Final answer:
Bacteria are classified by their traits, including shape and cell wall characteristics. The Gram stain and Bergey's Manual of Determinative Bacteriology are two classification systems widely used to classify bacteria. Scientists have also introduced additional groups based on various features.
Step-by-step explanation:
Bacteria are classified based on various traits, including shape, cell wall, and other characteristics. One classification method is the Gram stain, which differentiates bacteria based on their color when stained. Another classification system is found in Bergey's Manual of Determinative Bacteriology, which categorizes bacteria into major categories based on their cell wall nature, and then further groups them based on morphology and physiology. More recently, scientists have introduced additional groups based on physiological, biochemical, and genetic features.