Bacteria are composed of procaryotic cells. The thousands of species of bacteria are differentiated by many factors, including morphology (shape), chemical composition, nutritional requirements, and biochemical activities. The shape of bacteria is determined by heredity, called monomorphic bacteria. However, a number of environmental conditions can alter the shape of the organisms, causing a different shape, called pleomorphic bacteria. Unfortunately, if this happens identification becomes even more difficult. Bacilli are rod shaped bacteria, and when divided can be: Single. Diplobacilli, pairs. Streptobacilli, chains. Coccobacilli, oval shaped rods. Gram positive (Gr+) bacteria cell walls consist of several peptidoglycan layers with teichoic acids that might aid the transport of ions into and out of the cell. Gram negative (Gr-) bacteria also contain a peptidoglycan layer, but in very small amounts, located in periplasmic space and Gram negative bacteria contain no teichoic acid.