The cell structure is simpler than that of other organisms as there is no nucleus or membrane bound organelles. Bacteria are classified into 3 groups according to their basic shapes: spherical (cocci), rod (bacilli), and spiral. Bacteria are also classified by cell wall type: gram positive and gram negative. Gram positive is rigid, stronger, and has peptidoglycan. Gram negative has periplasmic space and lipopolysaccharides. Most bacteria reproduce by binary fission. During binary fission, the single DNA molecule replicates and both copies attach to the cell membrane. The cell membrane begins to grow between the two DNA molecules. Once the bacterium just about doubles its original size, the cell membrane begins to pinch inward. A cell wall then forms between the two DNA molecules dividing the original cell into two identical daughter cells. .
The economic importance of bacteria derives from the fact that bacteria are exploited by humans in a number of beneficial ways. Despite the fact that some bacteria play harmful roles, such as causing disease and spoiling food, the economic importance of bacteria includes both their useful and harmful aspects. Some bacteria play a helpful role in things like food, milk and cheese. Other bacteria aid in things such as digestion and pest controls. Bacteria cause illnesses in humans that can mostly be treated with antibiotics or other medications. Some bacterial infections cannot be cured or can only be treated with medicine to make it more bearable to deal with. To prevent any of these illnesses wash your hands, do not share food, drinks, or personal items, and cover your mouth when you cough or sneeze.
Viruses are the smallest of all the microbes. They are said to be so small that 500 million rhinoviruses (which cause the common cold) could fit on to the head of a pin. They are unique because they are only alive and able to multiply inside the cells of other living things.