What is salmonella?
Salmonella are gram-negative, motile, non-spore-forming, nonencapsulated, facultative-anaerobic rods that cause several diseases, primarily enteric (intestinal), in humans and other animals.
The genus Salmonella was named for Daniel E. Salmon, an American veterinary pathologist and bacteriologist. The type strain, originally named S. choleraesuis, was discovered by Salmon’s research associate, Theobald Smith. The genus is closely related to Escherichia coli (E. coli) and was initially subdivided into hundreds of species named for the diseases it caused and for the host organism: for example, S. typhi (typhoid fever), S. enteritidis (gastroenteritis), S. typhimurium (mouse typhoid), and S. choleraesuis (hog cholera).
After further genetic testing and after scientists determined that most Salmonella spp. are not very host specific, most of the original species were combined into a single species, S. enterica. This species was then divided into five subspecies and more than two thousand strains or serovars; for example, S. enterica subsp. enterica serovar Typhi has replaced S. typhi, S. enterica subsp. enterica serovar Enteritidis has replaced S. enteritidis, and S. enterica subsp. arizonae has replaced S. arizonae. Only S. bongori was deemed distinct enough to stand alone as a different species. The older designations are still frequently used in both professional journals and the popular press.
According to the Centers for Disease Control and Prevention (CDC) in 2015, 2,500 different strains (serotypes) of Salmonella had been identified, with fewer than 100 responsible for most human disease. S. enterica subsp. enterica contains the majority of disease-causing strains. Salmonellosis is the second most common cause of gastroenteritis, surpassed only by Campylobacter spp. infections. Infections caused by Salmonella spp. are considered zoonotic because many strains of this bacterium can be transferred from humans to animals and from animals to humans.
Most cases of salmonellosis are the result of fecal to oral contamination caused by ingestion of fecal-contaminated food. Because Salmonella spp. are so widespread and can survive several weeks in water or on vegetation and more than two years in soil, transmission is relatively easy. For example, during butchering and processing, raw meats can become contaminated with the intestinal contents of the butchered animals. Shellfish are easily contaminated when raw sewage makes its way into aquatic habitats. An infected chicken can deposit Salmonella into her eggs before shell deposition or on the shell as the egg is laid. Irrigating or washing crop plants in water contaminated by Salmonella can contaminate the crops. Food preparers with poor hygiene can also contaminate food.
After a multistate outbreak of Salmonella Heidelberg that involved more than six hundred Americans was sourced to chicken parts that had been packaged by Foster Farms, the US Department of Agriculture's Food and Safety Inspection Service (FSIS) determined that regulations and standards needed to be set for individual chicken parts as well as whole chickens. In 2015, the FSIS proposed the new standards designed to decrease consumer exposure to Salmonella in all chicken products.
In addition to food contamination, pets, especially birds, reptiles, and amphibians, can harbor Salmonella, which can easily be transferred from their cloacae to their feathers or skin.
The best way to prevent salmonellosis is to always wash one’s hands after using the toilet, handling raw meat, cleaning up feces, and handling a bird, reptile, or amphibian. In addition, cooking all food to an internal temperature of 167° Fahrenheit (75° Celsius) and boiling water for a minimum of one minute kills Salmonella. Freezing, however, will usually not kill all Salmonella in contaminated food or water. Cutting boards used for raw meat should also be cleaned thoroughly, preferably with bleach.
In humans, one of the most serious forms of salmonellosis is typhoid fever caused by S. enterica sub. enterica serovar Typhi, also called S. typhi. This bacterium is highly adaptable and can produce stress-related proteins that allow the bacterium to survive better under environmental stresses (such as increased temperatures, acidic conditions, and the presence of antibiotics). Unlike many Salmonella strains, this bacterium has only one animal reservoir: humans. It is usually transmitted through contaminated water and undercooked, contaminated food. Because of this, it causes most problems in developing countries with poor sanitation.
S. typhi can also be transmitted by food-service workers who were previously infected. About 5 percent of all persons who had typhoid fever retain infective bacteria and can pass these along. In the United States, food service workers who have had typhoid fever are required to be free of the typhoid bacterium (as measured by fecal swabs) before they can return to work.
Diarrhea is the most common symptom of salmonellosis, but bacteria can enter the intestinal epithelium and migrate to other areas of the body, causing fever, headache, rose-colored spots on the upper chest, and organ inflammation. Humans heterozygous for cystic fibrosis may have lowered susceptibility to typhoid fever because the changes in the cell membrane of heterozygotes decrease the likelihood of bacterial invasion.
As of 2013, the CDC reported that approximately 5,700 cases of typhoid fever occur in the United States each year, and most of these cases are contracted outside the country. Also according to the CDC, in the developing world, nearly twenty-two million cases of typhoid are seen each year. Most typhoid deaths are caused by dehydration, so rehydration therapy is critical in treating this or any other salmonellosis.
Bacterial strains resistant to ampicillin, chloramphenicol, trimethoprim/sulfamethoxazole, and streptomycin are so common that these antibiotics are no longer used. Ciprofloxacin is the drug of choice, but strains resistant to it are on the rise. Cephtriaxone and cephotaxime are being used more often, especially in areas with multiple resistance. Both oral and injectable vaccines are available, but these are only 50 to 70 percent effective. A very similar, but less common disease is paratyphoid fever, which is caused by the Paratyphoid serovar.
S. enterica serovar Enteritidis is the most frequent cause of Salmonella gastroenteritis in humans. Many other serovars can also cause gastroenteritis. The most common symptoms are diarrhea, abdominal cramps, and nausea. Unlike typhoid fever, gastroenteritis in healthy persons rarely lasts more than one week, although in rare cases it can become systemic. It is usually treated with rehydration therapy and is not always treated with antibiotics unless it is severe.
Braden, Christopher R. “ Salmonella enterica Serotype Enteritidis and Eggs: A National Epidemic in the United States.” Clinical Infectious Diseases 43 (2006): 512–17. Print.
Garrity, George M., ed. Bergey’s Manual of Systematic Bacteriology. 2nd ed. Vol 2. New York: Springer, 2005. Print.
Madigan, Michael T., and John M. Martinko. Brock Biology of Microorganisms. 12th ed. Upper Saddle River: Pearson, 2010. Print.
Romich, Janet A. Understanding Zoonotic Diseases. Clifton Park: Thomson, 2008. Print.
"Typhoid Fever." Centers for Disease Control and Prevention. US Dept. of Health and Human Services, 14 May 2013. Web. 31 Dec. 2015.
"USDA Proposes New Measures to Reduce Salmonella and Campylobacter in Poultry Products." United States Department of Agriculture. USDA, 21 Jan. 2015. Web. 31 Dec. 2015.