Micro-organisms and the Microbiome

The term ‘Micro-organism’ refers to any living thing that is too small to be seen with the naked eye. It covers an astonishing array of life forms that began with the earliest living occupants of our planet. Of the three recognized domains of life, two (Bacteria and Archaea) consist exclusively of micro-organisms. The third (Eukaryota) contains all of the macroscopic, multi-celled organisms that we recognize as plants and animals, but it also includes many micro-organisms. ‘Microbiome’ is a collective term for all of the micro-organisms belonging to these three domains, while the ‘Biome’ comprises all life on earth. For most of earth’s history until the evolution of multicellular Eukaryota around 600 million years ago, however, the earth’s Biome consisted only of micro-organisms (Fig. 1.1). Even now the Microbiome is estimated to represent more than half of the total living matter (biomass) on the planet [1]. Their invisibility makes it easy for us to overlook the vast impact that micro-organisms have on the sustainability of the planet and of life on earth. While this book will focus on the gut-associated Microbiome, it is important that we start with a look at the wider microbial world.