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This page is about microbiologic aspects of the organism(s).  For clinical aspects of the disease, see Salmonellosis.

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: João André Alves Silva, M.D. [2] Jolanta Marszalek, M.D. [3]

Overview[edit]

Salmonella is a rod-shaped, facultative intracellular, gram-negative enterobacteria.<ref name=Sherris>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> Salmonella species shows motility, produces hydrogen sulfide, and only 1% is able to ferment lactose.<ref name=Baron>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> 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[edit]

Cellular organism; Bacteria; Proteobacteria; Gammaproteobacteria; Enterobacteriales; Enterobacteriaceae<ref name=NCBI>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Biology[edit]

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Salmonella is a gram-negative, facultative intracellular, anaerobic, non-spore-forming bacillus. It measures 2 to 3 by 0.4 to 0.6 μm. Salmonella is a non-lactose fermenting bacterium. It reduces nitrates, produces acid on glucose fermentation and does not produce cytochrome oxidase.<ref>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> 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.<ref name="PerezCavalli2003">{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

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.<ref name="PerezCavalli2003">{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

Infectious Cycle[edit]

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:<ref name="pmid534385">{{#invoke:Citation/CS1|citation |CitationClass=journal }} </ref><ref name="pmid4630603">{{#invoke:Citation/CS1|citation |CitationClass=journal }} </ref><ref name="pmid3551701">{{#invoke:Citation/CS1|citation |CitationClass=journal }} </ref><ref name="pmid2919285">{{#invoke:Citation/CS1|citation |CitationClass=journal }} </ref>

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.<ref name="pmid8227148">{{#invoke:Citation/CS1|citation |CitationClass=journal }} </ref>

Classification[edit]

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[edit]

  • Contains six subspecies - I, II, IIIa, IIIb, IV, and VI:<ref name=Opinion80_2005>{{#invoke:Citation/CS1|citation

|CitationClass=journal }}</ref><ref name=Tindall_2005>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

    • 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[edit]

  • Formerly subspecies V<ref name="pmid15313257">{{#invoke:Citation/CS1|citation

|CitationClass=journal }} </ref>

Serovars[edit]

Salmonella subspecies may be serogrouped into more than 2500 serovars with polyvalent antisera. For this division the following bacterial structures are considered:<ref>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref>

Different salmonella serotypes may also be distinguished in culture, according to their different metabolism of sugars.<ref>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> 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[edit]

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:<ref name="pmid15532980">{{#invoke:Citation/CS1|citation |CitationClass=journal }} </ref>

Differential diagnosis[edit]

Salmonella gastroenteritis must be differentiated from other causes of viral, bacterial, and parasitic gastroentritis.

Organism Age predilection Travel History Incubation Size (cell) Incubation Time History and Symptoms Diarrhea type8 Food source Specific consideration
Fever N/V Cramping Abd Pain Small Bowel Large Bowel Inflammatory Non-inflammatory
Viral Rotavirus <2 y - <102 <48 h + + - + + - Mostly in day cares, most common in winter.
Norovirus Any age - 10 -103 24-48 h + + + + + - Most common cause of gastroenteritis, abdominal tenderness,
Adenovirus <2 y - 105 -106 8-10 d + + + + + - No seasonality
Astrovirus <5 y - 72-96 h + + + + + Seafood Mostly during winter
Bacterial Escherichia coli ETEC Any age + 108 -1010 24 h - + + + + - Causes travelers diarrhea, contains heat-labile toxins (LT) and heat-stable toxins (ST)
EPEC <1 y - 10 6-12 h - + + + + Raw beef and chicken -
EIEC Any ages - 10 24 h + + + + + Hamburger meat and unpasteurized milk Similar to shigellosis, can cause bloody diarrhea
EHEC Any ages - 10 3-4 d - + + + + Undercooked or raw hamburger (ground beef)  Known as E. coli O157:H7, can cause HUS/TTP.
EAEC Any ages + 1010 8-18 h - - + + + - May cause prolonged or persistent diarrhea in children
Salmonella sp. Any ages + 1 6 to 72 h + + + + + Meats, poultry, eggs, milk and dairy products, fish, shrimp, spices, yeast, coconut, sauces, freshly prepared salad. Can cause salmonellosis or typhoid fever.
Shigella sp. Any ages - 10 - 200 8-48 h + + + + + Raw foods, for example, lettuce, salads (potato, tuna, shrimp, macaroni, and chicken) Some strains produce enterotoxin and Shiga toxin similar to those produced by E. coli O157:H7
Campylobacter sp. <5 y, 15-29 y - 104 2-5 d + + + + + Undercooked poultry products, unpasteurized milk and cheeses made from unpasteurized milk, vegetables, seafood and contaminated water. May cause bacteremia, Guillain-Barré syndrome (GBS), hemolytic uremic syndrome (HUS) and recurrent colitis
Yersinia enterocolitica <10 y - 104 -106 1-11 d + + + + + Meats (pork, beef, lamb, etc.), oysters, fish, crabs, and raw milk. May cause reactive arthritis; glomerulonephritis; endocarditis; erythema nodosum.

