The first person to systematically classify bacteria was D.A. Bergey. He published ‘Manual of Determinative Bacteriology’ and kept bacteria under the class Schizomycetes.
Bacteria have been divided into six group:
- Modern bacteria (eubacteria)
The two major groups of monerans are, Archaebacteria (ancient bacteria) and eubacteria (true bacteria).Eubacteria is of further of two types – Bacteria and Cyanobacteria.
Archaebacteria represent a cell type that seems to possess the characteristics of both prokaryotes as well as eukaryotes.
In size, the archaebacteria are about 1μm in diameter, the size of typical prokaryotes lack membrane-bound organelles, nuclear bodies are bound by nuclear membranes as it is in eukaryotes and ribosomes are 70S, the size of those found in typical prokaryotes.
They have a unique cell wall that lacks peptidoglycan, closely resemble the eukaryotic cells in the mechanisms of protein synthesis, structural proteins, and RNA compliments of the ribosomes.
A very distinctive feature of archaebacterial genes is the presence of introns, elements that are unknown in other prokaryotes, though relatively common in eukaryotes. Archaebacteria also possess unique characteristic found in neither eukaryotes nor prokaryotes.
For example, their membrane contains branched-chain lipids with ether. This enables them to tolerate extremes of heat and pH.
Archaebacteria are of three types:
Methanobacteria are most primitive and lived on hydrogen.They are killed by oxygen and live in the digestive tract of grazing animals and rotten sewage (sludge digester) Example – Methanococcus, Methanosarcina.
Halophiles are chemo-organotrophic in nature. they require 17-23% NaCl. They are gram-negative and occur in salt lakes and are aerobic in nature. Example – Halococcus, Halobacterium.
Thermoacidophiles are a type of archaebacteria which live in an extremely acidic environment (pH-2) that have extremely high temperatures (upto 110oc). They are found in hot sulfur springs. Some of the eubacteria are also famous for living under the most hostile environment like salt pans, petroleum pans, spilled oil, hot springs, sulfur springs, snow, etc.example – Thermoplasma, Supholobus
The archaebacteria are the ‘ancient’ bacteria that include extremophiles like methanogens, halophiles, and thermophile. They represent some of the most ancient of like forms that persist today.
They have both eubacterial and eukaryotic characters besides the features unique to them. Their mode of reproduction, nutrition and cell shape and size resemble typical eubacteria.
Their cell walls are made of a variety of polymers but do not contain peptidoglycan, unlike eubacteria. Lipids of their cytoplasmic membranes are ether-linked, unlike eubacteria which contain glycerol ester lipids in their cell membrane.