* By Number of Species: The Proteobacteria are the largest and most diverse phylum of bacteria, with a vast number of species. They include a wide range of organisms like *E. coli*, nitrogen-fixing bacteria, and many disease-causing bacteria.
* By Biomass: It's estimated that the Actinobacteria make up the largest amount of bacterial biomass on Earth. They are responsible for many important functions in the environment, such as decomposing organic matter and producing antibiotics.
* By Abundance: The Cyanobacteria are extremely abundant in aquatic environments and are responsible for a significant amount of oxygen production on Earth.
In conclusion, there is no single "largest" group of bacteria. It depends on how you define "largest" and the specific measurement you are using.