can mimic appendicitis and mesenteric lymphadenitis.

Clostridium perfringens Any ages > 106 16 h - - + + + Meats (especially beef and poultry), meat-containing products (e.g., gravies and stews), and Mexican foods. Can survive high heat,
Vibrio cholerae Any ages - 106-1010 24-48 h - + + + + Seafoods, including molluscan shellfish (oysters, mussels, and clams), crab, lobster, shrimp, squid, and finfish. Hypotension, tachycardia, decreased skin turgor. Rice-water stools
Parasites Protozoa Giardia lamblia 2-5 y + 1 cyst 1-2 we - - + + + Contaminated water May cause malabsorption syndrome and severe weight loss
Entamoeba histolytica 4-11 y + <10 cysts 2-4 we - + + + + Contaminated water and raw foods May cause intestinal amebiasis and amebic liver abscess
Cryptosporidium parvum Any ages - 10-100 oocysts 7-10 d + + + + + Juices and milk May cause copious diarrhea and dehydration in patients with AIDS especially with 180 > CD4
Cyclospora cayetanensis Any ages + 10-100 oocysts 7-10 d - + + + + Fresh produce, such as raspberries, basil, and several varieties of lettuce. More common in rainy areas
Helminths Trichinella spp Any ages - Two viable larvae (male and female) 1-4 we - + + + + Undercooked meats More common in hunters or people who eat traditionally uncooked meats
Taenia spp Any ages - 1 larva or egg 2-4 m - + + + + Undercooked beef and pork Neurocysticercosis: Cysts located in the brain may be asymptomatic or seizures, increased intracranial pressure, headache.
Diphyllobothrium latum Any ages - 1 larva 15 d - - - + + Raw or undercooked fish. May cause vitamin B12 deficiency



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<ref name="pmid8209928">{{#invoke:Citation/CS1|citation |CitationClass=journal }} </ref><ref name="pmid16151544">{{#invoke:Citation/CS1|citation |CitationClass=journal }} </ref><ref name="pmid16698746">{{#invoke:Citation/CS1|citation |CitationClass=journal }} </ref><ref name="pmid12700377">{{#invoke:Citation/CS1|citation |CitationClass=journal }} </ref><ref name="pmid12700377">{{#invoke:Citation/CS1|citation |CitationClass=journal }} </ref>

Cause History Laboratory findings Diagnosis Treatment
Diverticulitis Abdominal CT scan with oral and intravenous (IV) contrast bowel rest, IV fluid resuscitation, and broad-spectrum antimicrobial therapy which covers anaerobic bacteria and gram-negative rods
Ulcerative colitis Endoscopy Induction of remission with mesalamine and corticosteroids followed by the administration of sulfasalazine and 6-Mercaptopurine depending on the severity of the disease.
Entamoeba histolytica cysts shed with the stool detects ameba DNA in feces Amebic dysentery

Luminal amebicides for E. histolytica in the colon:

For amebic liver abscess:

Natural Reservoir[edit]

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.<ref>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref><ref name=CDC>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Drug Side Effect[edit]

Related Chapters[edit]

Gallery[edit]

References[edit]

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