From the Cell to the Ecosystem: The Physiological Evolution of Symbiosis

Living organisms constantly interact with their habitats, selectively taking up compounds from their surroundings to meet their particular needs but also excreting metabolic products and thus modifying their environment. The small size, ubiquity, metabolic versatility, flexibility, and genetic plasticity (horizontal transfer) of microbes allow them to tolerate and quickly adapt to unfavorable and/or changing environmental conditions. The consumption of resources and the formation of metabolic products by spatially separated microbial populations constitute the driving forces that lead to chemical gradient formation. Communication and cooperation, both within and among bacterial species, have produced the properties that give these organisms a selective advantage. Observations of a wide range of natural habitats have established that bacteria do not function as individuals; rather, the vast majority of bacteria in natural and pathogenic ecosystems live in biofilms, defined as surface-associated, complex aggregates of bacterial communities that are attached to solid substrates and embedded in a polymer matrix of their own production. The spatial configurations of biofilms reach levels of complexity nearing those of multicellular eukaryotes. Microbial consortia have played important roles throughout the history of life on Earth, from the microbial mats (a type of biofilm) that were probably the first ecosystems in the early Archean, to the complex microbiota of the intestinal tract of different animals.