There’s a threat to our cave dwelling friend the bat. A disease called white-nose syndrome (WNS) is spreading through populations of bats and killing them while they hibernate. The fungus Pseudogymnoascus destructans is to blame. Once infected, bats experience fever, an increase in their arousal, and a decrease in fat stores; not what you need happening when trying to hibernate. The infected bats also suffer skin lesions where the fungus invades the skin. These lesions cause damage to their wings. Ultimately, WNS kills a bat by disturbing its hibernation enough that the bat uses up its carefully conserved energy stores. Without enough energy to survive its hibernation, it dies.

 White-nose syndrome affects bat species differently. Bats from the Palearctic region (Europe and Asia, excluding southeast Asia), such as Myotis myotis or the greater mouse-eared bat, show mild symptoms and can generally tolerate the fungus. Bats from the Nearctic region (North America to central Mexico) show varying levels of symptoms and susceptibility to the disease. This is where scientists are seeing an emerging and concerning level of bat mortality at the hands of WNS in North America.

 A large part of how badly a bat reacts to infection has to do with context. Has the bat species coevolved (meaning these species influenced the evolution of each other over time) with the fungus? Is the fungus just by chance adept at invading bat tissue, making it an “accidental pathogen”, or does P. destructans act differently on a bat wing than it would on the cave wall? How do different species of bats react to this fungus at a molecular level? To explore these questions and more, researchers Christina M. Davy , Michael E. Donaldson, et al. conducted a study “Transcriptional host–pathogen responses of Pseudogymnoascus destructans and three species of bats with white-nose syndrome” to explore the hypothesis that species specific interactions with the fungus Pseudogymnoascus destructans could partially explain the varying disease severity between species. 

It is worth mentioning just how this fungal infection is able to invade the bat’s skin at all.  P. destructans secretes enzymes that break down sebum, keratin, and Collagen, which are components of skin cells that offer protective barriers, structure and support. With these proteins broken down the fungus is able to spread into deeper layers of the tissue. 

To begin exploring the molecular response of these bat species and the invading fungus, the researchers conducted three experiments looking at three different species of bat. In the first and second experiment they exposed M. lucifugus, better known as the little brown bat and E. fuscus, the big brown bat, to a controlled amount of P. destructans and allowed the bats to hibernate under controlled conditions. These bats are from the Nearctic region, so having had no coevolution with the fungus the scientists were intrigued to see what kinds of molecular responses these species generated. When infected, the bat’s body will begin responding to the fungus invading its tissue. DNA will be transcribed, meaning it is coded into RNA, the string of nucleotides that cells can then read and turn into the proteins the body needs to mount its defense. Transcripts (messages within the cells that encode instructions to make new proteins) are present in the tissue of the bat wings and analysis of its code and frequency can lend insights into what kind of, and how severe of, a response the bat is having to the fungus. Inversely, sampling transcripts from the P. destructans can help explain how the fungus is able to target each bat species.

Samples from each bat’s wing were taken, a biopsy from a lesion positive area and a lesion negative area. The benefit of this method was it allowed the researchers to compare the amount of fungus present on the bat’s wing, when it simply had low fungal growth on the surface of its skin, versus the deeper more invasive presentation of the fungus. This method would also allow the researchers to explore the different levels of transcription happening at each skin site. The RNA from each biopsy was sequenced, trimmed for low quality reads, and mapped back onto a reference genome, or a known model of what the bat’s whole gene sequence should look like. This process separated what RNA sequences were bat, and what was fungal RNA.

In the third experiment the researchers observed and sampled from a wild greater mouse-eared bat population in the Czech Republic. These bats were exposed to the strain of P. destructans native to that area.

The first thing the researchers discovered was in all three bat species there was a higher level of genes belonging to the fungus in lesion- positive samples than lesion-negative, which can be visualized effectively in a box and whisker plot shown here from the primary primary research article linked below. The response to the fungal growth in bat lesions was the most similar between the little brown bat and the big brown bat, both Nearctic species. The P. destructans response, the genes it transcribed, as it infected the lesions were similar in all three bat species. 

The bats’ response to the fungal growth varied greatly between the species; there were very few overlapping transcripts, meaning the bats were coding different genes from each other in response to the fungal infection. In all, there were only five-transcripts, found in lesion-positive tissue, that were upregulated or expressed at a higher level, in all three bat species. With this, the researchers made some interesting discoveries as to how these bats respond to the fungus. 

