The bacterium Salmonella is a widespread pathogen responsible for millions of foodborne illnesses and numerous deaths globally each year. Its historical identification is intertwined with a classic case of mistaken credit in late 19th-century American science. The story of who truly discovered this microbe reveals a historical twist concerning the naming convention that has endured for over a century.
The Naming and the True Discoverer
The genus Salmonella is named in honor of Daniel Elmer Salmon, a respected veterinary surgeon and the first chief of the United States Department of Agriculture’s Bureau of Animal Industry (BAI). Salmon oversaw extensive research into animal diseases, including the devastating swine plague, or hog cholera, in the 1880s.
The actual isolation and identification of the first strain occurred in 1885, not by Salmon, but by his young colleague, Theobald Smith. Smith, a bacteriologist in Salmon’s laboratory, successfully isolated the organism from the intestines of pigs afflicted with hog cholera. The strain was initially named Bacillus suipestifer, later becoming Salmonella choleraesuis.
The researchers mistakenly believed this bacterium caused hog cholera, which was later proven to be a viral disease. Despite Smith performing the isolation work, Salmon, as the division chief, often took primary credit for the laboratory’s findings. The current genus name was established in 1900, when French bacteriologist Joseph Leon Lignières proposed Salmonella to honor Salmon’s administrative leadership.
What Exactly Is Salmonella?
Salmonella is a genus of rod-shaped, Gram-negative bacteria. It is generally motile due to long, whip-like appendages called flagella. It belongs to the Enterobacteriaceae family, a large group of bacteria that includes many common gut inhabitants. The bacteria are facultatively anaerobic, meaning they can survive and grow both with and without oxygen.
The genus is divided into two species, Salmonella enterica and Salmonella bongori, with S. enterica responsible for almost all human infections. Within S. enterica, scientists have identified over 2,600 distinct types, or serovars, based on surface antigens. These bacteria primarily colonize the intestinal tracts of animals, including poultry, swine, cattle, and reptiles, which serve as reservoirs for the pathogen.
Salmonella is known for its ability to survive in various harsh environments outside of a host for extended periods. It can tolerate a wide pH range, generally from 4.0 to 9.5, and demonstrates resilience in conditions of low water activity, such as in dried foods or chocolate. This hardiness contributes to its persistence in the food production chain, surviving even in cold storage temperatures.
Understanding Salmonellosis
In humans, infection with Salmonella causes salmonellosis, which manifests in two distinct forms depending on the serovar involved.
Non-typhoidal Salmonellosis
The most common illness is Non-typhoidal Salmonellosis, typically caused by serovars like S. Typhimurium and S. Enteritidis. This form presents as gastroenteritis, a localized infection of the small and large intestines. Symptoms usually appear six hours to six days after exposure and include diarrhea, abdominal cramps, and fever. This infection is generally self-limiting, resolving within four to seven days without antibiotic treatment, though dehydration can be a concern.
Typhoidal Salmonellosis
The second, far more severe form is Typhoidal Salmonellosis, known as enteric fever, caused by serovars like S. typhi and S. Paratyphi. Unlike non-typhoidal strains, these serovars are specifically adapted to humans and cause a systemic infection that spreads beyond the gut. The bacteria penetrate the intestinal wall and disseminate to organs like the liver, spleen, and gallbladder via the bloodstream.
Enteric fever is characterized by a gradual onset of sustained high fever, sometimes accompanied by a rash or extreme fatigue. The typhoidal serovars have specialized genetic factors, such as the Vi antigen in S. typhi, which helps the bacteria evade the host’s initial inflammatory response. Without proper treatment, this invasive illness can have a mortality rate as high as 30%.
Prevention and Public Health Impact
The primary transmission route for Salmonella is the fecal-oral pathway, often occurring through the consumption of contaminated food or water. Contamination frequently originates from animal products like raw or undercooked poultry, eggs, and meat, or from produce that has come into contact with animal feces. Cross-contamination in the kitchen, where raw food juices touch ready-to-eat items, is also a common cause of infection.
Public health efforts focus on preventing transmission through rigorous food safety practices. A fundamental measure is thoroughly cooking foods, as the bacteria are destroyed by heat; ground meats require an internal temperature of 160°F. Preventing cross-contamination involves:
- Using separate cutting boards for raw meats and produce.
- Meticulous hand washing with soap for at least 20 seconds.
- Washing hands particularly after handling raw ingredients or pets.
The global impact of Salmonella is substantial, making it one of the most monitored foodborne pathogens worldwide. Surveillance systems track outbreaks and identify emerging serovars and antibiotic resistance patterns. The understanding of the organism’s diversity laid the foundation for modern food safety regulations and the development of vaccines, particularly for the typhoidal strains prevalent in developing regions with poor sanitation.