General Introduction Bacteria (singular: bacterium) constitute a large domain of prokaryotic microorganisms. Found in water, soil, sewage, intestines of humans e.t.c. 0.1-10um long. Simple cell structure Multiply by binary fission. Classified By: Morphology, Staining reactions, Culture characteristics, Biochemistry, Antigenic structure & genetic Composition. General Morphology of Bacteria Cocci (spherical) Bacilli (Rod-shaped) Vibrio, (slightly curved rods or comma-shaped). Others: spiral-shaped, called spirilla or tightly coiled, called spirochaetes. Many bacteria can exists as: single cells, others associate in characteristic patterns: diploids (pairs) e.g. Neisseria chains, e.g. Streptococcus Staphylococcus group together in “bunch of grapes”. Bacteria can also be elongated to form filaments e.g. the Actinobacteria.