TABLE 1.

Overarching concepts and fundamental statements.

Evolution	Cells, organelles (e.g., mitochondria and chloroplasts), and all major metabolic pathways evolved from early prokaryotic cells. Mutations and horizontal gene transfer, with the immense variety of microenvironments, have selected for a huge diversity of microorganisms. Human impact on the environment influences the evolution of microorganisms (e.g., emerging diseases and the selection of antibiotic resistance). The traditional concept of species is not readily applicable to microbes due to asexual reproduction and the frequent occurrence of horizontal gene transfer. The evolutionary relatedness of organisms is best reflected in phylogenetic trees.
Cell Structure and Function	The structure and function of microorganisms have been revealed by the use of microscopy (including bright field, phase contrast, fluorescent, and electron). Bacteria have unique cell structures that can be targets for antibiotics, immunity and phage infection. Bacteria and Archaea have specialized structures (e.g., flagella, endospores, and pili) that often confer critical capabilities. While microscopic eukaryotes (for example, fungi, protozoa and algae) carry out some of the same processes as bacteria, many of the cellular properties are fundamentally different. The replication cycles of viruses (lytic and lysogenic) differ among viruses and are determined by their unique structures and genomes.
Metabolic Pathways	Bacteria and Archaea exhibit extensive, and often unique, metabolic diversity (e.g., nitrogen fixation, methane production, anoxygenic photosynthesis). The interactions of microorganisms among themselves and with their environment are determined by their metabolic abilities (e.g., quorum sensing, oxygen consumption, nitrogen transformations). The survival and growth of any microorganism in a given environment depends on its metabolic characteristics. The growth of microorganisms can be controlled by physical, chemical, mechanical, or biological means.
Information Flow and Genetics	Genetic variations can impact microbial functions (e.g., in biofilm formation, pathogenicity and drug resistance). Although the central dogma is universal in all cells, the processes of replication, transcription, and translation differ in Bacteria, Archaea, and Eukaryotes. The regulation of gene expression is influenced by external and internal molecular cues and/or signals. The synthesis of viral genetic material and proteins is dependent on host cells. Cell genomes can be manipulated to alter cell function.
Microbial Systems	Microorganisms are ubiquitous and live in diverse and dynamic ecosystems. Most bacteria in nature live in biofilm communities. Microorganisms and their environment interact with and modify each other. Microorganisms, cellular and viral, can interact with both human and nonhuman hosts in beneficial, neutral or detrimental ways.
Impact of Microorganisms	Microbes are essential for life as we know it and the processes that support life (e.g., in biogeochemical cycles and plant and/or animal microflora). Microorganisms provide essential models that give us fundamental knowledge about life processes. Humans utilize and harness microorganisms and their products. Because the true diversity of microbial life is largely unknown, its effects and potential benefits have not been fully explored.