Yersinia enterocolitica causes a type of food poisoning known as yersiniosis and produces a wide variety of unpleasant symptoms. Yersinia ruckeri causes redmouth disease in fish. The infection produces subcutaneous bleeding in the mouth and other body parts and has a high fatality rate. Y. ruckeri may have a very important benefit for humans, however. It produces a toxin that might be able to fight cancer.
Yersinia is named after Alexandre Yersin, a physician and bacteriologist who discovered Pasteurella pestis in 1894. The bacterium was renamed Yersinia pestis (or Y. pestis as it's sometimes abbreviated) in his honour.
Characteristics of Yersinia Bacteria
The members of the genus Yersinia are rod-shaped bacteria that sometimes develop an oval or spherical shape. They are facultative anaerobes—that is, they can survive without oxygen in the environment as well as with oxygen. They change their biology when the environment lacks oxygen.
Yersinia bacteria are also versatile in other respects. For example, Y. enterocolitica often has flagella and is motile. Once it has entered the body or when it's in an environment at body temperature, however, it loses its flagella. (Y. pestis doesn't produce flagella and is nonmotile.) Y. enterocolitica can survive at the low temperatures found in refrigerators as well as at body temperature.
Yersinia is gram negative. The gram stain is named after Hans Christian Gram, the bacteriologist who created the test in 1884. As a result of adding specific chemicals to the bacteria, gram positive bacteria appear purple and gram negative ones appear pink. The different results are due to the different amounts of peptidoglycan in the cell wall of the bacteria. Gram positive bacteria contain far more peptidoglycan than gram negative ones. Performing a gram stain is an important step in identifying a bacterium.
Yersinia is chemoorganotrophic. It gets its energy from the organic chemicals that it absorbs from its environment.
The Cause of Plague
The ultimate cause of plague is Yersinia pestis. The bacterium requires the help of rodents and fleas to cause an infection, however. The rodents act as a reservoir of bacteria. When a flea feeds on the blood of an infected rodent, it withdraws bacteria as well as blood. If the flea later bites a human to obtain blood, the bacteria travel into the person's body from the flea.
One reason for the transmission of bacteria from fleas to humans is the existence of so-called "blocked" fleas. The blockage occurs in the digestive tract. The digestive tract of a flea consists of a foregut, a midget, and a hindgut.
- The foregut begins with the mouth cavity and ends with a tubular region called the esophagus.
- A valve-like chamber called the proventriculus is located at the end of the esophagus where it joins the midgut.
- The midgut is the site of food digestion and absorption.
- The hindgut is the site of fecal pellet production from undigested food. The pellets are eliminated from the body.
Y. pestis bacteria obtained in the blood of a rodent multiply in the proventriculus and often in the midgut as well, creating a dense mass or blockage. As a result, digestion of blood and absorption of nutrients are hindered. The hungry flea frequently bites humans in an attempt to get food. When it tries to swallow blood after biting someone, the blockage causes the flea to regurgitate bacteria into the wound.
Bacteria on contaminated mouthparts of fleas also enter humans during a bite. This means that even fleas that aren't blocked or that are only partially blocked can infect humans with Yersinia.
The Secret Life of Fleas
Adaptations of Yersinia pestis for Infection
Y. pestis has some interesting features that help it to infect humans. Its cells secrete a slime while they are inside the flea's gut. The slime connects the bacteria and forms a structure called a biofilm. The biofilm helps to block the flea's digestive tract, which leads to the regurgitation that sends the bacteria into a human.
Once Y. pestis has entered a human, it produces a variety of chemicals that stop the person's immune system from attacking it. This enables the bacterium to survive and to cause disease.
The rodent that supplied fleas in the Black Death is commonly believed to be the black rat, or Rattus rattus. It's also known as the house rat and the roof rat. Recently, there has been a suggestion that the rodent was actually the great or giant gerbil (Rhombomys opimus).
Types of Plague
Yersinia pestis causes three types of plague—bubonic, septicemic, and pneumonic. All three types can be deadly. Modern antibiotics are often an effective treatment, however. The treatment needs to be started early in the infection, especially in the case of pneumonic plague.
Some frequent signs and symptoms of plague are listed below. The symptoms may be caused by other conditions, however. A doctor's diagnosis is required.
