Bacteria (singular: bacterium) are single-celled organisms which can only be seen through a microscope. They come in different shapes and sizes, and their sizes are measured in micrometer - which is a millionth part of a meter. There are several different types of bacteria, and they are found everywhere and in all types of environment.
Based on the morphology, DNA sequencing, conditions required and biochemistry, scientists have come up with the following classification with 28 different bacterial phyla:
Acidobacteria
Actinobacteria
Aquificae
Bacteroidetes
Caldiserica
Chlamydiae
Chlorobi
Chloroflexi
Chrysiogenetes
Cyanobacteria
Deferribacteres
Deinococcus-Thermus
Dictyoglomi
Elusimicrobia
Fibrobacteres
Firmicutes
Fusobacteria
Gemmatimonadetes
Lentisphaerae
Nitrospira
Planctomycetes
Proteobacteria
Spirochaetes
Synergistetes
Tenericutes
Thermodesulfobacteria
Thermotogae
Verrucomicrobia