The first bacteria are thought to have evolved 3.5 billion years ago, resulting in a long evolutionary history throughout which natural selection in different environments has led to the exceptional diversity seen in bacteria (Reece et al., 2011).
Bacteria are unicellular organisms composed of a single prokaryotic cell, differing to plants and animals which are multicellular organisms composed of many eukaryotic cells. Prokaryotic and eukaryotic cells share common features in that they both have a plasma membrane surrounding the cell while within the cell a fluid-like substance supports chromosomal and ribosomal organelles called cytosol (Reece et al., 2011). However apart from these features prokaryotic cells and eukaryotic cells are very different. The size of a prokaryotic cell for example is much smaller than that of a eukaryotic cell as they are typically 0.5-5μm compared to 10-100μm of a eukaryotic cell (Reece et al., 2011). Also prokaryotic cell walls contain a modified sugar polymer called peptidoglycan that may vary in thickness depending on the bacterium (Hoiczyk & Hansel, 2000) while eukaryotic cell walls are composed of cellulose of chitin. Surrounding the cell wall of a prokaryotic cell is a layer of polysaccharide or protein called a capsule or slime layer, which is sticky therefore allowing the cells to stick to a substrate or to other individuals. The biggest difference though is the location of their DNA. In eukaryotic cells the DNA is stored within a membrane bound organelle called the nucleus; however prokaryotic cells lack a nucleus and instead store their DNA in a nucleoid. Prokaryotic cells also have separate small, circular DNA molecules called plasmids that can replicate independently. Plasmids are different to the nucleoid as they contain fewer genes that are thought to be beneficial for survival (Fig. 2) (Reece et al., 2011).
Bacteria reproduce asexually by means of binary fission, where a single bacterial cell divides to produce two separate genetically identical bacterial cells (Margolin, 2000). Most bacteria can divide every few hours though some can divide every few minutes which can result in bacterial populations of many trillions of individuals (Reece et al., 2011). Binary fission however is not the cause of the vast genetic diversity seen in bacteria as the many trillions of individuals would all be genetically identical. Instead genetic diversity is promoted by the rapid reproduction rate of bacteria, mutations that occur during replication and genetic recombination (Papke & Gogarten, 2012).
Mutations arise due to errors in DNA replication, though the probability of a mutation arising in a bacteria’s DNA is approximately one in ten million (Reece et al., 2011). While this does not sound much when the rapid reproduction rate of bacteria is taken into account many mutations arise. For example, Escherichia coli bacteria can be found in the intestine (Hartl & Dykhuizen, 1984) where every day 2×10¹⁰ new individuals are produced from each individual bacterium. It would be expected then that 2,000 of these new individuals will have mutations, times this by E. coli’s 4,300 genes, 9 million gene mutations would arise in every human host every day (Reece et al., 2011).
Genetic diversity also arises from genetic recombination which is the transfer of genes between bacterial cells (Reece et al., 2011). The most well-known process is conjugation where a hollow strand called a pilus forms joining two bacterial cells together to transfer the plasmid or chromosomes from one cell to the other, introducing new genes within that cell which will be incorporated into its chromosome (Reece et al., 2011). Genetic recombination can occur within and between different bacterial species. Papke & Gogarten (2012) give their interpretation of gene transfer between different bacterial species as the exchange of DNA between a lion and an acacia tree so that the new tree produced would develop limbs that could be used to attack grazing giraffes. Yet this in a sense is what occurs between bacteria all the time.
It is this high genetic diversity that has evolved within bacteria that has allowed them to be successful in colonizing a variety of habitats on Earth. In 1977 bacterial communities were discovered flourishing in one of the most extreme habitats ever to have been discovered on Earth (Van Dover et al., 2002). Occurring thousands of meters below the surface of the ocean where light can no longer penetrate, super-heated, toxic plumes of water erupt from chimneys that stand tall in the darkness of the deep sea. These fissures in the Earth’s surface from which geothermally heated water issues are known as hydrothermal vents (Tunnicliffe, 1991; Van Dover et al., 2002).