Salmonella Infection (Salmonellosis)
Salmonella Infection (Salmonellosis)
Salmonellosis refers to a human infection that is caused by bacteria in a genus called Salmonella. Contamination of food by the bacteria is a common cause of salmonellosis.
Salmonellosis due to the contamination of food can be a food infection or a food intoxication, depending on the antigenic type (serotype) of Salmonella involved. A food infection relies on the growth of the bacteria to levels capable of causing symptoms. Growth of the contaminating strain is not necessary for a food intoxication since it is a toxin that has already been produced by the contaminating bacteria that cause the illness. Salmonellosis is most often a food infection, but if enough toxin-loaded bacteria are ingested, salmonellosis can be an intoxication.
Salmonellosis is common and widespread. Of particular concern, Salmonella have emerged that are resistant to many commonly used antibiotics.
Salmonella is a Gram-negative, rod-shaped bacterium. It is named after Daniel Salmon (1850–1914), who, with Theobald Smith (1859–1934), isolated the bacterium from pigs in 1885. Since then, over 2,500 different serotypes of the bacterium have been found; the term serotype indicates the protein composition of the bacterial surface, which produces a distinct immune response by the host. The many different serotype indicates that the surface of Salmonella is highly variable.
The bacterium is commonly found in the gastrointestinal tract of humans and other animals. In this environment it is of no concern. However, if food or water contaminated with Salmonella-containing feces are ingested, illness can result. Like other fecal bacteria, food contamination most often occurs when the food is handled by someone who has not properly washed their hands after having a bowel movement. Good hygiene is important in minimizing the risk of salmonellosis.
WORDS TO KNOW
CONTAMINATED: The unwanted presence of a microorganism or compound in a particular environment. That environment can be in the laboratory setting, for example, in a medium being used for the growth of a species of bacteria during an experiment. Another environment can be the human body, where contamination by bacteria can produce an infection. Contamination by bacteria and viruses can occur on several levels and their presence can adversely influence the results of the experiments. Outside the laboratory, bacteria and viruses can contaminate drinking water supplies, foodstuffs, and products, causing illness.
ENTEROTOXIN: Enterotoxin and exotoxin are two classes of toxin that are produced by bacteria.
LIPOPOLYSACCHARIDE (LPS): Lipopolysaccharide (LPS) is a molecule that is a constituent of the outer membrane of Gram-negative bacteria. The molecule can also be referred to as endotoxin. LPS can help protect the bacterium from host defenses and can contribute to illness in the host.
SEROTYPES: Serotypes or serovars are classes of microorganisms based on the types of molecules (antigens) that they present on their surfaces. Even a single species may have thousands of serotypes, which may have medically quite distinct behaviors.
TOXIN: A poison that is produced by a living organism.
Salmonellosis is caused most often by two strains: S. typimurium and S. enteritidis. Other serotypes of the bacterium usually cause disease in animals such as cattle and pigs. If these serotypes infect humans, the infection can be severe and even life-threatening.
Poultry carcasses can be contaminated with intestinal contents during slaughter of the bird. The bacteria can remain alive long enough for the carcass to be shipped to a grocery store and sold. The bacteria are readily killed by heat. But, if cooking is inadequate, the surviving organisms are capable of causing illness. Eggs can also be contaminated if the shell has a crack or break, which allows the bacteria to enter the inside of the egg. Other foods that are often involved in salmonellosis are raw meat (if it is undercooked), processed meat, dairy products, custards and cream-based desserts, and sandwich filling such as tuna salad or chicken salad.
Symptoms of salmonellosis develop within a few hours of eating contaminated food. The symptoms include abdominal cramping, nausea with vomiting, fever, headache, chills and sweating, a feeling of weakness, and loss of appetite. Some people also develop watery diarrhea or—if cells lining the intestine are damaged—bloody diarrhea. The rapid loss of fluids due to diarrhea can be dangerous to infants and the elderly. As well, less commonly the infection spreads to the bloodstream. Some people can develop a painful condition called Reiter's syndrome, which can persist for years and which can lead to arthritis.
For most people, the infection lasts 4–7 days, and most people recover without needing medical attention. However, severe diarrhea usually results in hospitalization.
Outbreaks of salmonellosis can occur, due to the consumption of contaminated food in a restaurant or at a social gathering. In recent example, an outbreak due to S. typhimurium that occurred in 21 of the United States in September, 2006 was traced to the consumption of contaminated tomatoes at restaurants. However, a number of studies have indicated that more than 80% of cases occur individually. This is unfortunate, according to the World Health Organization (WHO), as it diverts media attention from a serious global problem, especially in developing and underdeveloped countries.
The Salmonella that cause salmonellosis possess what are termed virulence factors; molecules that enable the bacteria to establish an infection. One important virulence factor is called adhesin. This is a molecule that can recognize a target site on the host cell and help the bacterium adhere to the host cell target. An example of a Salmonella adhesin are tubes called fimbriae that stick out from the bacterial surface. The end of each fimbriae contains a protein that can bind with a specific host cell surface protein.
