The salmonellae are gram-negative, non-spore-forming, facultatively anaerobic bacteria in the family Enterobacteriaceae. More than 2500 different serotypes of Salmonella have been identified.

Salmonellae are widely distributed in nature. They are generally found in the gastrointestinal (GI) tracts of the hosts with which they are associated. Some salmonellae, such as Salmonella typhi and Salmonella paratyphi, are found to colonize only the human GI tract. Other Salmonella serotypes, such as Salmonella typhimurium, have a wide range of hosts, including humans. Finally, some organisms, such as Salmonella dublin and Salmonella arizona, are rarely found in the GI tracts of humans. The specificity and range of the different serotypes helps to determine the epidemiology of infections caused by these bacteria.

Infections caused by the salmonellae are grouped into three major syndromes: gastroenteritis, typhoid or enteric fever, and localized infec- tion outside of the GI tract. Although there is considerable overlap between these syndromes, their epidemiology and clinical presentations are distinct enough to make discussion by syndrome useful.

GASTROENTERITIS

Gastroenteritis accounts for most Salmonella infections in humans. The incidence of Salmonella gastroenteritis in the United States doubled during the 1980s and 1990s. Much of this increase was attributed to the widespread contamination of chickens and eggs as the industry became increas- ingly centralized. While the rate of Salmonella gastroenteritis stabilized in the late 1990s, due to increased public awareness and improved sanita- tion during commercial processing, since then this rate has increased by 44%. It is estimated that there are 1.2 million episodes of Salmonella gastroenteritis annually in the United States, resulting in 20 000 hospitalizations and 400 deaths.