Yersinia pestis is the historical leader of all pathogenic-leaning bacteria. As a renowned zoonosis-oriented pathogen, his main non-human hosts being fleas and rodents. Once a terrible figure to humans and bacteria alike, he made a reputation for himself through relentless violence towards both humans and other bacteria.
Pestis cause three major pandemics throughout human history: first, the Justinian plague (541 to 767), and the Black plague (most casualties between 1340 and 1360, but lasting up to the 18th century, killing at least a third of Europe’s then population at its peak). A third pandemic spanned from the end of the 18th century to the 20th century, with modern means of transportation and industrialisation playing a key role in propagation. The mortality rates from this last pandemics were a lot lower than that of the previous two 1,2 .
The pandemics shaped Pestis’s evolution in major ways, especially the Black plague. Pestis’s age and evolution have been very tricky to trace back, because of discordances in molecular clock (that measures the number of mutations in a species over time). During infection, Y. pestis replicates his DNA very frequently to proliferate, which is not the case when living in his natural foci. Even though the mutation rate is constant in both cases, more DNA replication means more emerging mutations 2. This is why Pestis as a person underwent trendemous changes under a very short amount of time for a bacterium.
Though he hasn’t come to this realisation yet, Pestis exists as a part of a complex of strains having emerged from Yersinia pseudotuberculosis, known as the Yersinia pseudotuberculosis complex.
Background
Pestis is an exceptional evolutionary case for a pathogen. His ~5000 years lifespan makes him a very young individual 3 . Yersinia pestis as we know him initially diverged from Yersinia pseudotuberculosis 4, a known opportunistic pathogen of the gut to humans. Moreover, Pestis has undergone many kinds of evolutionary events at exceptional speed, given his young age.
According to Mycobacterium tuberculosis, as Pseudotuberculosis, he was shy, quite insecure, and easily swayed. He spent his early life admiring Tuberculosis and, even though it scared him somewhat, he marveled at other’s pathogenic abilities. Most things he did, he did for Tuberculosis’s attention, and then approval. Acting as a mentor, an older brother and a best friend, Tuberculosis actively trained and encouraged Pestis’s pathogeny.
At some point, Pestis realised his true potential for pathogeny was remaining untapped. As he started enjoying his abilities, he took it upon himself to organise his first massive epidemy. This is how the Justinian Plague took place, and it made Pestis determined to go farther beyond. After having proved his worth this way, and wanting to go farther and beyond, he ravaged Europe with the Black Plague. The Black Plague pandemic, to Pestis, was a way to assert himself once and for all, and to make himself feared all over the world. Encouraging hate for humans under flimsy pretences was just a way for him to justify his behaviour.
All of this mostly went as planned; plus, Pestis managed to unite many bacterial human pathogens, beyond his friends, around his own person (such as Vibrio cholerae, Pseudomonas aeruginosa, Streptococci and Staphylococci, Shigella… etc). Eventually, there was a cause linked to his name: killing as many humans as possible. It was then that Pestis lost Tuberculosis’s approval. Afraid of Pestis’s cruelty and arrogance, Tuberculosis’s attitude changed drastically, and the two of them got into a fight, injuring Tuberculosis in his left eye. This event left Pestis bitter, but harsher than ever. Despite claiming to act in the interest of all Bacteria, all Pestis did was foster the divide between pathogens to humans and others, especially human mutualists or commensals (such as Escherichia coli). These bacteria started being looked down on by Pestis’s entourage, even though they often populate other mammals as well 5.
After a few centuries, with support from many other pathogens, Pestis tried to engage in one last wide-scale operation: the 18th century Black Plague pandemic. This initiative turned out to be what gave him in, as Pestis was isolated for the first time by Dr. Kitasato Shibasaburou, and further described by Dr. Alexandre Yersin 6. Despite casualties in the millions, Pestis realised that his modus operandi being tapped into by humans, and the use of antibiotics, meant he wouldn’t be able to reengage in huge pandemics again for a while. This led him into profound torment, as his most trusted friend, Tuberculosis, wouldn’t agree to formally, and willingly, taking on his legacy. To this end, Pestis designated one of his most promising followers, Staphylococcus aureus, as a successor, for his keen ability to evolve genomic antibiotic resistances.
Nowadays, Pestis lingers in the wilderness. He inhabits arid meadows, steppes, and deserts throughout the world, with all kinds of small mammals as his hosts 7 , and fleas as vectors still.
Personality
Pestis’s personality nowadays is quite different from what he became famous for. Ever since diverging from Yersinia pseudotuberculosis, his personality underwent many successive changes, in a very short amount of (bacterial) time. At his core, Pestis is a very sensitive, but headstrong, person. He values loyalty, being true to one’s word, and doing what he thinks is right above all else (though through a very twisted moral compass). He likes to show off one way or another, and has a hard time admitting defeat, especially in front of others. As a person, he can be quite friendly, and likes humouring the people he meets if he deems them worthy.
In the past, Pestis was widely known as a tiny, yet fearsome, tyrant, cruel and unrelenting. To those close to him then, he hasn’t changed much, if maybe for boasting a bit less these days. To others, Pestis’s appreciation was a matter of life or death, be it social or literal. He often judged things based on instinct and impulsivity. Having very little empathy, or otherwise pity, the Pestis of his glory days was especially self-centered. Physical violence was his primary means of making himself heard and respected.
