Phylogenetic Diversity of Bacteria: Major Bacterial Phyla and the Proteobacteria

Phylogenetic Diversity of Bacteria

Introduction to Bacterial Phylogeny

The phylogenetic diversity of Bacteria is vast, encompassing numerous lineages that can be distinguished using molecular techniques, particularly 16S ribosomal RNA (rRNA) gene sequencing. Understanding these relationships is fundamental to microbiology, as it informs classification, ecology, and evolutionary biology.

• 16S rRNA gene sequencing is a molecular method used to identify and classify bacteria based on evolutionary relationships.

• There are over 30 recognized bacterial phyla, but only about 30 have species that have been successfully cultured in the laboratory.

• More than 90% of characterized bacterial genera and species belong to four major phyla:

  • Proteobacteria

  • Actinobacteria

  • Firmicutes

  • Bacteroidetes

Example: The use of 16S rRNA gene sequencing has revolutionized bacterial taxonomy, allowing for the identification of previously uncultured or unrecognized lineages.

Major Phyla of Bacteria

Overview of Major Phyla

Bacterial phyla represent broad evolutionary lineages. The four most prominent phyla, based on the number of characterized species, are:

• Proteobacteria: The largest and most metabolically diverse phylum.

• Actinobacteria: High G+C gram-positive bacteria, including many soil-dwelling species.

• Firmicutes: Low G+C gram-positive bacteria, including many important pathogens and industrial species.

• Bacteroidetes: Gram-negative, non-sporulating rods, often found in the human gut and environment.

Additional info: The diversity within these phyla includes a wide range of morphologies, metabolic strategies, and ecological roles.

Proteobacteria

General Characteristics

Proteobacteria is the largest and most metabolically diverse bacterial phylum. It includes many species of medical, industrial, and agricultural importance.

• Comprises more than one-third of all characterized bacterial species.

• All members are gram-negative.

• Exhibit a wide variety of energy generation mechanisms (e.g., phototrophy, chemolithotrophy, chemoorganotrophy).

• Display diverse relationships with oxygen (aerobic, anaerobic, facultative anaerobes).

• Show a variety of cell morphologies (rods, cocci, spirilla, etc.).

• Divided into six major classes:

  • Alphaproteobacteria

  • Betaproteobacteria

  • Gammaproteobacteria

  • Deltaproteobacteria

  • Epsilonproteobacteria

  • Zetaproteobacteria

• Horizontal gene flow (the transfer of genetic material between organisms other than by vertical transmission from parent to offspring) is important in shaping the metabolic diversity of this group.

Example: Escherichia coli (a Gammaproteobacterium) is a model organism in molecular biology and a common inhabitant of the human gut.

Classification of Proteobacteria

The six classes of Proteobacteria are distinguished by genetic, metabolic, and ecological characteristics:

• Alphaproteobacteria: Often oligotrophic, includes many plant-associated bacteria and some pathogens.

• Betaproteobacteria: Diverse in metabolism, includes important environmental and pathogenic species.

• Gammaproteobacteria: Includes many well-known human pathogens and industrially important bacteria.

• Deltaproteobacteria: Includes bacteria with complex life cycles and some that are important in sulfur cycling.

• Epsilonproteobacteria: Often found in extreme environments, includes some gastrointestinal pathogens.

• Zetaproteobacteria: Less well-known, includes iron-oxidizing bacteria.

Additional info: The classification is based on phylogenetic analysis of 16S rRNA gene sequences and supported by phenotypic characteristics.

Key Terms

• Gram-negative: Bacteria that do not retain the crystal violet stain in the Gram stain procedure, typically having a thin peptidoglycan layer and an outer membrane.

• Oligotrophic: Able to grow in environments with very low levels of nutrients.

• Horizontal gene flow: The movement of genetic material between organisms other than by descent from parent to offspring.