Practice question

Volume II of the second edition specifically treats phylum Proteobacteria, largest and most metabolically diverse bacterial phylum comprising five classes Alpha, Beta, Gamma, Delta, and Epsilon. It includes nitrogen-fixing rhizobia relatives of mitochondria, enteric pathogens such as Escherichia, environmental pseudomonads, and chemolithotrophic sulfur oxidizers. Introductory chapters discuss species concepts, numerical taxonomy, and polyphasic approaches. Allocation of entire volume reflects size, clinical importance, and phylogenetic breadth of Proteobacteria, separating it from Firmicutes in Volume III and Actinobacteria in Volume V for systematic clarity.