Salmonella (non-typhoidal)

Definitions & Key takeaways

Salmonella is a rod-shaped, gram-negative bacteria of the Enterobacteriaceae family, which is known to cause salmonellosis infection. Non-typhoidal Salmonella (NTS) infections are the most common type of salmonellosis and are caused by several different strains of the bacteria like S. Enteritidis, and S. Newport. NTS infections are foodborne diseases and primarily spread through contaminated food and water. They cause symptoms of food poisoning like fever, diarrhea, abdominal cramps, vomiting, and headache. These symptoms can be severe and may lead to dehydration, sepsis, or even death.

Watch video only
Salmonella is a bacterium belonging to the family Enterobacteriaceae. There are two main species: Salmonella bongori and Salmonella enterica, and the latter has six subspecies.
One of the subspecies is enterica, which has over 2500 serotypes that can be divided into two main groups based on the clinical symptoms they cause- so typhoidal or non-typhoidal Salmonella.
The non-typhoidal group, can infect humans and animals and cause a variety of disease states. But, the most common serotype, Salmonella enteritidis, causes intestinal inflammation, called gastroenteritis, or commonly called “food poisoning”.
OK, but generally, Salmonella are encapsulated gram-negative, rod bacteria – meaning, they have a polysaccharide layer outside the cell envelope and look like little red or pink sticks on a gram stain.
They’re facultative intracellular pathogens, meaning they can live both outside or inside of its host’s cells. And have flagella, making them motile, but don’t form spores.
They’re also facultative anaerobes, so they can undergo respiratory and fermentative metabolism; and they can ferment glucose but not lactose; are oxidase negative, and produce hydrogen sulfide gas.
And while a variety of media can be used to selectively identify Salmonella, among them is Triple Sugar Iron agar which produces a black precipitate when hydrogen sulfide is produced.
Now, once Salmonella is ingested and reaches the distal ileum of the small intestine, it tends to target the epithelial layer of the mucosal lining where it uses surface appendages to adhere to microfold cells, or M-cells.
And these M-cells eat, or phagocytose, the bacteria from the intestinal lumen and spit it out into the underlying Peyer’s patches - a type of mucosal immune tissue that extends into the submucosa.
When encountering non-typhoidal Salmonella, the immune system responds strongly by releasing proinflammatory cytokines that recruit additional immune cells, particularly neutrophils.
This causes inflammation of the small intestines and the colon, called enterocolitis. Responding immune cells can also damage the mucosa as they travel to the site of infection.
This can cause ulcers; and gastrointestinal tract dysfunction that leads to an efflux of water and electrolytes into the intestinal lumen, which causes diarrhea.
Now, usually, the infection is uncomplicated and limited to the mucosa, where it’s often destroyed by the local immune cells.
But, in some cases, the infection can become invasive, gaining access to nearby blood vessels, causing bacteremia.From the bloodstream, the bacteria can spread to other organs like the brain, bones, liver, or spleen.
Now, reservoirs for non-typhoidal Salmonella include infected humans and animals, particularly birds, reptiles, mammals, and amphibians.
So, transmission can be fecal-to-oral and foodborne through consumption of contaminated raw or undercooked animal products; especially poultry, meat, and eggs, or unpasteurized milk or milk-products.
Transmission can also occur through any contaminated consumable like water; fruits and vegetables; and even peanut butter.
Finally, the infection can also spread through direct contact with infected animals, if contaminated hands then reach the mouth.
Now, because Salmonella is sensitive to gastric acid, a large inoculum, meaning more than 100,000 microorganisms, is needed to cause an infection.
But certain factors; like being a young child, an older adult, having low gastric acidity, or being immunocompromised can cause an elevated risk of infection and development of invasive, complex cases.
Now, sometimes symptoms can be absent, even if a host is infected and a carrier capable of spreading non-typhoidal Salmonella.
When symptoms do occur, they start 24 to 48 hours, and they’re pretty typical for food poisoning: watery diarrhea, which may be bloody; abdominal cramps; nausea and vomiting; headache; chills; and a low-grade fever, which typically resolves within 2 days.
Dehydration may also occur if fluid loss from diarrhea has not been replenished. A definitive diagnosis can be made with a stool culture on selective media.
In invasive infections, additional cultures; like blood and bone marrow aspirate cultures may be necessary. Treatment involves managing symptoms, particularly fluid and electrolyte replenishment for diarrhea, suppressing nausea, and alleviating pain.
Antibiotics are typically not indicated for uncomplicated infections since infections tend to be self-limiting, and antibiotics can prolong a carrier state or even cause relapse.
However, antibiotics are usually given in invasive infections, especially in complicated cases like in immunocompromised individuals, and antibiotic selection is made based on sensitivity of the particular infecting strain.
Finally, an important part of treatment is practicing good hygiene and sanitation, including proper food and hand washing, and thoroughly cooking raw foods.##Summary So, to recap: Salmonella is a rod-shaped, gram-negative bacteria of the Enterobacteriaceae family.
After ingestion, Salmonella tends to infect M-cells of the small intestine epithelial mucosa, which transfer the bacteria to the underlying Peyer’s patches.
Serotypes that are non-typhoidal elicit a strong proinflammatory response which causes inflammation of the small intestine and colon.
Infections are usually foodborne from contaminated sources or from interactions with animal reservoirs, and cause the hallmark symptoms of “food poisoning”, including abdominal cramps, vomiting, headaches, fever and diarrhea.
Diagnosis can be made with a stool culture, and treatment is typically fluid and electrolyte replenishment, especially in uncomplicated or non-invasive cases.
But in complicated, invasive infections, antibiotic therapy may be necessary.