Symbiotic Interactions of Archaea in Animal and Human Microbiomes

Abstract Purpose of Review Since the introduction of Archaea as a new domain of life more than 45 years ago, progress in their phylogenetic classification and knowledge of their exclusive biological characteristics has identified archaea as unique microorganisms which are widespread in extreme but also in various moderate ecosystems, including eukaryotic hosts. However, archaea are still neglected players within microbiomes, and research on archaea-bacteria interactions is still in its infancy due to methodological challenges. Recent Findings This review summarizes the current knowledge of archaea as components within microbiomes and focuses on their interactions with their bacterial neighbors and the principles of archaeal interactions. Summary Archaea are common constituents of animal and human microbiomes, which are dominated by Euryarchaeota. The gastrointestinal tract is the most studied body site, where archaea account for up to 4% of all microorganisms, primarily represented by methanogens. No archaeal pathogen has yet been identified, although methanogens are hypothesized to be indirectly involved in pathogenicity. Archaeal interactions comprise symbiotic relationships, and the cell membrane and wall might be as crucial as quorum sensing/quenching for these interactions. Particularly, syntrophic interactions under energy-deficiency stress seem to be an essential strategy for archaea. However, more research is urgently needed to discover how archaea sense their environment, compete with bacteria, and interact within complex microbiomes associated with multicellular organisms.