Salmonella (cont.)

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How do Salmonella spp. cause disease in people?

Usually, Salmonella spp. are ingested. It is commonly accepted that at between 1 million to 1 billion bacteria are needed to cause infection although some investigators suggest some people may be infected by far fewer bacteria. Nevertheless, most data suggest food, water, or other sources of contamination contain large amounts of bacteria. Although human stomach acid can reduce and sometimes eliminate Salmonella spp., occasionally some bacteria get through to the intestine and then attach and penetrate the cells. Toxins produced by the bacteria (enterotoxin and cytotoxin) can damage and kill the cells that line the intestines, which results in intestinal fluid loss (diarrhea).

Some Salmonella can survive in cells of the immune system and can reach the bloodstream, causing blood infection (bacteremia). Other Salmonella spp. can enter the gallbladder, leaving the affected patient a chronic carrier of the organisms. Salmonella can then be shed with the bile from the gallbladder into the feces and then may infect other people.

How are Salmonella infections diagnosed?

Feces of patients that have symptoms of food poisoning or typhoid fever are cultured for Salmonella. Often, the preliminary diagnosis is first suggested by recognition of an outbreak (multiple patients having symptoms of a disease all at about the same time and often from the same food or water source). Again, many organisms and toxins can cause food poisoning, so it is sometimes difficult to determine the outbreak's cause. Usually, the laboratory needs to be notified that S. spp. is suspected as the cause so they will choose the correct testing media (selective agar media such as SS agar plates) to distinguish Salmonella from other potential bacterial pathogens such as E. coli, Shigella strains, Campylobacter, Staphylococcus food poisoning, or from toxins like botulism or pesticides. The majority of Salmonella isolates come from the feces of the infected person. Occasionally, Salmonella can be cultured from blood samples. Serovars are identified by serotyping (detecting bacterial proteins by using specific immunological tests). Definitive diagnosis usually requires that the bacteria be isolated and identified by these techniques. PCR tests also have been used to distinguish between bacterial strains.

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