Bubonic plague is the most common type of plague. In this form of the disease, bacteria enter the lymphatic system. Lymph nodes in the neck, armpits, or groin become swollen and painful. The swellings are known as buboes. The infected person also experiences symptoms that resemble those of the flu, such as fever, chills, a headache, and weakness. This form of the disease isn't contagious under normal conditions. However, touching fluid that has leaked from buboes may transmit the disease.
In septicemic plague, bacteria enter the blood. Bleeding occurs under the skin or in organs, which can lead to shock. The person may also experience abdominal pain, fever, chills, and weakness. Septicemic plague may occur on its own or may accompany one of the other types of plague. Sometimes gangrene (tissue death) occurs in the fingers, toes, and nose. These turn black as their cells die, giving the "Black Death" its name. Like bubonic plague, septicemic plague isn't contagious unless infected blood or tissue is touched.
In pneumonic plague, bacteria enter the lungs and cause pneumonia. This is the most serious form of the disease. The person experiences a cough, difficulty in breathing, and chest pain. They may also cough up blood. Unlike the other two forms of plague, pneumonic plague is contagious. The bacterium is spread via airborne droplets released from the lungs.
The Black Death
The Black Death was a widespread Yersinia pestis infection in Europe and Asia that had horrific results. Estimates of the percentage of Europeans that died from the infection range from one third to two thirds of the population. The largest outbreak occurred in the late 1340s and is the one that is generally referred to in descriptions of the Black Death, but smaller outbreaks occurred for centuries after. So many people died from the disease that it was hard to bury them fast enough. The situation must have been terrifying.
It's often reported that people who died of the Black Death had bubonic plague. If this was the case, the fact that the disease spread with such frightening rapidity over such a wide area is puzzling. Flea bites alone don't seem to be an adequate explanation. The bacterium can also be transmitted by direct contact with infected tissues, as might happen when handling the body of someone killed by the plague. Still, some researchers feeł that the disease must have been airborne to have spread so fast and that a large number of people must have had pneumonic plague.
The genomes of some strains of Yersinia pestis and Yersinia enterocolitica have been sequenced. This means that the order of DNA bases in their chromosome has been discovered. The bases form the genetic code of the bacterium. Now that the code has been discovered, researchers are trying to discover its meaning.
Plague of Justinian, the Black Death, and Yersinia pestis
The Bacterium Today
Yersinia pestis still exists today. Although the potential for harm is still present and small outbreaks do occur, the infection is currently much less serious than in the past. This is partly because we have antibiotics to treat the disease. The fact that many cities have fewer rats than in the past may also be helpful. Researchers think there must be other reasons why plague is less common and the bacterium is less dangerous today, however.
Scientists have found Y. pestis DNA from the time of the Black Death and are comparing it with the DNA in the current version of the bacterium to see if it has changed in a significant way. So far, however, no significant differences have been found. Scientists are also looking at the human genome. It's possible that our genome has changed in a way that gives us more resistance to the bacterium. The research is important and is of more than academic interest. If the plague threatens to become a major problem again, we need to know how to deal with it.
An outbreak of plague occurred in Madagascar in 2017. According to WHO (the World Health Organization), from August 1st to November 10th, 1618 cases of pneumonic plaque, 324 cases of bubonic plague, and one case of septicemic plague were diagnosed. In addition, 176 unclassified types of plague were reported to WHO. 72 people died from pneumonic plague. In late November, the outbreak was said to be contained. The CDC (Centers for Disease Control and Prevention) notes that cases of bubonic plague are diagnosed nearly every year in Madagascar, however. The agency maintains a web page that lists travel health notices. This page is referenced below.
Bubonic Plague in the Past, Present, and Future
Yersiniosis and Its Symptoms
Yersiniosis is a type of food poisoning or foodborne illness. It's caused by both Yersinia enterocolitica and Yersinia pseudotuberculosis. The symptoms of the illness depend on the patient's age. Children are generally affected more severely than adults.
Common symptoms of yersiniosis include abdominal pain, diarrhea, and fever. The diarrhea may be bloody. In older children and adults, the pain may be located on the right side of the abdomen and there may be no diarrhea. As a result, the illness may be confused with appendicitis. Occasionally, an infected person may develop a skin rash or experience joint pain.
The symptoms of yersiniosis generally last for one to several weeks. The rash or joint problems may last for even longer. Although most symptoms disappear after a month, joint pain may last for as long as six months.
