Chapter 10 Outline
Microbial Systematics and a Survey of the Domains Bacteria and Archaea
Introduction
10.1 Microbial Systematics
 There is an immense diversity of organisms on Earth
 Humans catalog these organisms using sets of characteristics based on
similarities and differences
 Systematics has two major goals
• Taxonomy
• Phylogeny
 Microbial evolution is very difficult to understand without an ability to
interpret phylogenetic trees.
 Many systematic studies rely on the principles of monophyly
• A clade is a group of organims that includes a common ancestor
 Mutations are a major source of species variation
• Account for the diversity within a clade
 Modern phylogenetic analyses use molecular characters
• Some used as molecular chronometers
 Ribosomal RNA is highly conserved
• The nucleotide base sequences of ribosomal RNA genes are a useful
molecular character
 Horizontal gene transfer complicates phylogenetic studies
• Horizontal gene transfer can confuse relationships in distantly related
organisms
 The great majority of bacterial and archaeal “species” have never been studied
• The definition of a bacterial or archaeal species does not follow traditional
definitions
10.2 The Domain Bacteria
 Several bacterial lineages branch off the phylogenetic tree very early
• Ancient hyperthermophiles
 The domain Bacteria contains the best studied prokaryotes
• Classified into several major clades based on 16S rRNA gene sequences
 The proteobacteria are a large and diverse group
• Contain autotrophic and heterotrophic species
• Alpha proteobacteria
• Beta proteobacteria
• Gamma proteobacteria
• Delta proteobacteria
 The gram-positive bacteria split into two phylogenetic subdivisions
• Contains either high or low GC content
• Actinobacteria
• Mycobacterium
• Firmicutes
• Bacillus
• Clostridium
• Staphylococcus
• Streptococcus
• Mycoplasma
 Cyanobacteria are important photoautotrophs
• Carry out oxygenic photosynthesis
• Stromatolites
• “Great oxidation event”
 The chlamydiae are extremely small
• Can’t grow in pure culture
 Spirochetes have diverse ecological niches
 The Bacteroidetes are widely distributed in the environment
• Some associated with human obesity
10.3 Major Groups of the Archaea
 The domain Archaea is truly unique
 The Crenarchaeota are a metabolically diverse group
 The Euryarchaeota are found in broad habitats
• Extreme halophiles
• Extreme methanogens
 The Euryarchaeota are found in broad habitats
• Methanogens
• Extreme halophiles
• Thermoacidophiles