SICK OFMEPATHOGENS? TOO.
Dave Warrell delves into the paradox of the good and evil of pathogens.
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ince the days of Louis Pasteur and Robert Koch, microbiologists have categorised microorganisms according to whether or not they cause disease. Any microorganism that does is a pathogen. Viewing microorganisms as binary, as disease-causing or not, is understandable. We see virtually everything in a similar way; in every story there are good guys and bad guys. It’s how we make sense of the world. In the case of pathogens, the result is a mindset that focuses on how pathogens damage us and how we might kill them. This tends to give pathogens agency; it makes their actions appear deliberate. We think of them as wanting to cause disease. The result is a series of war metaphors: we are currently battling COVID, we eradicated BSE, the fight against polio is nearly won. Yet, in the last 50 years, the likelihood of a decisive human victory over infectious disease has come into question. We have seen the rise of new pathogens and we have seen attempts to kill off many others fail. There are now calls to change the emphasis, to adopt a more ecological view of pathogens to help us understand them better. Taking a leaf out of ecology’s book Nature depends on symbiosis: all organisms have come to depend on organisms belonging to different species for their survival. Unlike the word pathogen, symbiont, which refers to any organism taking part in a symbiotic relationship, is neutral. Sometimes symbionts can have a positive effect on one another. For example, we provide a guaranteed source of food for the bacteria that live in our gut. In exchange, many of them help us to break down certain substances and help us build up our
So, is there a difference between a parasite and a pathogen?
“A mindset that focuses on how pathogens damage us and how we might kill them...tends to give pathogens agency; it makes their actions appear deliberate.” immune systems. Relationships like this are known as mutualism. On the other hand, there are many symbiotic relationships where one symbiont thrives to the detriment of the other. Plasmodium is a unicellular organism which can only reproduce inside other animals. In vertebrates, it primarily infects red blood cells, reproducing until the cells burst. In humans, this causes the group of symptoms that we call malaria. Because Plasmodium reproduces at our expense, it’s an example of parasitism. When there is a benefit to one symbiont but no clear effect on another, it is called commensalism. For example, many bacteria happily live on our skin with no ill effects for us. We often think of pathogens as separate entities from parasites, yet most definitions of parasites also apply to pathogens and vice versa. Think about malaria: we consider Plasmodium to be a parasite because it lives inside us to survive and it causes disease. How is this different to influenza viruses and the flu?
The answer isn’t clear-cut. Categorising microorganisms seems to be based on convention. Sometimes it’s a case of scale. For example, tapeworms are normally visible without a microscope, and are considered parasites rather than pathogens. However, many parasites, such as Plasmodium, are unicellular. At the microscopic level, the differentiator seems to be cell structure. Disease-causing eukaryotes, like Plasmodium, have cells with a nucleus, similar to ours and are considered parasites. Disease-causing prokaryotes, like bacteria such as E. Coli, don’t have a nucleus, and are considered pathogens. But wait, fungi, who are also eukaryotes, are generally considered pathogens? My point is that these divisions are arbitrary, so why don’t we place pathogens on the same scale as parasites? The term pathogen was coined specifically to describe an organism that causes disease. It doesn’t allow for any ambivalence. By contrast, the sliding scale of symbiosis allows us to capture the good, the bad, and everything in between. This ecological approach acknowledges that, regardless of what we call these microorganisms, their only imperative is biological: to replicate. It shifts the emphasis away from the actions of the pathogen and onto the relationship with its host or other microorganisms. Importantly, it acknowledges that there are many factors which influence whether a symbiont has a positive or negative effect on its host, that a parasite in some contexts is a mutualist in others.
Pull quote Artwork: Hermine de Clauzade de Mazieux
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I, SCIENCE