Another virulence factor is called lipopolysaccharide (LPS). There are many different structures of LPS. Those that are longer can help shield the bacterial surface from host compounds that can damage or kill the bacteria. Furthermore, a part of LPS called lipid A is a toxin.
Some strains of Salmonella also produce a toxin called enterotoxin. This toxin is located inside the bacteria, so as the numbers of Salmonella increase, the concentration of the enterotoxin in the food increases. Ingesting the food releases the enterotoxin in the intestine, where it ruptures the intestinal cells by forming a hole in their cell membrane.
Salmonellosis is global in occurrence and common. According to data from the United States Centers for Disease Control and Prevention (CDC), more than 40,000 cases of salmonellosis are reported each year in the United States. Since many more cases are never reported, the actual total is much higher—1.4 million cases, according to CDC. Approximately 1,000 people in the United States die of salmonellosis-related complications every year.
Diagnosis of salmonellosis relies on recognition of its symptoms and the identification of Salmonella from a stool (fecal) sample. Current tests that detect certain Salmonella proteins do not require growth of the bacteria, and thus can be completed within hours.
Identification of the type of Salmonella involved usually helps in determining which antibiotics to use. Salmonellosis usually responds well to antibiotics, however, serotypes of Salmonella that are resistant to a variety of antibiotics exist and are becoming more common.
Prevention involves good hygiene including handwashing and the cleaning of cooking utensils and equipment that have been used with foods such as poultry and ground meat before their re-use. Foods containing raw eggs should not be eaten; even if the eggs appeared intact, cracks that are not visible to the eye are large enough to allow bacteria to contaminate the egg.
Researchers are exploring the production of a vaccine against salmonellosis. The most promising strategy is to block the adhesion of the bacteria to the intestinal cells. This strategy has proven successful in developing a vaccine that appeared on the market in 2006 for another intestinal bacterium called Escherichia coli O157:H7 (E. coli O157:H7).
Salmonellosis has major economic impacts. Millions of people each year miss work and school because of the illness. Health care dollars are spent looking after those who become hospitalized. Exact figures are difficult to obtain, especially from developing countries, as they do not report on salmonellosis. But, in the United States, the estimated 1.4 million annual number of cases of salmonellosis results in the hospitalization of 15,000 people. The annual total medical cost of dealing with salmonellosis in the U.S. is estimated to be $1 billion. Other costs due to lost productivity and lost wages push the total cost to an estimated $3 billion. In Denmark, food-related salmonellosis cost the economy $14 million in lost wages and health care costs in 2001.
In February 2007, Salmonella-contaminated peanut butter was responsible for a nationwide salmonellosis outbreak in the United States. The FDA warned consumers not to purchase or eat certain brands of peanut butter manufactured at a facility in Georgia. Companies with brands associated with the salmonellosis outbreak recalled all potentially contaminated products, including peanut butter for home use and commercial peanut butter products used by some fast-food chains. The Salmonella-contaminated foods associated with outbreak affected approximately 370 people in over 40 states. While salmonellosis is typically associated with poultry products, the 2007 outbreak was not the first associated with peanut butter. A similar salmonellosis event that occurred in Australia in the mid–1990s was traced to contaminated peanut butter.
IN CONTEXT: EFFECTIVE RULES AND REGULATIONS
According to the Division of Bacterial and Mycotic Diseases at Centers for Disease Control and Prevention (CDC), the CDC “monitors the frequency of Salmonella infections in the country and assists the local and State Health Departments to investigate outbreaks and devise control measures. CDC also conducts research to better identify specific types of Salmonella. The Food and Drug Administration inspects imported foods, milk pasteurization plants, promotes better food preparation techniques in restaurants and food processing plants, and regulates the sale of turtles. The FDA also regulates the use of specific antibiotics as growth promotants in food animals. The US Department of Agriculture monitors the health of food animals, inspects egg pasteurization plants, and is responsible for the quality of slaughtered and processed meat. The US Environmental Protection Agency regulates and monitors the safety of drinking water supplies.”
SOURCE: Centers for Disease Control and Prevention (CDC), Coordinating Center for Infectious Diseases, Division of Bacterial and Mycotic Diseases.
The human suffering and economic consequences of salmonellosis is likely to increase with the continuing spread of Salmonella serotypes that are resistant to a variety of commonly used antibiotics. The WHO is trying to determine the global prevalence and antibiotic resistance patterns of the multi-drug resistant Salmonella through its Global Salm-Surv program.
See AlsoFood-borne Disease and Food Safety.
Prescott, Lansing M., John P. Harley, and Donald A. Klein. Microbiology. New York: McGraw-Hill, 2004.
Tortora, Gerard J., Berell R. Funke, and Christine L. Case. Microbiology: An Introduction. New York: Benjamin Cummings, 2006.
United States Food & Drug Administration. Bad Bug Book: Foodborne Pathogenic Microorganisms and Natural Toxins Handbook. McLean: International Medical Publishing, 2004.