Bacteria

Bacteria are prokaryotes and are not well represented in the fossil record. They evolved before the eukaryotes. Traditionally, bacteria have been characterized by using numerical taxonomy to compare staining reactions, cell shape, metabolic patterns, and mode of nutrition. Bacteria can sense and respond to light, nutrients, oxygen, toxins, and the like. Some imitate multicellular organisms by forming predatory colonies that entrap other microbes and digest them. Some even form fruiting bodies that release spores.

True classification based on evolutionary relationships is becoming possible due to comparative biochemistry studies.

Two branches of prokaryotic lineage are the eubacteria and archaebacteria.

Metabolism of Bacteria

Bacteria exhibit four types of metabolism or modes of nutrition: photoautotrophic, chemoautotrophic, photoheterotrophic, chemoheterotrophic.

Photoautotrophic bacteria synthesize their own organic compounds using sunlight as the energy source and carbon dioxide as the carbon source.

Chemoautotrophic bacteria utilize carbon dioxide and produce organic compounds using the energy in simple inorganic substances.

Photoheterotrophic bacteria use sunlight as an energy source but their carbon must come from organic compounds, not CO2.

Chemoheterotrophic bacteria include parasitic types that draw nutrition from living hosts, and saprobic types that obtain nutrition from products, wastes, or remains of other organisms.

Bacterial Cell Sizes and Shapes

Typically, the length or width of bacteria falls between 1 and 10 micrometers.

Three basic shapes are common:

Structural Features

 Bacteria are prokaryotes which means they have no nucleus or other membrane-bound organelles.

Metabolic reactions take place in the cytoplasm or at the plasma membrane. Proteins are assembled on floating ribosomes. Nearly all bacteria have a cell wall, usually containing a tough mesh of peptidoglycan which is peptides cross-linked with polysaccharides.

Historically, the Gram stain (developed by a person named Gram) has been an important tool to distinguish two main categories of bacteria. Cell walls of Gram-positive bacteria retain a deep purple stain. Gram-negative bacteria lose the purple color when washed with alcohol and stain pink with a counterstain.

This animation (Audio - Important) describes gram staining

Exterior to the cell wall is the glycocalyx, a jellylike capsule that helps bacterial cells attach to a substrate or deter the host's infection-fighting cells.

Two kinds of filamentous structures may be attached to the cell wall:

REVIEW: A rod shaped bacterium is called a

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