Salmonella

# Overview
Salmonella is a rod-shaped, facultative intracellular, gram-negative enterobacteria. Salmonella species shows motility, produces hydrogen sulfide, and only 1% is able to ferment lactose. It may be isolated from the stool of infected patients, and grown in culture media, such as MacConkey agar and deoxycholate agar. Salmonella enters the body through contaminated food or water, and invades the intestinal epithelial cells, causing inflammation.
This bacterium may be classified into 2 different species: Salmonella enterica and Salmonella bongori. Salmonella enterica is divided in 6 different subspecies, of which I, contains most pathogenic serotypes for humans. Salmonella may be serogrouped into more than 2500 serovars with polyvalent antisera, according to the capsular antigen, polysaccharide O antigens, and flagellar antigens. The bacteria show tropism for the epithelial cells of the gastrointestinal tract, macrophages, dendritic cells, and neutrophils. Different serotypes of Salmonella may have different natural reservoirs, some have humans as their only natural reservoir (serotypes Sendai and Typhi), while others (serotype Dublin) may infect humans and cattle.

# Taxonomy
Cellular organism; Bacteria; Proteobacteria; Gammaproteobacteria; Enterobacteriales; Enterobacteriaceae

# Biology
Salmonella is a gram-negative, facultative intracellular, anaerobic, non-spore-forming bacillus. It measures 2 to 3 by 0.4 to 0.6 m. Salmonella is a non-lactose fermenting bacterium. It reduces nitrates, produces acid on glucose fermentation and does not produce cytochrome oxidase. Due to the presence of flagella, almost all salmonella species are motile. 1% of the bacteria is able to ferment lactose, which may be responsible for its non-detection in some culture media.
For the isolation of salmonella in culture, freshly passed stool are preferred. Common media for the growth of salmonella include: MacConkey agar, deoxycholate agar, and xylose-lysine-deoxycholate agar.
When the sample has a low number of bacteria, special enrichment broths, such as the selenite-based enrichment broth, may be used to raise the number of bacteria.

## Infectious Cycle
Salmonella enterica enters the body through the mouth, by ingestion of contaminated food and water. For the bacteria to cause disease, an inoculum of about 50 000 pathogens is often required. Once in the intestine, the bacteria will first infect the apical epithelium. Salmonella will then initiate bacterial mechanisms that allow invasion of the host cells, inducing inflammatory changes, such as:
- Diffuse and focal infiltration of PMN
- Crypt abscesses
- Necrosis of the epithelium
- Fluid secretion
- Edema
Different serovars will have different preferable intestinal locations. An example is the enterocolitis at the terminal ileum, cecum, and proximal colon caused by serovar Typhimurium. Intestinal disease is marked by neutrophil migration to the intestinal epithelium. This recruitment is done by the secretion of interleukin-8, induced by Salmonella.

# Classification
Before 1983 Salmonella was classified in several species. However, its genome study has shown high levels of DNA similarities among different types of salmonella, which leads to the actual classification of salmonella in 2 different species:

## Salmonella enterica
- Contains six subspecies - I, II, IIIa, IIIb, IV, and VI:
I - enterica
II - salamae
III - arizonae
IIIb - diarizonae
IV - houtenae
VI - indica
- I - enterica
- II - salamae
- III - arizonae
- IIIb - diarizonae
- IV - houtenae
- VI - indica
- Subspecies I contains most pathogenic serotypes for humans
- Subspecies IIIa and IIIb, formerly belonging to the genus Arizonae, are responsible for rare human infections

## Salmonella bongori
- Formerly subspecies V

## Serovars
Salmonella subspecies may be serogrouped into more than 2500 serovars with polyvalent antisera. For this division the following bacterial structures are considered:
- Capsular antigen
- Polysaccharide O antigens
- Flagellar antigens
Different salmonella serotypes may also be distinguished in culture, according to their different metabolism of sugars. Different serovars may also have different disease manifestations. For example, Salmonella enterica, serovar Typhimurium is the causative agent of typhoid fever, and is not associated with classical salmonellosis.

# Tropism
In vitro, Salmonella is able to interact with different types of cells. However, in vivo, the bacteria was found to enter only certain human cells, namely:
- Epithelial cells of the gastrointestinal tract - first cells to be infected by Salmonella
- Macrophages - where Salmonella survives and replicates
- Dendritic cells - relevant for dissemination of bacteria in the initial stages of infection, yet, they are not considered adequate reservoirs for Salmonella at latter stages
- Neutrophils - interaction is based on the immune response against the bacteria, hence this might not be considered true tropism

# Differential diagnosis
Salmonella gastroenteritis must be differentiated from other causes of viral, bacterial, and parasitic gastroentritis.
8Small bowel diarrhea: watery, voluminous with less than 5 WBC/high power field
Large bowel diarrhea: Mucousy and/or bloody with less volume and more than 10 WBC/high power field
 It could be as high as 1000 based on patient's immunity system.
The table below summarizes the findings that differentiate inflammatory causes of chronic diarrhea

# Natural Reservoir
Different salmonella serovars may have different natural reservoirs. Common types of serovars of salmonella enterica that infect the human gastrointestinal tract include serovars Sendai, Typhi, and Paratyphi. Humans are their only natural reservoir. Other serotypes, such as serotype Dublin, have cattle as their natural reservoir, but has also the capacity to cause infection in humans.

# Drug Side Effect
- Vedolizumab

# Related Chapters
- 1984 Rajneeshee bioterror attack
- List of foodborne illness outbreaks
- Food Testing Strips

# Gallery
- Salmonella bacteria in tetrathionate enrichment broth stained using direct FA staining technique. From Public Health Image Library (PHIL). 
- Salmonella bacteria in tetrathionate enrichment broth stained using direct FA staining technique. From Public Health Image Library (PHIL). 
- Triple sugar iron agar (TSI) tested for Salmonella (H2S+) and (H2S-); Citrobacter sp. and S. arizonae. From Public Health Image Library (PHIL). 
- Gram-negative bacilli, or rod-shaped Salmonella sp. bacteria. From Public Health Image Library (PHIL). 
- SEM depicts a number of red-colored Salmonella sp. bacteria invading a mustard-colored ruffled immune cell. From Public Health Image Library (PHIL).