Now, with his reputation with humans having caught up to him, Pestis was forced to take a step back. He doesn’t feel regret for most of his past, but became more contemplative, and overall calmer. He realised how easily his short-temper could backfire, and tries to think more before acting. He tires of big displays of strength faster, and, ironically, values his close relationships more than ever. He will still take on a fight willingly, but not needlessly.
Relationships
As the leader of all bacterial pathogens, Pestis also acts as their emissary, be it to men or microbes. As such, he has been in contact with very many people; those who have never heard of, or seen, Yersinia pestis are probably to be counted on the fingers of a hand. Despite his firm beliefs and his commitment to his cause, every person he met influenced him in some way – though some more than others.
Deinococcus radiodurans
Radiodurans is like a pebble in Pestis’s shoe. By preaching ideas completely opposite to that of Pestis – that is, peace among all species, across all phyla –, Radiodurans effectively created a popular movement against that of Pestis.
Even now, Pestis considers Radiodurans an enemy, though he knows how silly he looks trying to take him down (Radiodurans just happens to be very good at dodging).
Escherichia coli
to be completed
Homo sapiens
to be completed
Mycobacterium
Mycobacterium tuberculosis
Pestis’s feelings towards Tuberculosis are too intense for their bond to be called simple friendship. But they are, in nature, entirely different from what romantic love is. In short, Pestis has an extreme attatchment to Tuberculosis, even beyond that of a mentor. To each other, Tuberculosis and Pestis are like platonic soulmates.
Despite this unequivocal fact, their relationship has been very tumultuous. Pestis, in particular, has been very controlling of Tuberculosis, stemming from their disagreement on humans. He is convinced Tuberculosis has been “lost” to them, and has felt helpless and desperate about it for many centuries now. Now that he’s living in relative isolation, Pestis somewhat regrets his behaviour towards him, though he may not admit to it.
Curiously, despite existing in a complex himself, Pestis has always been completely oblivious to the Mycobacterium tuberculosis complex, and talks to each member as if they were entirely separate individuals.
Mycobacterium leprae
Leprae and Pestis are long time friends, though they don’t see each other that often anymore. In spite of Leprae’s physical weakness, Pestis never gave up on supporting him, and is always proud to talk about him as a friend.
Vibrio
Vibrio cholerae
These two have a lot in common. Pestis knows he can always trust Cholerae to be there for him, and will defend him against anything if need be. Cholerae is his best friend, Tuberculosis put aside. He watches out for Cholerae in his brothers’ stead when possible.
Vibrio harveyi
Harveyi and Pestis get along really well, largely due to their mutual curiosity with one another. Both being pathogenic in their own way, and knowing great fame in their respective communities, their goofy temperament only brought them even closer together.
Vibrio vulnificus
Pestis approves of Vulnificus’ pathogenic power, and doesn’t really understand what Vulni’s got against him. He wishes Vulnificus would come to his senses, but Harveyi having warned Pestis about him, he doesn’t have any expectation for the situation to resolve anytime soon.
Wolbachia
Pestis tends to think of the Wolbachia as “business partners”. Both have been in touch through flea hosts on the regular.
At first, Wolbachia pipientis was disapproving of Pestis setting up fleas to his own profit. Eventually, they came to an agreement, allowing Pestis to develop his activity (though it heavily leaning in the Wolbachia’s favour). A major part of this agreement was to have Pestis give up all use of his Type 3 Secretion System in the fleas 8 . Nowadays, they are still very positively acquainted to each other. Pestis is one of the few outsiders that the Wolbachia actually tolerate, and feels very flattered about it, deep down.
As Wolbachia strains have been identified in Xenopsylla cheopis 9, the two of them may occasionally meet in the flea ovaries, more specifically the oocytes 10, 11.
Yersinia pseudotuberculosis complex
Yersinia pestis is part of the Yersinia pseudotuberculosis complex, as he and Pseudotuberculosis are still so close on a genomic level that they could be classified as a single species. Making them two separate subspecies of Y. pseudotuberculosis has been proposed by several authors before 4 . The phylogeny of the complex is currently being investigated 12.
Pestis and Pseudotuberculosis are the same person, with different temperaments and some shared memories. Pseudotuberculosis remained joyful and naive, and didn’t experience the plague pandemics the same way Pestis did. Recently described Yersinia similis, and member strains of the Korean group, may also be other incarnations of this individual.
Trivia
- Pestis has incredible physical strength; it’s very surprising to witness, as it doesn’t show in his build at all.
- Pestis’s cytoplasm is black and thicker/stickier than average. It can clog up his eyes when he gets angry or sad, making it difficult for him to see. This is why the darkness of the gray in his eyes tends to vary. A happier Pestis has lighter-tinted eyes.
- Pestis’s T3SS is very famous in microbiology and biochemistry, as it was the first T3SS to ever be studied 13.
- Despite his hatred of humans, he is actually quite fond of his other hosts, especially the smaller mammals like mice or rats.
References
Disclaimer
Reference 1 has Didier Raoult on it, but also an anthropologist that I was lucky enough to meet IRL in uni once (Michel Signoli). He gave a very interesting presentation about the plague epidemic that my home city endured back in the days of the second pandemic. I am choosing to cite this paper for its historical aspect. Not all of Raoult’s publications were frauds, but I hate Raoult with a burning passion for dishonoring our microbio scene on an international scale and for his complete disregard for ethics in clinical trials (and I am not even alluding to COVID here). This is why I try my best to steer clear of any publication with his name (and some of his colleagues’) on it.