Yersinia pestis It is a Proteobacteria of the Enterobacteriaceae family that is characterized by being a pleomorphic coccobacillus, 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 can also affect humans (zoonosis). Use the rat flea (Xenopsylla cheopis) as a vector and in addition to rodents and humans, it can also affect domestic animals, especially cats.
Plague is a reemerging disease, that is, researchers considered it controlled or disappeared. However, it has appeared again, 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.
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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.
Yersinia pestis it is non-mobile in a temperature range between 25 and 35 ° C and, although it can present motility when isolated, it does not form spores. Its optimum growth temperature is 28 ° C, although it can grow up to 40 ° C and develops faster at a pH of 7 to 7.2, with a much wider tolerance range (5 to 9.6)..
It is characterized by being:
- A facultative anaerobic bacteria.
- With fermentative metabolism.
- It is nitrate reductase and catalase positive.
- It is also positive for the test of or -nitrophenyl-b-D-galactopyroniside (ONPG) without the production of lactose.
- It is oxidase negative.
Yersinia pestis It is a natural disease of rodents, which expresses its main virulent factors at 37 ° C. It also presents in its cell wall some complexes of proteins and lipoproteins, called VW and F1 antigens, which prevent the bacteria from being phagocytosed.
The bacteria Yersinia pestis It has an intermediate shape between elongated (bacillus) and rounded (coco), which is why it is defined as a coccobacillus that reaches a maximum length of 3 µm and a maximum diameter of 0.8 µm..
This bacterium has the typical cell wall of Enterobacteriaceae, with the same lipid composition and a common enterobacterial antigen. Its polysaccharides lack side chains of O groups.
There is no true capsule, however, when the bacteria grow above 38 ° C it forms a carbohydrate and protein envelope that is called the capsular antigen or fraction 1 (F1).
Its genome varies between 4,600,755 and 4,653,728 base pairs.
The colonies appear translucent white-gray after 24 hours of growth at 35-37 ° C on blood agar (BA) or on chocolate agar, after 48 hours they reach 1-2 mm in diameter and turn opaque yellow. Older colonies in BA take on the appearance of fried egg or hammered copper. On MacConkey (MAC) agar they are white after 48 hours.
Yersinia pestis is taxonomically located in the phylum Proteobacteria, class Gammaproteobacteria, order Enterobacteriales, family Enterobacteriaceae and genus Yersinia.
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 Y Y. pestis, the latter selected as a type species. Some researchers suggest that Y. pestis it is a clone of Y. pseudotuberculosis that appeared shortly before the first plague pandemic.
The bacterium was originally named Bacterium pestis, name used until 1900, then it was sequentially relocated in the genres Bacillus Y Pasteurella, until in 1970 he received Yersinia pestis.
Currently, three subspecies or biovaries of Y. pestis based on minor phenotypic differences: Y. pestis antiqua, Y. pestis medievalis Y Y. pestis orientalis.
Yersinia pestis it is maintained in nature thanks to the 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 1500 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 flea of humansPulles 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 stomach of the flea, which when it becomes hungrier, will bite a new host in search of food. But due to the blockage of the proventriculus, he will vomit the freshly ingested and infected blood with Yersinia pestis, which the new host will invade. The flea will continue repeating the cycle until it dies of hunger.
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, 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.
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 break and the bacteria come out again, in large numbers into the bloodstream, causing great septicemia..
It may also be that the bacteria reproduce so quickly in the bloodstream that you don't have a chance to form buboes. If the bacteria enter the host via the respiratory system, they will reproduce in the lungs..
The bacteria Yersinia pestis is the etiological agent of plague, which can present in three different ways: bubonic, septicemic, pneumonic and minor 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, with the femoral and inguinal nodes being the most affected, followed by the axillary, cervical or other.
The spread of the bacteria through the bloodstream allows it to quickly reach any part of the body, including the lungs, and that bubonic plague can be complicated by a secondary pneumonic plague of blood origin.
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 and which occurs as a complication of bubonic 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 due to how quickly the disease progresses.
This is a less virulent form of bubonic plague, generally occurring in regions where Yersinia pestis it is endemic. In these cases there is a remission of the disease after a week of presenting symptoms.
Symptoms of the disease will change depending on the type of plague that occurs.
The first symptoms of the disease may include skin lesions (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 appear surrounded by edema and are painful, with reddened skin, but without a rise in temperature, they can suppurate in 14 days.
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..
This infection is asymptomatic until the last two days of the disease, when there is a large bloody sputum emission. There is a rise in 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.
Due to how quickly the infection develops, death usually comes before the first symptoms of it can appear..
This presents some of the symptoms of bubonic plague, such as swelling of the lymph nodes, fever, headache and general body pain..
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 can also be used (100 mg IV or PO every 12 h).
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