The most useful laboratory classification of bacteria involves a threefold distinction: the Gram stain characteristics (gram positive or gram negative), morphology (coccus or bacillus), and oxygen requirements for growth (aerobic or anaerobic). Species exist that are morphologic exceptions, such as spirochetes; others are intermediate in oxygen requirements; still others are identified by other techniques, such as the acid-fast stain. Reaction to Gram stain has long been correlated with bacterial sensitivity to certain classes of antibiotics. A classic example is the susceptibility of most gram-positive organisms to penicillin. Morphology, when used in conjunction with this primary reaction, greatly simplifies identification of large bacterial groups, and oxygen growth requirements narrow the possibilities still further. The interrelationship of these characteristics also helps to control laboratory error. For example, if cocci seem to be gram negative instead of gram positive, a laboratory recheck of the decolorization step in the Gram procedure is called for since nearly all cocci are gram positive. If the staining technique is verified, the possibility of a small bacillus (Coccobacillus) or a Diplococcus must be considered.