Shigella flexneri is a species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 of Gram-negative

Gram-negative

Gram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color...

 bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 in the genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 Shigella

Shigella

Shigella is a genus of Gram-negative, nonspore forming, non-motile, rod-shaped bacteria closely related to Escherichia coli and Salmonella. The causative agent of human shigellosis, Shigella causes disease in primates, but not in other mammals. It is only naturally found in humans and apes. During...

that can cause diarrhea in humans. There are several different serogroups

Serotype

Serotype or serovar refers to distinct variations within a subspecies of bacteria or viruses. These microorganisms, viruses, or cells are classified together based on their cell surface antigens...

 of Shigella; S. flexneri belongs to group B. S. flexneri infections can usually be treated with antibiotics although some strains have become resistant

Antibiotic resistance

Antibiotic resistance is a type of drug resistance where a microorganism is able to survive exposure to an antibiotic. While a spontaneous or induced genetic mutation in bacteria may confer resistance to antimicrobial drugs, genes that confer resistance can be transferred between bacteria in a...

. Less severe cases are not usually treated because they become more resistant in the future.

Discovery

The species was named after the American physician Simon Flexner

Simon Flexner

Simon Flexner, M.D. was a physician, scientist, administrator, and professor of experimental pathology at the University of Pennsylvania . He was the first director of the Rockefeller Institute for Medical Research and a trustee of the Rockefeller Foundation...

, in addition to the Japanese physician Kiyoshi Shiga

Kiyoshi Shiga

was a Japanese physician and bacteriologist.-Biography:Shiga was born in Sendai, Miyagi Prefecture, though his original family name was Satō. He graduated from the Medical School of Tokyo Imperial University in 1896 and went to work at the Institute for the Study of Infectious Diseases under Dr....

, who researched the cause of dysentery.

Infectious cycle

S. flexneri contians a virulence plasmid that codes for three virulence factors: a type-3 secretion system (T3SS), invasion plasmid antigen proteins (ipa proteins), and IcsA (used for cell-to-cell spread).
Upon infection, S. flexneri injects the host cell cytoplasm with ipa proteins using the T3SS--a needle-and-syringe like apparatus common to many gram negative pathogens. These ipa proteins induce "membrane ruffling" by the host cell. Membrane ruffling creates membrane pockets which capture and engulf the bacteria. Once inside, S. flexneri utilizes host cell actin for propulsion to move directly from cell to cell using a cellular mechanism known as paracytophagy

Paracytophagy

Paracytophagy is the cellular process whereby a cell engulfs a protrusion which extends from a neighboring cell. This protrusion may contain material which is actively transferred between the cells...

, similarly to the bacterial pathogen Listeria monocytogenes.