Overview of bacteriology

Overview of Bacteriology (page 4)

(This chapter has 6 pages)

© 2008 Kenneth Todar, PhD

BACTERIAL REPRODUCTION AND GENETICS

Most bacteria reproduce by a relatively simple asexual process called binary fission: each cell increases in size and divides into two cells. During this process there is an orderly increase in cellular structures and components, replication and segregation of the bacterial DNA, followed by formation of a septum or cross wall which divides the cell into two. The process is evidently coordinated by activities associated with the cell membrane. The DNA molecule is believed to be attached to a point on the membrane where it is replicated. The two DNA molecules remain attached at points side-by-side on the membrane while new membrane material is synthesized between the two points. This draws the DNA molecules in opposite directions while new cell wall and membrane are laid down as a septum between the two chromosomal compartments. When septum formation is complete the cell splits into two progeny cells. The time interval required for a bacterial cell to divide or for a population of cells to double is called the generation time. Generation times for bacterial species growing in nature may be as short as 15 minutes or as long as several days.

Figure 13. A pair of dividing streptococci. The chromosome has been replicated and is partially segregated as septum formation is beginning. Electron micrograph of Streptococcus pyogenes by Maria Fazio and Vincent A. Fischetti, Ph.D. with permission. The Laboratory of Bacterial Pathogenesis and Immunology, Rockefeller University.

Genetic Exchange in Bacteria

Although procaryotes do not undergo sexual reproduction, they are not without the ability to exchange genes and undergo genetic recombination. Bacteria are known to exchange genes in nature by three fundamental processes: conjugation, transduction and transformation. Conjugation requires cell-to-cell contact for DNA to be transferred from a donor to a recipient. During transduction, a virus transfers the genes between mating bacteria. In transformation, DNA is acquired directly from the environment, having been released from another cell. Genetic recombination can follow the transfer of DNA from one cell to another leading to the emergence of a new genotype (recombinant). It is common for DNA to be transferred as plasmids between mating bacteria. Since bacteria usually develop their genes for drug resistance on plasmids (called resistance transfer factors, or RTFs), they are able to spread drug resistance to other strains and species during genetic exchange processes. The genetic engineering of bacterial cells in the research or biotechnology laboratory is often based on the use of plasmids as vectors. The genetic systems of the Archaea are poorly characterized at this point, although the entire genome of Methanosarcina has been sequenced which opens up the possibilities for genetic analysis of the group.

Evolution of Bacteria and Archaea

For most procaryotes, mutation is is a major source of variability that allows the species to adapt to new conditions. The mutation rate for most procaryotic genes is in the neighborhood of 10^-8. This means that if a bacterial population doubles from 10⁸ cells to 2 x 10⁸ cells, there is likely to be a mutant present for any given gene. Since procaryotes grow to reach population densities far in excess of 10⁹ cells, such a mutant could develop from a single generation during 15 minutes of growth. The evolution of procaryotes, driven by such Darwinian principles of evolution (mutation and selection) is called vertical evolution.

However, as a result of the processes of genetic exchange described above, the bacteria and archaea can also undergo a process of horizontal evolution, also called horizontal gene transfer (HGT). In this case, genes are transferred laterally from one organism to another, including between members of different Kingdoms, which allows the recipient to experiment with a new genetic trait. Horizontal gene transfer is becoming realized to be a significant force in driving cellular evolution.

The combined effects of fast growth rates, high concentrations of cells, genetic processes of mutation and selection, and the ability to exchange genes, account for the extraordinary rates of adaptation and evolution that can be observed in the procaryotes.