Archaebacteria have been many times classified with bacteria because of all fundamental similarities. These are supposed to be a very ancient group of bacteria. Their cell wall is unique and consists of polysaccharides and protein. Archaebacteria contain two sub-groups:
(1) Methanogens and (2) Thermoacidophiles
Thermo-acidophiles are found in hot sulphur springs. Under aerobic conditions and at a temperature around 80 degree Centigrade, these oxidise sulphur to sulphuric acid and can make the environment highly acidic (pH 2). Archaebacteria live in extreme environments and were separated from the main line of bacterial evolution very long ago.