What are Bacteria?

Bacteria are microscopic organisms whose single cells have neither a membrane-enclosed nucleus nor other membrane-enclosed organelles like mitochondria and chloroplasts. Another group of microbes, the archaea, meet these criteria but are so different from the bacteria in other ways 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! [View]

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:
Gram-positive bacteria are encased in a plasma membrane covered with a thick wall of peptidoglycan. Gram-negative bacteria are encased in a triple-layer. The outermost layer contains lipopolysaccharide (LPS).
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 additional insights into the evolutionary relationships among the bacteria.


Comparison of their sequenced genomes reveals that all the Gram-positive rods and cocci as well as the mycoplasmas belong to a single clade that has been named the Firmicutes.

Gram-Positive Rods

Aerobic Gram-Positive Rods

Anaerobic Gram-Positive Rods:

Gram-Positive Cocci

The bacteria in this group grow in characteristic colonies.


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 their peptidoglycan wall as well as 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 (now known as the J. Craig Venter Institute — JCVI) 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 485 protein-encoding genes, they have concluded that only 381 of them are essential to life.

Workers at the JCVI have also succeeded in synthesizing the complete genome of one species of mycoplasma, inserted this into a second species, which converted the second species into the first. Read more about this remarkable achievement.


Most of these Gram-positive organisms grow as thin filaments — like a mold — rather than as single cells. In fact, they were long thought to be fungi and were called actinomycetes. But fungi are eukaryotes and the actinobacteria are not.

Actinobacteria 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 actinobacteria and often classified with them. They include three important human pathogens:

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:


A diverse group that includes Bacteroides fragilis, one of the most abundant organisms in the human large intestine. It has been estimated that these Gram-negative anaerobes make up >30% of the dry weight of human feces. While they can be pathogenic elsewhere in the body, they live as commensals in the intestine digesting otherwise-indigestible polysaccharides and providing us with some fatty acids, sugars, and perhaps vitamin K.


These are thin, corkscrew-shaped, Gram-negative 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]


Chlamydiae are also obligate intracellular parasites (they cannot make their own ATP).

Cyanobacteria (blue-green algae)

Unlike other photosynthetic bacteria, cyanobacteria

It is estimated that cyanobacteria are responsible for ~ 25% of the photosynthesis occurring on our planet.

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

Cyanobacteria also contain two antenna pigments:

These two pigments also occur in red algae. Their chloroplasts (in fact probably all chloroplasts) evolved from an endosymbiotic cyanobacterium.

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 a discussion of the endosymbiosis theory of the origin of eukaryotes.

An Endosymbiotic Origin of the Gram-negative Bacteria?

Why are the Gram-negative bacteria encased in two membranes while the Gram-positives have only one? Evolutionary biologist James Lake has proposed that the Gram-negatives arose by one single-membrane bacterial ancestor engulfing another. His analysis of many genes in the various bacterial groups indicate that the most probable ancestors of this possible endosymbiosis were a clostridium and an actinobacterium. Clostridia are the only Gram-positive bacteria that have photosynthetic members and because the photosynthetic apparatus in all photosynthetic Gram-negative bacteria is in the inner membrane, perhaps the actinobacterium was the host and the clostridium the endosymbiont.

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7 May 2018