Salmonella

Salmonella species

Organism

Salmonella are moderately-sized gram-negative bacilli.
Possess a peritrichous (def) arrangement of flagella.
Facultative anaerobes (def).

Habitat

The intestinal tract in humans and animals.

Source

Human or animal feces containing Salmonella; transmitted by the fecal-oral route. Since many different animals carry Salmonella in their intestinal tract, people usually become infected from ingesting improperly refrigerated, uncooked or undercooked poultry, eggs, meat, or dairy products contaminated with animal feces.

Epidemiology

Around 50,000 cases of salmonellosis are reported annually in the U.S. but most cases go unreported. An estimated 2,000,000 - 3,000,000 people a year in the U.S. become infected with Salmonella and at least 500 die.
The majority of Salmonella cause diarrhea, but one species, S. typhi, frequently disseminates into the blood and causes a severe form of salmonellosis called typhoid fever. In 1998, 375 cases of typhoid fever were reported in the U.S. but most of these cases were acquired during foreign travel

Clinical Disease

Enteritis (def) is the most common form of salmonellosis. Symptoms generally appear 6-48 hours after ingestion of the bacteria and include vomiting, nausea, nonbloody diarrhea, fever, abdominal cramps, myalgias (def), and headache. Symptoms generally last from 2 days to 1 week followed by spontaneous resolution.
All species of Salmonella can cause bacteremia (def) but S. typhi, S. paratyphi, and S. choleraesuis are the most common species to cause bacteremia.

Pathogenicity

In the outer membrane of the gram-negative cell wall, the lipopolysaccharide functions as an endotoxin . Endotoxin, especially when in the blood, can lead to inflammation, high fever, hypotension, capillary damage, intravascular coagulation, tissue degradation, and irreversible shock. Death is a result of what is called the shock cascade (see Fig. 1).
Cell wall adhesins (def) enable the bacterium to make a more intimate contact with the mucous membranes.
Invasins (def) enable Salmonella to invade M cells located in the Peyer's patches of the small intestines. (M cells are phagocytic cells in the mucous membrane whose function is to sample microbes from the intestinal lumen and pass them on to the lymphoid tissue of the Peyer's patch in order to activate the immune defenses against intestinal microbes). Once inside the M-cell, the Salmonella replicate within the phagosome, subsequently killing the cell and spreading to adjacent cells.
Salmonella are protected from the acid pH of phagosomes and stomace acids by an acid tolerance response (ATR) gene.

Treatment

Antibiotic treatment is not recommended for Salmonella enteritis. Infections with S. typhi and S. paratyphi or other Salmonella septicemia can be treated, based on antibiotic susceptibility testing, with antibiotics such as fluroquinolones, chloramphenicol, trimethoprim-sulfamethoxazole (TMP-SMX), or a broad-spectrum cephalopsporin* (see antibiotic table).

*Drugs may change with time.

For a more detailed article on Salmonella infections, see Salmonellosis, by Michael Zapor, MD, PhD, Fellow, Infectious Disease Section, Department of Medicine, Walter Reed Army Medical Center and David P Dooley, MD, Chief, Associate Fellowship Director, Department of Medicine, Infectious Disease Service, Brooke Army Medical Center; Associate Professor, Department of Medicine, University of Texas at San Antonio.