A doctor must be consulted for a diagnosis and treatment recommendation. Mild cases of the illness may not need treatment, but this decision is up to the doctor. Antibiotics may be prescribed for more serious cases.
Causes of Yersiniosis
In North America, yersinioisis is generally caused by eating food contaminated by Y. enterocolitica. These foods include meat from pigs, cows, and sheep that is either raw or inadequately cooked. Pork seems to be the most problematic meat. Raw chitterlings (pig intestines) are especially dangerous. The symptoms don't appear for four to seven days after eating the food, so it may be hard to link a food to the infection.
There are other causes of yersiniosis, included drinking unpasteurized milk and untreated water. Inadequate food hygiene can also result in transmission of the bacterium. The bacterium lives in the intestine of its host, so anything that is contaminated by intestinal contents or the stool of infected animals or humans may be dangerous. The infection is usually diagnosed by a stool test.
Enteric Redmouth Disease in Fish
Enteric redmouth disease, or simply redmouth disease, is most common in fish belonging to the family Salmonidae, Fish in this family include salmon and trout. The disease is caused by a Yersinia ruckeri infection.
The bacterium causes subcutaneous bleeding in a fish's mouth and on its tongue, making these body parts appear bright red. Subcutaneous bleeding or haemorrhages may also occur in the eyes, fins, and skin. In the late stage of the disease the abdomen may become swollen with fluid.
Redmouth disease affects fish that are used for food, especially those in fish farms, so it has economic importance. It can be treated with antibiotics. The mortality rate is high, however.
Microfilaments are long, thin strands of protein that form part of a cell's cytoskeleton. The microfilaments are made of actin and are especially important in the process of cell division. Y. ruckeri produces a substance that interferes with the production of actin and may be useful to humans.
A Yersinia Toxin That May Fight Cancer
Yersinia ruckeri produces a toxin called Afp18. The toxin is an enzyme that deactivates a protein called RhoA. The protein is present in both fish and humans and has similar functions in each organism. It plays a role in the production and breakdown of actin filaments in a cell. Actin filaments are required in the process of cell division. Cancer cells have a much higher rate of cell division than normal cells and have a high demand for actin. This has led scientists to suspect that AFP18 may be helpful in treating the disease.
Afp18 inactivates RhoA by attaching a sugar molecule called n-acetyglucosamine to tyrosine, an amino acid present in the protein. The researchers say that this is an unusual reaction. When they injected the toxin into zebra fish embryos, which had rapidly dividing cells, actin filaments in the embryos collapsed, the cells stopped dividing, and the embryos failed to develop. It's possible that this destructive process may also happen to cancer cells when Afp18 is added to them. This remains to be seen, however. Some of the current cancer drugs work by interfering with cell division, although they don't target actin.
In principal, the use of Afp18 for treating cancer by stopping cancer cell reproduction sounds great. There may be a problem with interfering with cell division, though. Some cells in the human body have a relatively high rate of cell division, so they may affected by the toxin as well. Two locations of these cells are the lining of the intestine and the red bone marrow. Nevertheless, the toxin could be very useful, especially if it affects only cancer cells.
Yersinia's Role in the Future
Yersinia has been interacting with humans for a very long time. It's an old enemy that still has the power to destroy. We are slowly starting to understand its complex and fascinating biology. It would be wonderful if the bacterium became a helper instead of a foe.
References and Resources
- Plague information from the CDC
- Facts about plague in Madagascar from WHO
- Sun, Y.-C., Jarrett, C. O., Bosio, C. F., & Hinnebusch, B. J. (2014). Retracing the Evolutionary Path that Led to Flea-borne Transmission of Yersinia pestis. Cell Host & Microbe, 15(5), 578–586. http://doi.org/10.1016/j.chom.2014.04.003
- Travel health notices from the CDC (At the time when this article was last updated, the plague situation in Madagascar was classified as "Watch Level 1, Practice Usual Precautions".)
- Yersiniosis facts from HealthLinkBC (A Government of British Columbia website)
- Albert-Ludwigs-Universität Freiburg. (2015, July 24). Toxin from salmonid fish has potential to treat cancer. ScienceDaily. Retrieved July 24, 2017 from www.sciencedaily.com/releases/2015/07/150724093733.htm
© 2015 Linda Crampton