Diversity of Bacteria.

Bacteria exhibit extraordinary evolutionary and ecological diversity. They range from dominant, well-characterized phyla to rare lineages that are known only through environmental sequencing. This chapter reviews four key bacterial phyla, including Pseudomonadota, Bacillota, Actinomycetota, and Bacteroidota. These phyla are widely distributed, metabolically versatile, and play a central role in ecosystem functioning and human health. We discuss unique phyla within the PVC superphylum (Planctomycetota, Verrucomicrobiota, Chlamydiota) for their unusual cell biology, compartmentalization, and host associations. We also highlight hyperthermophilic phyla, such as Thermotogota, Aquificota, and Thermodesulfobacteriota, that thrive in geothermal ecosystems and drive sulfur and carbon cycling. We consider less-cultivated lineages, including Deinococcota, Acidobacteriota, Nitrospirota, Fusobacteriota, Fibrobacterota, Synergistota, Deferribacterota, and Chrysiogenota, in terms of their ecological niches, metabolic specializations, and roles in biogeochemical cycles, symbiosis, and disease. Collectively, these examples demonstrate the remarkable metabolic flexibility and ecological impact of bacteria, ranging from host-associated commensals and pathogens to free-living autotrophs in extreme environments. Despite advances in genomics and cultivation-independent methods, vast portions of bacterial diversity remain uncultured and poorly understood. Continued exploration of both dominant phyla and rare lineages promises to refine bacterial taxonomy, expand our understanding of microbial evolution, and reveal novel metabolic pathways with implications for ecology, medicine, and biotechnology.