Salmonella bacteria are known to cause salmonellosis, typhoid fever, and paratyphoid fever in humans. Salmonella infection is usually caused by ingesting large amounts of the bacteria in contaminated food or water.
Dr. Charles "Pat" Davis, MD, PhD, is a board certified Emergency Medicine doctor who currently practices as a consultant and staff member for hospitals. He has a PhD in Microbiology (UT at Austin), and the MD (Univ. Texas Medical Branch, Galveston). He is a Clinical Professor (retired) in the Division of Emergency Medicine, UT Health Science Center at San Antonio, and has been the Chief of Emergency Medicine at UT Medical Branch and at UTHSCSA with over 250 publications.
Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.
Salmonella (S.) is the genus name for a large number (over 2,500) of types of bacteria. Each type is distinctly identifiable microscopically by its specific protein coating. The types are otherwise closely related. Salmonella bacteria are rod-shaped, flagellated, Gram stain-negative, and are known to cause disease in humans, animals, and birds (especially poultry) worldwide. The two major diseases caused by Salmonella spp. are gastroenteritis (also termed non-typhoidal salmonellosis or Salmonella poisoning) and typhoid fever (typhoid and paratyphoid fevers) in humans. Infections caused by these bacteria or their toxins are called salmonellosis, a general term. This article will present both the non-typhoidal and typhoidal salmonellosis, which are closely related.
The terminology that identifies the particular protein coats, or serovars, is complex, and what previously were thought to be various species of the genus Salmonella are now thought by many researchers to be serovars of only two species,
S. enterica and S. bongori. However, these designations are not always accepted in the scientific literature. Therefore, common serovars that have been named in the past are still used (for example,
S. typhi, S. typhimurium, S. paratyphi, S. enteritidis,
S. cholerasuis, S. saintpaul). Minor variations in some serovars are termed subspecies and assigned a number. The serovars are identified by the Kauffman-White classification that uses two major types of antigens (somatic O, along with envelope antigens that may mask O antigens, and flagellar or H antigens) to distinguish the over 2,500 types of Salmonella bacteria. Sometimes laboratories or other reporting agencies identify isolates simply as Salmonella spp. (species) and do not identify the serovars. Nomenclature of these closely related bacteria is likely to remain in flux, even in the current literature. For example, a proposed correct taxonomic name for the organism that causes typhoid fever is Salmonella enterica ssp. enterica, serovar typhi. The simplified version is Salmonella typhi.
The impact of Salmonella bacteria in history is substantial. After examining descriptions of his illness and death, investigators suggest that Alexander the Great died from typhoid in 323 BC. The bacteria seem to thrive when sanitary conditions decline, especially in wars. The bacteria were first isolated
from pigs by Theobald Smith in 1885. The genus name Salmonella was derived from the last name of D.E. Salmon, who was Smith's director. In 1896, diagnosis of Salmonella spp. infection in humans was accomplished. One of the first recognized outbreaks happened in 1899 when British troops in South Africa were decimated by typhoid. Of those troops, about 13,000 deaths were due to the disease while 8,000 were due to warfare! The first vaccine available in the U.S. was administered to U.S. troops in the early 1900s.
A famous carrier of Salmonella was Mary Mallon, a cook who was found to be the source of several typhoid outbreaks in the U.S. (1906-1907). At the time, the typhoid "carrier" situation (a carrier sheds the pathogen but is not sick) was not widely understood in the early 1900s. She was known as "Typhoid Mary" and was forced to stop being a food handler. After being out of her job and quarantined on an island for about
two years, she was set free and was instructed again not to be a food handler. Nevertheless, she changed her name and did become a food handler and eventually caused several other outbreaks of typhoid with resulting deaths. She was quarantined to an island for over 20 years and died in 1938.
In addition, this highly contagious bacterium has been used as a terrorist tool. In1984, salad bars in Oregon were intentionally contaminated with the bacteria. Studies of Salmonella pathogenic mechanisms have given insight to how bacteria cause disease and are ongoing.
Medical Author: Melissa Conrad Stöppler, MD
Medical Editor: William C. Shiel Jr., MD, FACP, FACR
The pistachio nut recall in March 2009 is only one example of numerous product recallsin recent years due to fears of contracting Salmonella food poisoning. Similarly, this year products processed by the Peanut Corporation of America (PCA) were found to be the source of a Salmonella outbreak. In 2008, an outbreak arose from the consumption of certain jalapeño and serrano peppers from Mexico. As these and numerous other outbreaks illustrate, virtually any food can become contaminated with one of the many species of Salmonella.
Salmonella is a bacterial infection that is passed to humans from animals, including poultry, cattle, pigs, and domestic animals. Eating undercooked poultry and drinking unpasteurized milk are among the ways humans can acquire the infection. But increasing media coverage has focused on vegetable products as the source of many Salmonella outbreaks. When vegetables or fruits are the source of an outbreak, it means that these products have been handled unsafely, such as processing or preparation on surfaces that have become contaminated with animal feces or raw poultry. Another way for vegetables to become contaminated is by an infected food handler.