Bacteria are one celled microorganisms that lack the green pigment chlorophyll.
Four hundred million (400,000,000) of these cells are same size as a single grain of granulated sugar. Reproducing by dividing in half (fission) to produce two identical cells. Under ideal conditions, they can reproduce every 15-30 minutes, one bacterium could become 70 billion bacteria in only 12 hours.
Bacteria are divided into two major groups, based on laboratory staining technique called gram staining. Those stain stain violet are called “gram positive”; examples of these are (staphylococcus aureus) which cause acne, and (clostridium tetany) which cause tetanus.
Those that do not accept violet (but except a counter stain) are called “gram negative”; examples of these are (pseudonymous aeruginosa) which break down and contaminates both living and nonliving matter, (salmonella typhosa) which causes typhus, and (escherichia coli) a bacteria normally present in human intestinal tracts which when found in milk or water in proof of fecal contamination causing diarrhea or dysentery.
In addition to their staining characteristics, they may be classified by their shape or form.