Salmonella typhi (typhoid fever)

Salmonella is a bacterium belonging to the family Enterobacteriaceae.

There are two main species: Salmonella bongori and Salmonella enterica, which itself 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 typhoidal or non-typhoidal Salmonella.

The typhoidal group, which includes serotype Salmonella typhi (S. typhi), specifically infects humans and causes enteric fever, which is more commonly called typhoid fever.

If left untreated, it can be fatal and throughout history, has been the cause of death of aviator and engineer Wilbur Wright, one of the Wright brothers; Dr. Hashimoto, the first to describe Hashimoto's thyroiditis; and several people in New York City infected by “Typhoid Mary” in the first documented asymptomatic carrier of the disease in the United States.

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 targets the epithelial layer of the mucosal lining.

Here, 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.

And, well, S. typhi has a Vi capsular polysaccharide antigen virulence factor, which helps protect the bacteria from being tagged with antibodies that signal leukocytes like neutrophils to come in and phagocytose the bacteria.