Yersinia pastis is an alternate spelling of Yersinia pestis, and refers to a strain of bacteria. This bacteria has been linked to the black plague and remains active in the modern world, although it is generally confined to a limited number of animal reservoirs. Yersinia pastis is anaerobic and gram-negative. Infection with this microbe, while very serious, can be treated successfully with modern antibiotic drugs, although some strains appear to be developing antibiotic resistance.
This microbe was first identified and linked to the plague in the late 19th century, a linkage that has since been proven conclusively with the aid of DNA testing. Discovery of Yersinia pastis allowed researchers to piece together a complete understanding of the typical disease cycle of the black plague. The bacteria is normally carried by a host species of small mammal, typically rats, marmots, or similar animals. Fleas then transfer the microbe to human hosts. Human-to-human transmission of the plague is also possible but is generally a secondary form of transmission.
Three forms of infection by Yersinia pastis are generally seen in humans. The most common variety of infection, directly transmitted by flea bites, leads to the bubonic plague, the most common form of plague, which produces painful swelling in the lymph nodes and many general systemic symptoms. Septicemic plague occurs when the microbe is able to directly infect the bloodstream and is a very dangerous form of the disease, especially when untreated. Pneumonic plague is an infection of the lungs and also extremely serious, not least because it produces a wracking cough that allows the microbe to infect new hosts through airborne transmission.
Yersinia pastis is endemic in many regions of the globe, including Southeast Asia and the American Southwest. Major plague outbreaks are rare in the modern world, and none have occurred in the United States since the 1920s. This microbe, however, remains a threat, infecting 1,000 to 2,000 people per year and killing about one in seven of those.
Antibiotics are currently highly effective in dealing with Yersinia pastis infection, but recent research suggests that resistance may be increasing. Bacteria are often able to exchange elements of DNA with one another in the wild, and Yersinia pastis seems to have been able to acquire genes that produce resistance to antibiotics through exchange with other, much more common, bacteria. These bacteria have evolved drug resistance in response to the widespread use, frequent overuse, and misuse of antibiotics.