Yersinia pestis

*Yersinia pestis* is a gram-negative bacterium that causes the disease known as the plague. This organism is a non-motile coccobacillus, which means it is a small, rod-shaped cell that cannot move on its own. It is also a facultative anaerobe, meaning it can survive with or without oxygen. *Y. pestis* is classified as a hyperparasite. This is because it lives inside a parasite, such as a flea, which is itself a parasite of a host, such as a rodent. This complex relationship allows the bacterium to move between different species.

The bacteria cause three distinct clinical forms of the disease. The bubonic plague is the most well-known form. The septicemic plague occurs when the bacteria enter the bloodstream. The pneumonic plague is a version that affects the lungs. This form is particularly dangerous because it can spread through the air via respiratory droplets. If a person catches the pneumonic form, they can pass it to others through coughing or sneezing. This cycle of infection makes the pneumonic form a significant threat to public health.

The mechanism of transmission often involves the oriental rat flea, *Xenopsylla cheopis*. When a flea feeds on an infected animal, the bacteria begin to multiply in the flea's digestive tract. The bacteria produce proteins that lead to the formation of a biofilm. This biofilm creates a physical blockage in the proventriculus, which is a valve in the flea's throat. Scientists call this accumulation "Bacot's block." Because the flea cannot swallow blood, it attempts to feed more aggressively. During these attempts, the flea regurgitates the bacteria back into the host's wound. This process effectively injects the infection into the new host.

*Y. pestis* uses several advanced tools to survive inside a host. It possesses plasmids, which are small pieces of DNA that carry specific instructions. The plasmid pPCP1 codes for a protease called Pla. This protein is a virulence factor that helps the bacteria cause pneumonic plague. Another plasmid, pMT1, helps the bacteria survive within the flea. The bacteria also use a type-III secretion system, or T3SS. This is a molecular machine that allows the bacteria to inject proteins, called Yops, directly into host immune cells. These proteins help the bacteria avoid being destroyed by the host's immune system.

History shows that *Y. pestis* has shaped human civilization through massive pandemics. It was the cause of the Plague of Justinian and the Black Death. One of the deadliest events in recorded history was the Black Death. The bacterium was discovered in 1894 by Alexandre Yersin. He was a Swiss and French physician working during an epidemic in Hong Kong. At the time, Kitasato Shibasaburō was also searching for the cause of the plague. Yersin originally named the organism *Pasteurella pestis*. It was renamed *Yersinia pestis* in 1944 to honor his discovery.

While the plague is an ancient disease, it remains a modern concern. The World Health Organization reports between 1,000 and 2,000 cases every year. Today, the disease is most commonly found in Madagascar, Peru, and the Democratic Republic of the Congo. During the Vietnam War, cases in Asia increased five to sixfold. This increase was likely due to ecosystem disruptions and closer contact between people and animals. Fortunately, the prognosis for patients is much better now than in the past. This is because doctors can use antibiotic treatments to fight the infection effectively.

The bacterium also impacts the natural world and various animal populations. Over 200 species of rodents serve as reservoirs for the disease. In North America, common reservoirs include prairie dogs, chipmunks, and squirrels. In Asia, the bacteria are often found in gerbils and susliks. The plague can be devastating to certain species, such as the black-tailed prairie dog. In these colonies, the disease can cause a total colony collapse. This collapse can have a massive effect on the entire prairie food web.