What are Bacteria?

Until recently, the term bacteria was used for all microscopic prokaryotes. But, it turns out that there are two groups of prokaryotes that differ from each other in just about every way except size and lack of a nucleus.

These are now distinguished as the:

  • Bacteria; the "true" bacteria (also known as Eubacteria)
  • Archaea; (also known as Archaebacteria)

The archaea are so different from the bacteria that they must have had a long, independent evolutionary history since close to the dawn of life. In fact, there is considerable evidence that you are more closely related to the archaea than they are to the bacteria!

Link to page devoted to the Archaea.

Index to this page

Properties of Bacteria

Classification of Bacteria

Until recently classification has done on the basis of such traits as: The Gram stain is named after the 19th century Danish bacteriologist who developed it. Although the Gram stain might seem an arbitrary criterion to use in bacterial taxonomy, it does, in fact, distinguish between two fundamentally different kinds of bacterial cell walls and reflects a natural division among the bacteria.

More recently, genome sequencing, especially of their 16S ribosomal RNA (rRNA), has provided new insights into the evolutionary relationships among the bacteria.

The Proteobacteria

This large group of bacteria form a clade sharing related rRNA sequences. They are all gram-negative but come in every shape (rods, cocci, spirilla).

They are further subdivided into 5 clades: alpha-, beta-, gamma-, delta-, and epsilon proteobacteria.

Alpha (α) Proteobacteria.

Some examples:

Beta (β) Proteobacteria

Gamma (γ) Proteobacteria

The largest and most diverse subgroup of the proteobacteria.

Some examples

Delta (δ) Proteobacteria

This group contains the myxobacteria. They are found in vast numbers in soil and are major players in the decay of organic matter. [Link to discussion]

Epsilon (ε) Proteobacteria

Two members of this small group that are human pathogens:

Some Gram-Positive Bacteria

Gram-Positive Rods

Aerobic Gram-Positive Rods

Anaerobic Gram-Positive Rods:

Gram-Positive Cocci

The bacteria in this group grow in characteristic colonies.


Most of these organisms grow as thin filaments — like a mold — rather than as single cells. In fact, they were long thought to be fungi. But fungi are eukaryotes and the actinomycetes are prokaryotes. Actinomycetes dominate the microbial life in soil where they play a major role in the decay of dead organic matter. Many of them have turned out to be the source of valuable antibiotics, including streptomycin, erythromycin, and the tetracyclines.

Mycobacteria and Corynebacteria

These gram-positive organisms are closely related to the actinomycetes and often classified with them. They include three important human pathogens:


These are thin, corkscrew-shaped, flexible organisms that range in length from a few to as many as 500 µm.

Two notorious examples:

Both these organisms have had their complete genomes sequenced. [Link]


Mycoplasmas have the distinction of being the smallest living organisms. They are so small (0.1 µm) that they can be seen only under the electron microscope.

Mycoplasmas are obligate parasites; that is, they can live only within the cells of other organisms. They are probably the descendants of gram-positive bacteria who have lost much of their genome — now depending on the gene products of their host.

The DNA sequences of the complete genomes of seven mycoplasmas have been determined, including

How many genes does it take to make an organism?

The scientists at The Institute for Genomic Research (TIGR) who determined the Mycoplasma genitalium sequence followed this work by systematically destroying its genes (by mutating them with insertions) to see which ones are essential to life and which are dispensable. Of the 480 protein-encoding genes, they conclude that only 265–350 of them are essential to life.


Chlamydiae are also obligate intracellular parasites.

Cyanobacteria (blue-green algae)

Unlike other photosynthetic prokaryotes, cyanobacteria Cyanobacteria also contain two antenna pigments:

These two pigments also occur in red algae. Perhaps their chloroplasts evolved from endosymbiotic cyanobacteria. In fact, probably all chloroplasts evolved from endosymbiotic cyanobacteria.

The micrograph is of Oscillatoria, a filamentous cyanobacterium (magnified about 800 times). Each disk in the chains is one cell.

Mitochondria and Chloroplasts

There is now lots of evidence that both of these eukaryotic organelles evolved from once free-living bacteria. that took up an endosymbiotic way of life in the ancestors of the eukaryotes.
Link to discussion of the endosymbiosis theory.
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15 October 2004