Microorganisms are microscopic, living one-cell organisms. They can not be see with naked eye. Bacteria are one of five types microorganisms, with a body size of 3-5 µm. It’s know more than 400,000 species of microorganisms, but we estimate a total of 4 – 5 million in our world. Bacteria are divided into two groups: gram positive – the good bacteria and Gram negative bad bacteria – harmful – pathogenic.