YERSINIA PESTIS: CHARACTERISTICS, MORPHOLOGY, DISEASES - BIOLOGY - 2025

Yersinia pestis is a Proteobacterium of the Enterobacteriaceae family that is characterized by being a pleomorphic cocobacillus, with a size that ranges between 1 and 3 µm in length and between 0.5 and 0.8 µm in diameter; it is also Gram negative with a bipolar staining with Giemsa, Wright's and Wayson stains and its metabolism is facultative anaerobic.

It is the etiological agent of plague, which is a natural disease of rodents and which can also affect humans (zoonosis). It uses the rat flea (Xenopsylla cheopis) as a vector and in addition to rodents and humans, it can also affect domestic animals, especially cats.

Scanning electron micrograph of Yersinia pestis, which causes bubonic plague, on proventricular spines of the flea Xenopsylla cheopis. Taken and edited from: National Institutes of Health (NIH).

Plague is a re-emerging disease, that is, researchers considered it controlled or disappeared. However, it has reappeared, being highly contagious and with the ability to develop as bubonic, pneumonic, or septicemic plague.

In addition to the bite of an infected insect, contact with liquids or tissues, or the consumption of an infected animal can transmit the infection, contagion can even occur through the inhalation of respiratory particles from sick people or animals.

General characteristics

Plague bacteria is a Gram negative organism that exhibits bipolar staining when treated with Giemsa, Wright's or Wayson stains, although this staining pattern is sometimes not very evident. Bipolar staining means that the staining is stronger at the ends of the bacteria than at the center.

This genus was erected in homage to the French-Swiss bacteriologist Alexandre Yersin who was co-discoverer of the bacterium in 1894, independently with the Japanese bacteriologist Kitasato Shibasaburō.

The genus comprises eleven species, including three pathogens: Y. enterocolitica, Y. pseudotuberculosis and Y. pestis, the latter selected as the type species. Some researchers suggest that Y. pestis is a clone of Y. pseudotuberculosis that appeared shortly before the first plague pandemic.

The bacterium was originally baptized as Bacterium pestis, a name used until 1900, then it was sequentially relocated to the genera Bacillus and Pasteurella, until in 1970 it received Yersinia pestis.

Three subspecies or biovars of Y. pestis are currently recognized based on minor phenotypic differences: Y. pestis antiqua, Y. pestis medievalis, and Y. pestis orientalis.

Lifecycle

Yersinia pestis is maintained in nature thanks to transmission between blood-sucking fleas and different species of rodents and lagomorphs. However, there is evidence showing that virtually all mammals are susceptible to being affected by this enterobacteria.

There are more than 1,500 species of fleas, however, only about 30 species are proven vectors of the disease, mainly the rat flea (Xenopsylla cheopis), as well as Nosopsylla fasciatus and the human flea (Pulles irritans).

When a flea sucks blood from an infected mammal, it acquires some bacteria along with the blood. Once inside the flea, the bacteria may reproduce so quickly that it blocks the proventriculus, a portion of the digestive tract between the esophagus and the stomach.

If this happens, the blood will not reach the flea's stomach, which, as it becomes hungrier, will bite a new host in search of food. But due to the blockage of the proventriculus, it will vomit the blood recently ingested and infected with Yersinia pestis, which will invade the new host. The flea will keep repeating the cycle until it starves.

Another possibility is that when ingested by the flea, the bacteria lodge and multiply in its digestive tract without affecting it, and that it is injected into a new host when the flea feeds again.

In the definitive host, Yersinia pestis lodges in the lymphatic system, the blood, or the reticuloendothelial system. The bacteria can reproduce at a generation time of 1.25 hours, the generation time is the time required to double the size of a colony.

In the human being

When infecting a human being, the bacteria can take different routes, when transmission is by bite, it travels through the bloodstream to the lymph nodes, where it reproduces and causes an inflammation called bubo that is loaded with bacteria.

Subsequently secondary buboes form, until the buboes rupture and the bacteria come out again, in large numbers into the bloodstream causing great septicemia.

It may also be that bacteria grow so quickly in the bloodstream that they don't have a chance to form buboes. If the bacteria enter the host via the respiratory system, it will reproduce in the lungs.

