Bacteria

Evolution of taxonomic schemes

Aristotle & early biologist: 2 Kingdoms – plants and animals

Plants are:

· Stationary

· Green in color

· Possess a cell wall, therefore includes fungi, even though fungi are not green

Animals are:

· Motile

· Heterotrophic

Robert Whitaker (1969): 5 kingdoms

· Monera: Prokaryotic cells; unicellular and colonial; includes autotrophic and heterotrophic spp.

· Protista: Eukaryotic cells; unicellular & colonial; autotrophic, internal digestion (animal-like) & external digestion (fungi-like)

· Fungi: External digestion

· Plantae: Autotrophic

· Animalia: Internal digestion

Current theory is 3-domain system, which overlaps kingdoms

A Comparison of the three domains of life, from Campbell & Reese, et. al., 2008)
Character	Domain
	Bacteria	Archaea	Eukarya
Nuclear envelope	Absent	Absent	Present
Membrane-enclosed organelles	Absent	Absent	Present
Peptidoglycan in cell wall	Present	Absent	Present
Membrane lipids	Unbranched hydrocarbons	Some branched hydrocarbons	Unbranched hydrocarbons
RNA polymerase	One kind	Several kinds	Several kinds
Initiator amino acid for protein synthesis	Formyl-methionine	Methionine	Methionine
Introns in genes	Very rare	Present in some genes	Present
Response to the antibiotics streptomycin and chloramphenicol	Growth inhibited	Growth not inhibited	Growth not inhibited
Histones associated with DNA	Absent	Present in some species	Present
Circular chromosome	Present	Present	Absent
Growth at temperatures >100^oC	No	Some species	No

Comparison of prokaryotic and eukaryotic cells

Bacteria are prokaryotes

• Lacking membrane-bounded organelles, functions of mitochondria, chloroplasts, ER, Golgi complexes and lysosomes are taken over by the plasma membrane, which is extensively folded inward.

Bacteria have several common shapes

• Spherical are called cocci
• Rod-shaped are called bacilli
• Helical have several names. Very short helices are called vibrio. Longer helices, if rigid, are called spirillum, and if flexible, are called spirochete.

Bacteria are typically covered by a cell wall

• Details of cell wall chemistry determine staining characteristics and sensitivity to drugs.
• Eubacterial cell wall is composed of peptidoglycan
• Gram-positive bacteria have very thick cell walls composed primarily of peptidoglycan.
• Gram-negative bacteria have cell walls composed of two layers, a thin peptidoglycan wall and a thick outer membrane containing polysaccharides bonded to lipids.
• Some species may also produce a capsule or slime layer which protects them from phagocytosis.
• Some species have hundreds of hairlike appendages known as pili, which are often involved in conjugation, a form of genetic exchange.

Bacteria have a single DNA molecule, and they may have additional plasmids

• Single DNA molecule and plasmids are circular molecules of DNA
• There is little, if any, protein, associated with bacterial DNA.

Although bacteria undergo binary fission, exchange or transfer of genetic material is possible through three mechanisms.

• Transformation, fragments of DNA released by a cell are taken in by another bacterial cell.
• Transduction, a phage carries bacterial genes from one bacterial cell to another following lysis of the original cell.
• Conjugation, cytoplasmic bridges form via pili which allow plasmids to migrate from one cell to another.

Some bacteria form endospores to withstand unfavorable conditions

Bacteria undergo a variety of metabolic strategies

• Heterotrophs
• Saprobes (saprotrophs)
• Autotrophs (can be photoautotrophs or chemoautotrophs)
• Aerobic, facultative anaerobes, obligate anaerobes

Archaea and Eubacteria are as different from each other as Eubacteria are from Eukaryota

• Fundamental differences in plasma membranes, cell wall chemistry, metabolic strategies.
• Archaea include methanogens, extreme halophiles and extreme thermophiles
• Eubacteria include the familiar prokaryotes, both beneficial and pathogenic.