Yersinia pestis

# Overview
Yersinia pestis (Y. pestis), a rod-shaped facultative anaerobe with bipolar staining (giving it a safety pin appearance) causes the infection in mammals and humans.  The bacteria maintain their existence in a cycle involving rodents and their fleas.  The genus Yersinia is gram-negative, bipolar staining coccobacilli, and, similarly to other Enterobacteriaceae, it has a fermentative metabolism. Y. pestis produces an antiphagocytic slime. The organism is motile when isolated, but becomes nonmotile in the mammalian host.

# Taxonomy
Bacteria; Proteobacteria; Gammaproteobacteria; Enterobacteriales; Yersinia; Yersinia pestis

# Biology
Yersinia pestis is a nonmotile, non-spore-forming, Gram-negative, non-lactose fermenting, ovoid and "safety-pin-shaped" (bipolar appearance when stained) bacillus.  It is an enterobacteriaceae commonly measuring 0.75x1.5 m. On cell cultures, it grows on sheep-blood agar, as grayish translucent colonies. It also grows on MacConkey and nutrient-rich broths.
Yersinia pestis only survives for a few hours on physical surfaces, being very sensitive to high temperatures, sunlight and disinfectants.
Yersinia pestis is thought to have evolved from Yersinia pseudotuberculosis about 1500 - 2000 years ago. According to its behavior towards nitrate and glycerol, Yersinia pestis may be classified as:
- Biovar Antiqua
- Nitrate reduction: positive
- Glycerol reduction: positive
- Biovar Medievalis
- Nitrate reduction: negative
- Glycerol use: positive
- Biovar Orientalis
- Nitrate reduction: positive
- Glycerol use: negative

## Genome
The complete genomic sequence is available for two of the three sub-species of yersinia pestis:
- Strain KIM - Biovar Medievalis
- Strain CO92 - Biovar Orientalis
As of 2006, the genomic sequence of a strain of biovar Antiqua has been completed. Similar to the other pathogenic strains, there are signs of mutations causing loss of function. The chromosome of strain KIM is 4,600,755 base pairs long; the chromosome of strain CO92 is 4,653,728 base pairs long.
Similarly to other enterobacteriaceae (Yersinia pseudotuberculosis and Yersinia enterocolitica), Yersinia pestis is host to the plasmid pCD1.  In addition, it also hosts two other plasmids, pPCP1 (also called pPla or pPst) and pMT1 (also called pFra) that are not carried by the other Yersinia species.
- pFra codes for a phospholipase D that is important for the ability of Yersinia pestis to be transmitted by fleas.
- pPla codes for a protease, Pla, that activates plasminogen in human hosts and is a very important virulence factor for pneumonic plague.
Together, these plasmids, and a pathogenicity island called HPI, encode several pathogenic proteins, characteristic of Yersinia pestis. Among other things, these virulence factors are required of:
- Bacterial adhesion
- Injection of proteins into the host cell
- Invasion of the host cell (via a Type III secretion system)
- Acquisition and binding of iron from red blood cell, via siderophores
A comprehensive and comparative proteomics analysis of Yersinia pestis strain KIM was performed in 2006. The analysis focused on the transition to a growth condition mimicking growth in host cells.

# Tropism
Yersinia pestis shows tropism for lymphoid tissue.

# Natural Reservoir
Plague is primarily a disease of rodents. The infection is maintained in natural foci of the disease in wild rodent colonies, through transmission between rodents, by their flea ectoparasites.

# Gallery
- Gram-negative Yersinia pestis bacteria, which was grown on a medium of sheeps blood agar (SBA) 72hrs. From Public Health Image Library (PHIL). 
- Ulcerated skin lesion at a plague inoculation site caused by the Gram-negative bacterium, Yersinia pestis. From Public Health Image Library (PHIL). 
- Blood smear reveals presence of Gram-negative Yersinia pestis plague bacteria. From Public Health Image Library (PHIL). 
- Gram-negative Yersinia pestis bacteria, which were cultured on a sheep blood agar (SBA) medium 120 hrs (20x mag). From Public Health Image Library (PHIL). 
- Gram-negative Yersinia pestis bacteria, which were cultured on a sheep blood agar (SBA) medium 120 hrs (20x mag). From Public Health Image Library (PHIL). 
- Gram-negative Yersinia pestis bacteria, which were cultured on a sheep blood agar (SBA) medium 24 hrs (10x mag). From Public Health Image Library (PHIL). 
- Gram-negative Yersinia pestis bacteria, which were cultured on a sheep blood agar (SBA) medium 72 hrs (10x mag). From Public Health Image Library (PHIL). 
- Hematoxylin-eosin stained (H&E) photomicrograph reveals  histopathologic changes in a lymph node tissue sample in a case of fatal human plague. From Public Health Image Library (PHIL). 
- Hematoxylin-eosin stained lung tissue sample revealed the histopathologic changes indicative of what was diagnosed as a case of fatal human plague from the country of Nepal (125x mag). From Public Health Image Library (PHIL). 
- Chest x-ray of a plague patient revealing bilateral infection, greater on the patient's left side, which was diagnosed as a case of pneumonic plague, caused by Yersinia pestis. From Public Health Image Library (PHIL). 
- Fingertips of patients right hand exhibited the signs of what is known as acral gangrene caused by the bacterium, Yersinia pestis. From Public Health Image Library (PHIL). 
- Patient acquired a plague infection through abrasions on his upper right leg. From Public Health Image Library (PHIL). 
- Lung tissue sample revealed the histopathologic changes indicative of fatal human plague (125x mag). From Public Health Image Library (PHIL). 
- Purple-colored Yersinia pestis bacteria on the proventricular spines of a Xenopsylla cheopis flea. From Public Health Image Library (PHIL).