Diseases

The bacterium Yersinia pestis is the etiological agent of plague, which can present in three different ways: bubonic, septicemic, pneumonic and minor plague.

Bubonic plague

It is the most common form of infection, with an incubation period from a few hours to 12 days, although it generally takes between 2 and 5 days to manifest. It is produced by the bite of an infected flea.

In this type of plague there is the appearance of buboes, the femoral and inguinal ganglia being the most affected, followed by the axillary, cervical or others.

The spread of the bacteria through the bloodstream allows it to quickly reach any part of the body, including the lungs, and bubonic plague can be complicated by secondary pneumonic plague of blood origin.

Yersinia pestis culture on chocolate agar medium. Taken and edited from: Department of Health and Human Services.

Pneumonic plague

This can have two origins. One considered primary, caused when a healthy person inhales respiratory particles from another infected person. The other form, considered secondary, is the hematogenous one mentioned above, which occurs as a complication of bubonic plague.

Septicemic plague

It is the most violent form of infection and also occurs from the bite of infested fleas. In this type of plague, you do not develop bubo formation because of how quickly the disease progresses.

Minor plague

This is a less virulent form of bubonic plague, generally occurring in regions where Yersinia pestis is endemic. In these cases there is a remission of the disease after a week of presenting symptoms.

Symptoms

Symptoms of the disease will change depending on the type of plague that occurs.

Bubonic plague

The first symptoms of the disease may include lesions on the skin (papule, pustule, ulcer or eschar) due to the flea bite. Sudden high fevers, with or without chills, may also occur.

The liver, spleen, and peripheral lymph nodes become swollen. In the latter, buboes are formed, which are surrounded by edema and are painful, with reddened skin, but without a rise in temperature, they can suppurate in 14 days.

Thrassis bacchi johnsoni flea, infected with Yersinia pestis. Taken and edited from: Centers for Disease Control and Prevention.

Other symptoms include a racing pulse, low blood pressure, agitation, delusions, and lack of coordination.

If left untreated, the infection can progress to generalized septicemia, bleeding, increased pain in the lymph nodes, delirium, shock, and even death after a period of 3 to 5 days.

Pneumonic plague

This infection is asymptomatic until the last two days of the disease, when there is a large bloody sputum emission. There is elevation of the body temperature, chills, tachycardia, headaches and shortness of breath.

Death usually occurs after 48 hours of onset of symptoms if there is no adequate treatment.

Septicemic plague

Due to how quickly the infection develops, death usually comes before the first symptoms of it can appear.

Minor plague

This presents some of the symptoms of bubonic plague, such as swelling of the lymph nodes, fever, headache and general body pain.

Treatments

Penicillin is useless in these cases, as the bacteria show a natural resistance to this antibiotic. The most appropriate treatment consists of streptomycin, chloramphenicol, gentamicin, or tetracyclines.

Alternatively, fluoroquinolone or doxycycline can be used.

The main factor that affects the treatment of the disease is time, if it is started quickly, mortality can be reduced by 95-99%. In the case of pneumonic and septicemic plague, they progress so rapidly that treatment is not effective.

If these last two forms of plague are detected within 24 hours of the onset of symptoms, the recommended treatment is streptomycin, at a dose of 7.5 mg / kg IM every 6 h for 7-10 days or up to 0.5 g IM every 3 h for 48 h. Doxycycline (100 mg IV or PO every 12 h) can also be used.

References

1. Yersinia pestis. Recovered from: LabCe.com.
2. Yersinia pestis. On Wikipedia. Recovered from: en.wikipedia.org.
3. RD Perry, & JD Fetherston (1997). Yersinia pestis –etiologic agent of plague. Clinical Microbiology Reviews.
4. M. Achtman, K. Zurth, G. Morelli, G. Torrea, A. Guiyoule & E. Carniel (1999). Yersinia pestis, the cause of plague, is a recently emerged clone of Yersinia pseudotuberculosis. PNAS.
5. PP Flaquet (2010). The plague, a reemerging infectious disease. Cuban Journal of Comprehensive General Medicine.
6. T. Butler (1983). Plague and other Yersinia infections. Plenum Press (New York).