Different species of bacteria are classified into five categories according to their shapes: cocci, bacilli, vibrios, spirochaetes or spirilla. Cocci are spherical, while bacilli are rod-shaped. Vibrios are shaped like commas, spirochaetes like corkscrews and spirilla like spirals.
The majority of bacteria are in the cocci, bacilli or spirilla categories. In nature, bacteria can exist either as single cells or in pairs, clusters or chains. A colony of bacterial cells of a certain shape is named based on how the cells are arranged within the group. For example, the term diplococci refers to a coccus, or spherical, bacterium arranged in pairs. Meanwhile, streptococci are spherical bacteria arranged in chains, and staphylococci are spherical bacteria arranged in clusters.
Streptococcus pyogenes and Streptococcus pneumoniae are examples of cocci that can cause illness in humans. Streptococcus pyogenes is the bacteria species responsible for the contraction of strep throat, while Streptococcus pneumoniae can infect a person's lungs, causing pneumonia and spreading when the infected individual coughs. Streptococcus thermophilus, however, is a beneficial coccus that helps enable milk to transform into yogurt. Meningitis and the sexually transmitted infection gonorrhea are also caused by bacteria in the cocci category.
Escherichia coli is a bacillus that can be either innocuous or harmful, depending on its environment. While inside of the large intestine, it behaves as a normal component of the intestinal flora. However, it can cause illness elsewhere in the body, such as in the bloodstream, lungs or gastrointestinal tract.