There are thousands of species of bacteria on earth, many of which have not yet been identified. When attempting to classify a bacterium, a variety of characteristics are used, including visual characteristics and laboratory tests. Bacteria are simple, unicellular organisms. Most are free-living organisms, but a few require animal or plant hosts for survival. Bacteria absorb nutrients from their environments, excrete waste products, and secrete various toxins that help them invade tissues. Bacteria have no enclosed nucleus. Their chromosomal material is in the form of a large loop, packed into the cytoplasm of the cell.
Some bacteria can be identified through a simple visual perusal. First, the operator considers the appearance of the bacterial colony (a group of the same kind of bacteria growing together, often on a petri dish.) The operator also views individual bacteria under a microscope, considering their shape, groupings, and features such as the number and location of flagella.
A variety of laboratory techniques can be used to narrow down the identity of a bacterial species if a visual survey is not sufficient. The operator can stain the bacteria using a gram stain or an acid-fast stain. The bacteria can be cultured on a specific medium which promotes the growth of certain species, as in the membrane filter method of testing for coliform bacteria. Other tests can detect bacterial by-products, while yet more advanced tests actually analyze the DNA of the bacteria.