COS 19-4 - Effects of Sceloporus virgatus cloacal microbiota on the growth of pathogenic fungi

Background/Question/Methods

Many oviparous species do not provide parental care for their offspring after laying eggs. For instance, striped plateau lizards (Sceloporus virgatus) lay eggs in dirt nests at the start of monsoon season and then abandon their brood. Left unattended, nests may offer ideal growing conditions for pathogenic fungi that grow on eggshells, feed off embryonic nutrients, and kill eggs. How might parents be able to protect their eggs from infection in the absence of parental care behaviors such as egg-tending? One mechanism may be providing protective microbes to eggs. In S. virgatus, females transfer symbiotic bacteria to their eggs as they pass through the cloaca (the opening of the reproductive tract). We hypothesize these bacteria inhibit fungal growth. Pathogenic fungus strains Neocosmospora rubicola, Purpureocillium lilacinum, Aspergillus sp., and Fusarium sp. were cultured from soil collected at putative nest sites and exposed to 7 cloacal bacteria species: Enterobacter ludwigii, Citrobacter werkmanii, Enterococcus faecalis, Citrobacter amalonaticus, Serratia 1353, Serratia 41 and Serratia 31. We conducted plate-based challenges to visualize the effects of each bacteria isolate on fungal growth. To quantify fungal growth, we counted the number of spores in liquid co-cultures for each fungal strain once a control plate started to sporulate.

Results/Conclusions

Across all 7 bacterial strains, the cloacal microbiota significantly reduced the number of P. lilacinum, Aspergillus sp., and Fusarium sp. spores (p < 0.001), and tended to reduce the number of N. rubicola spores (p = 0.058). The bacteria strains Serratia 1353, Serratia 41, and Citrobacter amalonaticus exhibited broad antifungal properties, reducing the number of spores for each fungus. These results support our hypothesis that the cloacal microbiota does reduce the growth of pathogenic fungi. Some mechanisms by which fungal growth can be limited are if (1) the bacteria outcompete the fungi for space and nutrients in the cocultures or (2) the bacteria actively degrade fungal cells. Regardless of the mechanism, the cloacal microbiome may ultimately increase hatch success and fitness of untended eggs. These results also suggest that some pathogenic fungal species, like N. rubicola, may be more resistant to the cloacal microbiota and thus could be a greater threat to hatch success and fitness of reptilian offspring. To shed more light on the degree of pathogenicity for these soil strains, future research could directly explore the effects of N. rubicola, P. lilacinum, Aspergillus sp., and Fusarium sp. on the hatch success and fitness of S. virgatus eggs.