A Basic Introduction
All of the organisms on earth have been classified into one of three major divisions.
Most of the genetic diversity on Earth is represented by microbes. Recent data have shown that humans differ from chimpanzees by only 1.5% of their DNA sequences. Eukaryotic DNA sequences are so similar that the genome of one species can be used to predict the genome of others. In a typical bacterium, however, 25-50% of the genes are unique to that species.
Archaea (formerly known as the archaebacteria) are single-celled microbes mostly represented by extremophiles, organisms that live under conditions where other types of organisms cannot survive. Until recently the archaea were classified in the same group as eubacteria; however, comparisons of transcription factors and membrane ATPases suggest that archaea are more closely related to eukaryotes than eubacteria. Substantially less is known about the archaea than about the eubacteria which are thus often referred to as just ‘bacteria’.
The eubacteria consist of a wide variety of single- and multi-cellular species, some with complex developmental cycles. They are divided into two subgroups based on the results of a Gram stain test.
- gram-negative cells (stain pink)
- gram-positive cells (stain purple)
There are four basic steps of the Gram stain. A primary stain (crystal violet) is applied to a heat-fixed smear of a bacterial culture, followed by the addition of a mordant (Gram’s iodine), rapid decolorization with alcohol or acetone, and counterstaining with safranin or basic fuchsin. Gram-negative bacteria are surrounded by thin inner and outer membranes which retain little of the dye resulting in a pink cell. Gram positive bacteria have a thick cell wall made of peptidoglycan which retains much of the dye resulting in a deep blue or purple cell.
The genus Myxococcus consists of gram-negative bacterium that exist as free-living single-celled organisms during part of their lifecycle and can aggregate and self-organize to form fruiting bodies in response to environmental cues. Epulopiscium fishelsoni is one of the largest bacterium known, and because of this it has evolved some curious adaptations. The gram-positive Epulopiscium reproduces exclusively through an unusual form of sporulation reminiscent of vivipary. Anywhere from one to twelve daughter cells are grown inside of the parent cell until the cell eventually lyses and the new bacteria burst through the cell wall. Another member of the eubacteria, the actinomycetes, are used by the Attine tribe of ants for the antibiotics produced by the fungus-like bacteria. The actinomycete-produced antibiotics protect the ants’ fungal gardens from infection by a parasitic fungus (Escovopsis).
Chloroplasts and Mitochondria
Current evidence indicates that eukaryotic mitochondria and chloroplasts are descended from free-living eubacteria that formed a symbiosis with eukaryotes. Comparisons of highly conserved ribosomal RNA (rRNA) gene seuences suggest mitochondria are descended from the proteobacteria and chloroplasts are descended from the cyanobacteria. In fact, some dinoflagellates (eukaryotes) are known to engulf cyanobacteria when it is light out, allowing the dinoflagellates to photosynthesize, then discard the cyanobacteria at night when they have no use for them.
Why more is known about some bacteria than is known about any other type of organism.
What is currently know about the basic molecular mechanisms in cells is a result of studies of bacteria. This is because bacteria are relatively easy to manipulate genetically. Bacteria are haploid organisms. In diploid organisms most mutations are recessive making them difficult to identify. Mutations in haploid organisms usually have an immediate, easily identifiable effect. Bacteria also hav short generation times. The shorter the generation time the more experiments that can be done. Some strains of Escherichia coli (E. coli) can reproduce every 20 minutes under ideal conditions. The asexual reproduction used by bacteria makes it is easy to obtain a large number of identical organisms to perform experiments on. Every daughter cell is a clone of the mother. With bacteria there is no sex to complicate genetic experiments. The ability to grow colonies on agar plates allows researchersto produce a large number of organisms in a small place, and no matter how crowded the bacteria are on the original agar plate, it is possible to isolate a pure strain of the bacterium in one or a few steps of colony purification. Using serial dilutions a measurable number of discrete colonies can easily be obtained from a densely concentrated culture. One of the major advantages of bacterial genetics is the opportunity to isolate rare mutants or other strains of bacterium. Using the proper selective growth conditions a single bacterium can be seleted from among billions placed on an agar plate. Finally, the three methods of genetic exchange between bacteria (transformation, conjugation, and transduction) allow for the possibility of all the genetic exepriments performed on microbes.
(Header Image: Epulopiscium)