Yersinia pestis

Yersinia pestis is rod-shaped, gram-negative, non-spore-forming and non-motile Coccobacillus bacteria. It was isolated by a French bacteriologist Alexandre Yersin^​1​ and a Japanese bacteriologist Kitazato Shibasaburo almost at the same time in 1894. Y. pestis became a well-known bacterium after it was identified to be the causative agent of plague, one of the worst epidemics in human history, and became well known. The strain has been renamed quite a number of times; from Bacterium pestis to Bacillus pestis which has been renamed again as Pasteurella pestis, and finally in 1970, the name Yersinia pestis was determined^​2​. There have been proposals made in an attempt to include Yersinia pestis as a subspecies of Y. pseudotuberculosis. However, those have not been adopted for the public health safety and historical reasons.

Y. pestis is a pathogen which primarily infects rodents. A typical vector responsible for the transmission of this pathogen is Xenopsylla cheopis, a type of fleas. When these fleas uptake Y. pestis infected blood, a lump of bacteria including Y. pestis is aggregated between their esophagus and the stomach. This prevents X. cheopis from ingesting blood and these unconsumed blood is mixed with the bacteria and enters the host again. This has ultimately been spread to humans which have led to a worldwide pandemic. Europe has suffered greatly due to Justinian’s plague in the 4th century and the Black Death in 14th to 18th centuries. The Black Death especially ravaged in China as well. The pest emerged in China in 1894 has been spread from the Yunnan Province to Hong Kong, and across the world including India, Europe, Africa and the Americas.^​2–4​

There are three types of Y. pestis infected plagues(5). The most common form is the bubonic plague. After 1 to 10 days of incubating period, patients develop high fever which is accompanied by headaches and chills, and with one or more swollen lymph nodes called buboes. This generally occurs when bitten by a flea infected with Y. pestis. The bacteria multiply in the closest lymph nodes from where it was entered. The pathogen can be spread to other parts of the body if not treated with appropriate antibiotics.

Septicemic plague is usually developed from untreated bubonic plague, and it can also be caused by a primary infection. The spread of Y. pestis through the bloodstream causes bleeding in skins and organs which may lead to necrosis. The name, Black Death was derived from the symptoms that turn the skin and sores into black. When bacteria infect lungs, they develop to pneumonic plague which shows symptoms of pneumonia such as dyspnea, chest pain, and coughings. Any lung-related plague is the most serious form of the disease and is also the only plague that can be transmitted from person to person through airborne droplets.

Researchers tried to understand the genetic characteristics of Y. pestis by sequencing its genome. Y. pestis strains are largely classified into three subtypes or biovars: Antiqua, Mediaevalis, and Orientalis^​6​. Each has been historically associated with a major fest pandemic. Among the genomes of Y. pestis, strain KIM belongs to Mediaevalis and strain CO92 belongs to Orientalis have been well characterized. The sizes of the chromosome for KIM and CO92 are 4,653,728 bp, and 4,600,755 bp respectively. It was confirmed that 11kb and many smaller insertions in the strain CO92 made it larger by 50kb than strain KIM. Other Y. pestis strains including KIM have three plasmids pPCP1, pMT1, and pCD1 with the sizes of 9.5, 100.9, and 70.5 kb, respectively. All these plasmid sequences have been decoded​^​7–9​, and each gene has been found to contribute to pathogenicity. There has been ongoing genome sequencing projects for Y. pestis strains and as of March 2020, EzBioCloud Genome Database includes 286 strains of Y. pestis^​10​.

The Y. pestis has been spread a number of times throughout the human history. It has caused deaths of tens to hundreds of millions of people and has also changed our lifestyles. There was a very interesting article that the author introduced the 14 cultural changes of human lives brought by the Black Death. Please click the link for details, https://www.ranker.com/list/how-black-plage-changed-the-world/kellen-perry?ref=collections_btm&l=2580660&collectionId=2162​8​^​11​. Currently, we are undergoing a very difficult time as the whole world declared an emergency due to COVID-19. We believe that everyone is realizing more than ever, the microorganism could have the power to affect life. However, with the advanced technologies and sciences, we also clearly know how to fight against the pathogen unlike in the past. We hope the treatments and vaccines for COVID-19 can be made available as soon as possible to overcome this pandemic crisis.