Editorial: Insights into Microbe–Microbe Interactions in Human Microbial Ecosystems: Strategies to Be Competitive

The human body is colonized by trillions of commensal microorganisms (bacteria, archaea, viruses, and microscopic eukaryotes) that are collectively referred to as the human microbiota. The microbiota colonizes the skin and mucosal body surfaces of humans and animals, where they are engaged in a constant crosstalk with the host immune system and metabolism. This human microbiota displays a vast genetic catalog, the so called microbiome, contributing functions that are not encoded by our own human genome (Li et al., 2014). The classical tools to analyze its taxonomy and diversity, such as microscopy and cultivation, have been gradually replaced by culture-independent approaches. Initially the study of the human microbiota focused on taxonomy but interests have shifted to understanding the functional role of these human microbial ecosystems and their implications for the host (Salazar et al., 2014). It is also well established that the composition and functionality of this microbiome is essential for maintaining a " healthy status. " The microbes living on and within the human body inhabit competitive and complex environments, and deploy different ecological strategies for survival, which may imply notable changes on these microorganisms at metabolic, physiological and genetic level. The current Research Topic covers a collection of reviews, mini-reviews and original research articles that discuss how bacteria adapt to the specific human niches by competing, or otherwise co-existing with other bacteria and host cells. Recent development of high-throughput analytical tools and " meta-omics " technologies has allowed us to obtain complete overviews of community composition and diversity as well as inferred functionality of genes and metabolic pathways in a wide range of body habitats. The mini-review by Palau-Rodríguez et al. discusses the use of the metabolomic approach as a powerful tool for exploring the crosstalk between microbial and host metabolism in order to identify human gut microbial-host co-metabolites in the context of metabolic diseases such as obesity and type 2 diabetes. The laboratory mice are useful experimental models for the study of microbial communities in a mammalian host. The review by Martín et al. provides a comprehensive overview of the application of gnotobiotic animals as tools to decipher the mechanisms underlying microbe–microbe and microbe–host interactions. In addition, the combination of gnotobiotic techniques with new approaches (" omics " and genetic engineering) has revealed causative associations between alterations in the commensal microbiota and several diseases.