In terms of cellular structure, how are bacteria classified?

Bacteria are primarily classified based on their shape and cell wall composition because these characteristics provide critical information about their identity and potential behavior in various environments. The shape of bacteria can vary widely, commonly categorized as cocci (spherical), bacilli (rod-shaped), and spiral (helical). This morphological characteristic can often help in initial identification and understanding of their ecological roles.

Additionally, the composition of the cell wall is a significant determinant in bacterial classification. The presence of peptidoglycan in varying amounts distinguishes two major groups: Gram-positive bacteria, which have a thick peptidoglycan layer that retains the Gram stain, and Gram-negative bacteria, which have a thinner peptidoglycan layer surrounded by an outer membrane. This structural difference not only affects the staining properties but also influences the bacteria's susceptibility to antibiotics and their role in disease.

While size, metabolic rate, and even color can provide useful information, they are not as foundational for the classification of bacteria as shape and cell wall composition. Size alone does not account for the diverse forms bacteria can take, metabolic rates vary widely within the same species, and color under the microscope can be influenced by various factors, such as staining techniques and does not contribute to a systematic classification scheme.