Alteration of the Genetic Characteristics of Bacteria

I. Often one gets the impression that the genetic material of an organism is constant. Though, for the most part, DNA remains unchanged, several processes result in alterations to the genetic make-up of an organism. When discussing infectious disease, these processes have significance with regard to acquisition of pathogenic characteristics by pathogenic bacteria and spread of antibiotic resistance. These processes include exchange, recombination and mutation.

A. Mutations are changes in the DNA sequence of an organism. These changes can result in alteration in proteins. In most cases when mutations have affected a gene coding for a protein, it is harmful and results in inappropriate functioning of that protein. But from time to time a mutation increases an organism's ability to survive in its environment. In the absence of agents that cause mutation rates to increase (mutagens), mistakes in DNA replication are extremely rare.

1. Chemicals that interact with DNA or with the enzymes active in replication often cause mistakes in replication. More often, chemicals that harm DNA lead to the activation of enzymes which repair the damaged DNA.  These are thousands of times more error prone than the replication enzymes. Consequently repairing the damage often leads to mutations.

2. Radiation (ultraviolet light, X-rays, gamma rays, etc.) can similarly damage DNA. In an effort to repair the damage errors are introduced. Many of the chemical changes that radiation induces are not repairable and lead to the death of the cell. Use of ultraviolet lights in surgical suites to sterilize the environment and gamma rays to sterilize food is based on this phenomena.

B. Within many bacterial cells are small circles of DNA known as plasmids which carry limited amounts of DNA. These plasmids can be transferred from one cell to another by a process known as conjugation. This requires that contributing organisms have a sex pili and the recipients be receptive to the pili. In some cases the plasmid is integrated into the recipient's chromosome but often the plasmids remains extrachromosomal.

C. Bacteria are also capable of picking up DNA from the environment. So when one cell dies its DNA is fragmented. The fragments are released as the dead cell decomposes. These pieces of DNA are picked up at a very low rate by other bacteria. This is referred to as transformation. This will often lead to transfer of certain traits from the dead cells to the living ones.

D. Viruses that infect bacterial cells are referred to as bacteriophages. When these viruses invade a cell they can pick up pieces of that host cell's DNA as they mature and leave that cell. Infection of a neighboring cell can lead to introduction of that DNA (and any genes that it carries) into the newly infected cell. This is referred to as transduction. 

Here are some good links!!!

Grapes of Staph    Mutation and Recombination  http://www.cat.cc.md.us/courses/bio141/lecguide/unit4/u4ib68.html

Very good discussion of the mechanisms  by  which the genetic information of a bacteria can be altered.

WSU  DNA alteration in bacteria      http://www.wsu.edu:8080/~hurlbert/pages/Chap9.html

If you haven't been to this link yet go now.  Great graphics and self evaluation questions!