A fungus among us
As I am constantly addicted to browsing the Internet even while on break, I came across two articles about fungal biology over the holidays. Fungi are eukaryotes, and the cells of fungi are very similar from a structural and biochemical standpoint in comparison to human cells. As we’ll learn in Chapters 9 and 10, this can lead to a difficult situation: antibiotics which target fungal infections can have with them serious side-effects, which must be taken into consideration when being prescribed. The most desirable antibiotics exhibit the principle of selective toxicity, where the antibiotic is more toxic to the pathogen than to the host that is infected. This is generally accomplished by the antibiotic affecting something about the pathogen that is different from us, and in the case of fungi this turns out to not be a trivial problem.
In the first article, the similarity between fungi and humans is demonstrated in a very profound way: fungi have a daily biological cycle just like humans. The experiments were done in a very cool way. Scientists found proteins in the common bread mold Neurospora crassa that showed a daily up and down cycle in their abundance. They used one of these proteins and joined it to a firefly protein called luciferase, which releases light that can be measured in the laboratory. They grew the mold in the lab with a set 24 hour cycle of light and dark. When the mold was moved from a light/dark cycle into complete darkness, the mold continued to glow with a characteristic daily cycle of 22 hours instead of 24 hours which resembles a circadian value close to that observed for bacteria. Mammalian circadian rhythms are very close to the 24 hour period, and cannot vary far from that median value. The biological value of a sun-based clock for the bacterial example is clear: the organism is photosynthetic, and therefore is dependent upon sunlight for it’s lifestyle. In the case of the bread mold, the biological value is open to conjecture.
Fungal infections are common among many eukaryotic organisms, including plants, insects, amphibians, and fish. Interestingly, fungal infections in mammals are far less common, with fewer than 400 species of fungi able to cause disease in humans, and only four species that represent the most significant fungal pathogens. The second article proposes a hypothesis as to why mammals are relatively resistant to fungal disease. Researchers at Albert Einstein College of Medicine compared the relative fitness of an organism (as measured by it’s innate resistance to a fungal infection) at a given core body temperature to the metabolic cost of maintaining that body temperature. Although there are significant benefits to elevated body temperature in protecting an organism from infection by pathogens, the increased body temperature comes at a cost: the organism must expend calories from its diet to generate heat. The authors Bergman and Casadevall conjecture that the observed 37°C body temperature of mammals hits the “sweet spot” where the benefits (due to innate resistance to a particular type of pathogen) and drawbacks (due to the need to constantly take in calories in the form of food to generate heat) are balanced out. If the core body temperature of mammals was just a few degrees lower than 37°C, it is likely that they would exhibit a higher number of infections due to fungi. Conversely, if the core body temperature was a few degrees warmer, the organism would have to spend significantly more time consuming calories just to maintain that temperature.
So here is our first Bonus opportunity: based on what you know about how cells work and the second article, why do you think that fungi do not grow very well on humans in comparison to insects, fish, or amphibians? That is; why is the human body at 37°C not an optimal place for fungi to grow? Might there be aspects of human anatomy that might be more susceptible to a fungal infection?