Primary Source: Charlotte Rafaluk-Mohr, Michael Gerth, Jordan E. Sealey, Alice K.E. Ekroth, Aziz A. Aboobaker, Anke Kloock, Kayla C. King, Microbial protection favors parasite tolerance and alters host-parasite coevolutionary dynamics, Current Biology, Volume 32, Issue 7, 2022, Pages 1593-1598.e3, ISSN 0960-9822, https://doi.org/10.1016/j.cub.2022.01.063.
While it may not be the most glamorous fields in biology, the study of parasites is a crucial aspect to understanding the ecosystems we live in. Through studying parasites, we further explore the spread of disease, resource management, and prevention of infection, or even death, due to parasitic infections. Of course, the study of parasites goes hand in hand with studying the host species that these parasites inhabit. The relationship between host and parasite, while very specific, can be generalized into one of two types. The first, known as directional, is a dynamic defined by the evolutionary arms-race between the host, developing better and better protection, and the parasite, developing better and better infectability. The second, known as a fluctuating dynamic, depends on the health of the host population. Under a fluctuating dynamic, when the host population does well, the parasite has high infectability. Conversely, when the host population suffers, the parasite’s infection rates go down accordingly. These evolutionary dynamics shape the future for both the parasite and host species. As such, manipulating these dynamics may be key to the conservation of both host species and their parasites. A recent study by Oxford researchers explores these interactions and the potential of host microbiomes influencing the long-term coevolution between host and parasite species.
Learning to be Tolerant
In order to study the evolutionary trends of a parasite and its host, researchers used a species of roundworms, Caenorhabditis elegans, as a model host organism, along with the bacteria responsible for staph infections, Staphylococcus aureus, acting as a parasite. The microbial lifestyle of these organisms means that they reproduce rapidly, allowing many generations to come and go within the span of the study. Additionally, one group of C. elegans was colonized during development by bacteria Enterococcus faecalis. While common in the human microbiome, this bacteria’s introduction to C. elegans was novel. E. faecalis offers protection by producing anti-microbial substances and preventing the theft of nutrients from an initial parasite infection to propagate further parasite generations. The C. elegans colonized by this bacteria were referred to accordingly as the protected group. Typical response to parasite infection, on the part of the host organism, typically consists of one of two strategies. The first strategy, more frequently present in organisms with a short life span, is to ramp up reproduction to ensure that their genes are passed along before parasitism kills them. This type of parasite tolerance is known as fecundity tolerance. The second, less bleak strategy is to survive through the infection. This second strategy, called mortality tolerance, benefits both the parasite, which now has more chances at infecting the individual host, as well as the host, who now has more opportunities to pass on its genes.
Nematodes: The Next Generation
After 15 generations of nematodes and parasites, researchers found that both groups had increased fecundity tolerance. The researchers also tested the ability of the protected group and unprotected group to be colonized by the beneficial E. faecalis bacteria after 15 generations. Notably, the protected group was colonized by significantly more of the protective bacteria than the unprotected group, meaning that the protected group had evolved to recruit more of the protective bacteria. While both groups developed fecundity tolerance, the group protected with a bacterial microbiome of E. faecalis were more prone to mortality tolerance than the unprotected group. Mortality tolerance typically develops in response to less harmful infections, as surviving through parasite development is less costly than preparing an immune response. Seeing this response occur in response to the introduction of protective bacteria suggests that this introduction to the microbiome shifted evolutionary dynamics between the parasite and host by reducing the harm caused by parasite infection.