Salmonella and Salmonella Food Poisoning
█ BRIAN HOYLE
Salmonella is the name of a group, or genus, of bacteria that live in the intestinal tract of warm-blooded animals, including humans, as well as in cold-blooded animals such as turtles. The name of the microbe comes from its discoverer. In 1885, American veterinary scientist Daniel Salmon isolated the first strain ( Salmonella cholerasuis ) from the intestine of a pig.
Since his discovery, more than 2,300 different types of Salmonella have been discovered. While many of these can be innocuous in their normal intestinal environment, if they infect another area of the body (i.e., a cut) or contaminate food, illness can result.
Salmonella food poisoning, salmonellosis, affects two to four million Americans each year. The number of cases has been increasing in recent years, due in part to the increasing resistance of Salmonella to the antibiotics commonly used to treat the illness. It has been estimated by the Centers for Disease Control and Prevention that the economic cost of Salmonella food poisoning in the U.S. alone is between five and 17 billion dollars annually.
This economic burden, increasing prevalence of Salmonella food-borne illness, and the ease by which disease-causing strains of Salmonella could be acquired and deliberately added to food supplies, have made Salmonella one of the microorganisms that is regarded as being a potential threat to national security.
Salmonella food poisoning results from the growth of the bacterium in food. The rapid increase in the number of bacteria in the intestinal tract overwhelms the defensive capabilities of the host and produces the symptoms of food poisoning. Symptoms include nausea, vomiting, abdominal cramps, diarrhea, fever, and headache. Typically, the symptoms last for a few days.
Prolonged diarrhea can be dangerous. The body loses more fluids and salts than can be replaced, which can threaten the health of various organs and tissues in the body. In severe cases, especially in the young and the elderly, the resulting shock can cause permanent damage. As well, Salmonella can spread from the intestinal tract to the bloodstream, leading to more widespread infections.
The food poisoning caused by Salmonella is one of about ten bacterial causes of food poisoning. Other responsible bacteria include Staphylococcus aureus , Clostridium perfringens , Vibrio parahaemolyticus , and specific strains of Escherichia coli .
A number of foods are especially susceptible to contamination. Chicken carcasses and the outer surface of eggs are frequently contaminated with Salmonella present in the poultry feces. However, proper cooking will destroy the bacteria. The bacteria can gain entry to eggs through cracks in the shell. If the egg or meat is prepared at too low a temperature, the bacteria can survive and can multiply during subsequent storage at room temperature, or even at refrigeration temperature. Other targets for contamination include cream-based desserts, milk and dairy products, shrimp, salad dressing, cocoa, chocolate, and salad-type sandwich filling (such as tuna salad or chicken salad).
An important route of contamination is the handling of food by people who have not washed their hands properly after using the bathroom. This "fecal to oral" route of transmission can be prevented by hand washing, and proper kitchen hygiene (e.g., cleaning cutting boards after cutting raw poultry, storage of prepared foods in the refrigerator).
The thousands of different strains of Salmonella are also known as serotypes. These designations indicate that the differences between the strains lie mainly in the chemically different composition of the outer surface of the bacteria. The differences elicit different immune responses. The immune response for a particular strain is characteristic and can be useful in identifying the strain of Salmonella that is causing the malady. Salmonella enteriditis is of particular concern in food poisoning. This strain causes gastroenteritis and other maladies.
Strains like Salmonella enteriditis can establish infection because they have components that contribute to the infection. These components are classified as virulence factors. One of these factors is called adhesin. An adhesin functions in adhesion of the bacterium to a receptor on the surface of a host cell. An example of an adhesin is the tube-like structures called fimbriae that stick out from the surface of the cell.
Another virulence factor is lipopolysaccharide (LPS for short). LPS can help shield the surface of the bacterium from host antibacterial compounds. A part of the LPS called lipid A can cause a fever and a subcomponent of lipid A called endotoxin is harmful to the host. Salmonella also produces other toxins (e.g., enterotoxin). Because these toxins remain inside the bacterial cell, the increased number of bacteria that results from multiplication in the contaminated food increases the amount of the toxin ingested, and so increases the severity of symptoms.
The identification of Salmonella is not particularly difficult. Well-known lab media of defined composition exist, on which the bacteria grow and produce characteristic colours. For example, Salmonella grows on bismuth sulfide media and produces jet-black colonies, due to the production of hydrogen sulfide.
Unfortunately, however, the accurate diagnosis of Salmonella food poisoning usually comes after the fact, if the illness is diagnosed at all.
A vaccine that blocks the adhesion of the bacteria to the intestinal epithelial cells, which is a crucial part of the infection, could conceivably be developed. However, even if such a vaccine is possible, other vaccine needs that are more pressing currently occupy dedicated research resources. For the foreseeable future, the best strategy in preventing Salmonella food poisoning will remain the cooking of foods such as meat to the proper temperature for a recommended time, the proper storage of prepared foods, and good hygiene.
█ FURTHER READING:
Fox, Nicols. Spoiled: Why Our Food Is Making Us Sick and What We Can Do about It. New York: Penguin USA 1998.
Salyers, Abigail, A., and Dixie D. Whitt. Bacterial Pathogenesis: A Molecular Approach. Washington, DC: American Society for Microbiology Press, 2001.
Centers for Disease Control and Prevention. "Salmonella Enteritidis." Division of Bacterial and Mycotic Diseases. April 25, 2001. < http://www.cdc.gov/ncidod/dbmd/diseaseinfo/salment_g.htm > (08 March 2003).
United States Food and Drug Administration. "Salmonella spp." Center for Food Safety and Applied Nutrition. January 10, 2003. < http://vm.cfsan.fda.gov/~mow/chap1.html > (02 March 2003).