In M. lucifugus the positive and negative lesion samples were most similar to each other in individual bats. In M. lucifugus, the little brown bat, it appears the bat mounts a systemic response to infection, meaning reaction to the infection is spread throughout the entire body. The appearance of up-regulation, the body's way of making more of the gene product, was seen in genes related to immune response and inflammation, indicating the bat’s entire body was driven to fight the fungus. On the other hand, E. fuscus, the big brown bat, showed the greatest similarity between positive samples from all big brown bats and the same for negative samples. This indicates a localized response to the fungus, one in which the body’s reaction stays in one area; the lesions. Here the researchers saw an upregulation of genes related to interleukins, chemokines, and protein kinases. These proteins are important in cell communication and cell regulation/motility, especially in immune responses, supporting the localized immune response. For example, interleukins travel to the target cell, bind and change the cell’s behavior. This finding in E. fuscus was novel to this study. The researchers’ findings support the idea that Myotis myotis, the greater mouse-eared bat that exhibits a lower level of symptoms to infection, also mounts a systemic response to the fungus. This clashes with another published work, mentioned in this study, that argues these bats have no immune response to the fungus. But in that study the experimental bats never developed clinical symptoms of infection. There were two ideas presented as to how this bat is able to better tolerate the fungus, one being a systemic or whole body response, the other being a baseline response that they maintain while hibernating. This baseline theory was discarded when researchers compared the expressed lesion-positive bat genes to previously published transcriptomes of Myotis myotis that were never exposed to the fungus. Genes from infected Myotis myotis matched better to similar immune response genes in M. lucifugus, supporting a systemic response instead of a tolerance one. The greater mouse-eared bat doesn’t appear to be generating some kind of low-level constant response while hibernating, whether infected or not.

The conditions between the 3 bats were slightly altered so a direct comparison of their gene expression should be processed with that in mind but the sequencing method was the same for the fungal transcripts across species so a comparison of fungal gene expression could be made. 

 The varying response of the bat species to P. destructans is certainly an area of further interesting research. The scientists from this study noted that species specific wing-chemistry or microflora could play a role in influencing the bats response to a fungus eating away at its tissue. In E. fuscus there were a few microflora (microorganisms that live on the skin) that appeared to offer potential protection from fungal growth. Mainly, they noted that a systemic response does not appear to be a guarantee for reduced reaction to WNS. Both the Myotis species in this study mounted that type of response yet had very different symptom levels. Looking at the fungus, P. destructans, the researchers noted that it is more likely the fungus is an “accidental pathogen” as its gene expression was highly similar between species and when grown on a bat wing versus not.

 In all, as WNS makes its way through North American bat species it is possible that bats susceptibility to the pathogen will decrease. Sadly, this will occur as bats susceptible to the syndrome die off, leaving those who by chance possess genes that make them better suited to dealing with the fungus to repopulate their communities. WNS though, if spreading too rapidly, could devastate bat populations before they have a chance to adapt. This is where studies like this are key. Better understanding of how one species can defend against a pathogen can be applied to treatments and protective measures to try and save susceptible species. Keeping in mind the importance as well to not lump all species together, as this study has shown, one explanation can not be laid upon all bats. Beyond the population decline and dangers of local species extinction from WNS, reduced bat populations pose greater ecological threats. Bats are an often feared and misunderstood creature and the important roles they play are not often talked about mainstream. Bats are key pest controllers and pollinators in many ecosystems. Those bats flying over your head at night aren’t trying to bite you but rather eat the bugs that bothered you all day. Many bats also eat nectar and fruits, playing a key role in pollination and seed dispersal. Protection of bat populations connects directly with protection of the larger workings of the ecosystems many of us are dependent on.

References 

The main article: 

Davy, Christina M., Michael E. Donaldson, Hana Bandouchova, Ana M. Breit, Nicole A. S. Dorville, Yvonne A. Dzal, Veronika Kovacova, et al. “Transcriptional Host-Pathogen Responses of Pseudogymnoascus Destructans and Three Species of Bats with White-Nose Syndrome.” Virulence 11, no. 1 (December 2020): 781–94. https://doi.org/10.1080/21505594.2020.1768018.

Other helpful links: 

https://evolution.berkeley.edu/evolibrary/article/evo_33

https://link.springer.com/referenceworkentry/10.1007%2F0-306-48380-7_2036

https://www.usgs.gov/news/how-does-white-nose-syndrome-kill-bats

https://my.clevelandclinic.org/health/articles/10978-skin

https://www.verywellhealth.com/systemic-reaction-1298693

https://www.britannica.com/science/interleukin

https://www.immunology.org/public-information/bitesized-immunology/receptors-and-molecules/chemokines-introduction

https://www.cellsignal.com/learn-and-support/protein-kinases

https://www.fws.gov/midwest/news/ImportanceOfBats.html

Alexis Tierney (‘22) is a graduate of Seneca High school and is from Shamong NJ. She will be a senior at Bucknell in the fall and is looking forward to her final year of school. Alexis is a double major in biology and psychology and plans to study clinical psychology after college. In her free time Alexis enjoys listening to music, playing with her dog and spending time with her friends and family. At school she is involved in flavor nutrient learning research in the psychology department and is excited about more opportunities to become involved in volunteer work in the fall.