Bacteria have a protective covering called 'cell wall'. Beneath this. there is a plasma membrane which encloses protoplasm of the bacterium containing various types of RNA, DNA, proteins and organic molecules. In bacteria, DNA molecule and enzymes involved in oxidation of food are found associated with th-e plasma membrane (Figure shown below).
Bacteria, though simple in their structure, are the most abundant of all the life forms on earth today. This is due to their rapid rate of multiplication and the ability to adapt to any nutrient and environment.
Viruses are on the borderline of life between non-living and the living organisms. They have some characteristics of a living organism such as the ability to reproduce while some of non-living objects, like the absence of metabolic processes and ability to crystallise. Viruses are very simple organisms with a circular nucleic acid molecule (DNA or RNA) surrounded by a protein coat (Figure shown below).