Salmonella

GENERAL CONCEPTS:

The diseases produced by different species of Salmonella are collectively known as "salmonelloses". These diseases occur worldwide and are most generally manifested as a self-limiting gastroenteritis.
Salmonellae are pathogenic because of their capacity to i) invade intestinal mucosa and ii) produce toxins.
The salmonellae infect a variety of animals, resulting in a large animal reservoir. S. typhi is more specific to humans, however.
Approximately 2000 serotypes of Salmonella are known.

DISTINCTIVE PROPERTIES:

The genus Salmonella is a member of the family Enterobacteriaceae. The genus is composed of Gram-negative bacilli that are facultative and flagellated (motile).
Salmonellae possess 3 major antigens; the "H" or flagellar antigen (phase 1 & 2), the "O" or somatic antigen (part of the LPS moiety) and the "Vi" or capsular antigen (referred to as "K" in other Enterobacteriaceae).
Salmonellae also possess the LPS endotoxin characteristic of Gram-negative bacteria. This LPS is composed of an "O" polysaccharide ("O" antigen) an "R" core and the endotoxic inner "Lipid A". Endotoxins evoke fever and can activate complement, kinin and clotting factors.

PATHOGENESIS:

Salmonellosis may present as one of several syndromes including gastroenteritis, enteric (typhoid) fever or septicemia.
Disease is initiated by oral ingestion of the bacteria followed by colonization of the lower intestine. The bacteria are capable of mucosal invasion, which results in an acute inflammation of the mucosal cells. This then leads to the activation of adenylate cyclase, increased fluid production and release of fluid into the intestinal lumen, resulting in diarrhea.
Salmonella gastroenteritis is the most common form of salmonellosis and generally requires an 8-48 hour incubation period and may last from 2-5 days. Symptoms include nausea, vomiting and diarrhea. Salmonella enteritidis is the most common isolate.
Enteric or typhoid fever occurs when the bacteria leave the intestine and multiply within cells of the reticuloendothelial system. The bacteria then re-enter the intestine, causing gastrointestinal symptoms. Typhoid fever has a 10-14 day incubation period and may last for several weeks. Salmonella typhi is the most common species isolated from this salmonellosis.
Salmonella septicemia (bacteremia) may be caused by any species but S. cholerae-suis is common. This disease resembles other Gram-negative septicemias and is characterized by a high, remittent fever with little gastrointestinal involvement.

HOST DEFENSES:

Several factors are important in preventing Salmonella infections including:
- Gastric factors such as the acidity or rate of emptying of the stomach contents. These factors affect the number of organisms that reach the intestine.
- Intestinal factors such as motility, the presence of normal flora, the amount of mucus, the presence of secretory IgA specific for Salmonella and genetic components. These factors affect the organism's ability to colonize and penetrate the intestinal mucosa.
- Nonspecific factors such as nutrition, the presence of iron-binding proteins and the enzyme lysozyme. These factors also affect the survivability of ingested Salmonella.

EPIDEMIOLOGY:

The source of organisms for Salmonella gastroenteritis include contaminated food or water. Most commonly, persons acquire Salmonella from contaminated poultry (turkeys and chickens). S. enteritidis or S. choleraesuis are the most commonly isolated species.
Enteric fever, in contrast, is generally transmitted from person to person and involves S. typhi(no animal reservoirs). Contamination of food or water with human feces and an asymptomatic human carrier state provide the reservoir.

DIAGNOSIS:

Clinical: Clinical diagnosis of the salmonelloses is often difficult because the symptoms closely resemble other diarrheal diseases. Isolation of the organism is required for positive identification.
Laboratory: Salmonella can be readily isolated and characterized using standard bacteriologic media or rapid identification systems. Salmonellae are motile, incapable of fermenting lactose and produce H₂S. Serological techniques may be used for epidemiological characterization.

CONTROL:

Sanitary: Since salmonellae are acquired through ingestion of contaminated foodstuffs, sanitary means of control are most important. Treatment of animal feeds reduces the overall level of organisms in the animal population, improved slaughtering practices prevent cross-contamination of animal products and proper hygienics by food-handlers prevents contamination at the consumer level.
Immunological: A vaccine for typhoid is available, since only one serotype is responsible for the disease. However, the vaccine is not very effective because the bacteremic stage (i.e. where bacteria contact the vaccine-induced antibody) is brief.
Chemotherapeutic: Typhoid fever and Salmonella septicemia may be treated using moderate to broad spectrum antibiotics. Gastroenteritis should only be treated by replacing lost fluids, since antibiotic therapy does not affect the course of the disease and may only increase the number of resistant species.