They live in communal nests by the hundreds and have extremely low genetic diversity. Nevertheless, social spiders successfully manage to repel attacks on their nests by dangerous pathogens. Previously, researchers thought that the secret was inside the spiders. However, new research suggests that the spiders’ nest material is the key to the strong antimicrobial defence. The nests contain very special bacteria and fungi, and the researchers hope that these include antibiotics that can alleviate the growing antimicrobial resistance threatening people and health systems worldwide.
The COVID-19 pandemic has taught us how crucial distance can be for the transmission of disease. Social insects and arthropods face the same challenge. Perhaps not surprisingly, only 76 of 93,000 known species of arachnids (including spiders, scorpions, ticks, mites, harvestmen and solifuges) live in social groups. Studying how these rare social arachnids defend their nests from being attacked by bacteria, viruses or fungi is therefore especially interesting.
“Previous studies of the social spider species Stegodyphus dumicola have shown that neither the spiders’ immune system nor the microbiome provides special protection. Instead, our genetic analysis shows that the special bacterial and fungal nest microbiome may help to protect them. We hope that some of these antimicrobial agents will also be able to help people to combat increasing antimicrobial resistance,” explains lead author Seven Nazipi, PhD in microbiology and previously from the Department of Biology – Section for Microbiology, Aarhus University.
Re-searching the nests
Social insects such as bees, ants, and termites are not rare. The benefits gained from group living involve higher reproductive success because of parental care, protection from predation by nest building and nest-guarding behaviour, and improved foraging success. This also applies to Stegodyphus dumicola,which live in female-dominated communities with up to several hundred individuals cooperating on most activities.
“Nevertheless, these very closed communities have extremely low genetic diversity, which poses a potential threat for infection by external pathogens. The theory therefore is that specific bacteria and fungi inhabiting the spider nests may provide antimicrobial defence. Since no evidence indicates that the microbiome of the spiders themselves has a protective effect, we attempted to investigate the microbiome of the nest material in the new study,” says Seven Nazipi.
Stegodyphus dumicola spiders build compact tunnelled nests comprising dense layers of spider silk that are often built on branches of savanna shrubs such as acacia trees in central and southern Africa. The nests often also incorporate the remains of their prey, and the researchers believe that this may be the source of the spiders’ defences.
“Previous studies of insects have shown that specific microorganisms that live in the nests can provide antimicrobial defence against other pathogens. We sequenced nest material to characterise the bacterial and fungal diversity of 17 nests in three locations in Namibia. We hoped to find a microbiome pattern in these bacterial nests that could determine whether the spiders lived in symbiosis with or were aided by specific bacteria or fungi,” explains Seven Nazipi.
Based on the genetic data, the researchers mapped the composition of the microbiomes and discovered that they were largely determined by the local soil or plant environment, meaning that the geographically separated spider populations also surrounded themselves with different types of microorganisms.
“Nevertheless, we identified a core microbiome that was similar for all three geographical locations comprising four bacterial genera and four fungal genera, which probably originate from the surrounding soil and plants near the nest environment. Although we did not find any evidence of symbiosis between bacteria, fungi and spiders in the nests, we did find antimicrobial activity in bacteria and fungi that may protect the spiders from external pathogens,” says Seven Nazipi.
Very interesting suggestion
The researchers had hoped that the new study could identify symbiosis between spiders and bacteria, such as the symbiosis between leafcutter ants and Actinobacteria (Pseudonocardis), which produces antibiotics that aid in controlling the parasitic Excovopis fungi. A chemical analysis from these specific bacterial symbionts led to the discovery of the fungicide dentigerumycin.
“Several symbioses exist between insects and bacteria that have provided knowledge about new and effective substances such as candicidin D, used for treating the pathogenic yeast Candida albicans. Although we cannot identify any antimicrobial compound excreted by the bacteria in the nests that helps the spiders in their defence, bioinformatics tools can enable us to determine that some very interesting regions in the bacteria’s genomic sequences could encode completely new antibiotics that can help people,” explains Seven Nazipi.
Whether the bacteria and fungi that create the effective defence for the social spiders originate from desert soils, plants or prey capture, the researchers are convinced that they are specifically selected from the local environment and then possibly enriched in the nests.
“We still need to clarify whether and how the nest microbiome actually provides antimicrobial defence for the spiders, but the genetic potential in both bacterial and fungal isolates may indicate this. The next step is to isolate and test each of the bacterial strains, and ultimately we hope to find the compounds that help the social spiders and to determine whether these can also help people to achieve a healthier and safer life,” concludes Seven Nazipi.