These are due to dissimilar development conditions, found quite frequently in bacteria.
By Samra Idrees
- Non hereditary variations
- Hereditary variations
Non hereditary variations:
It occurs under various effects of environment. For example a deficiency of calcium in the medium provokes an increase in spore production and a slimy growth in anthrax bacilli. Similarly a decrease in oxygen lowers the degree of pigmentation and increases the number of smooth colonies in tubercle bacilli. Under the influence of unfavorable conditions some species of bacteria undergo profound changes with the formation of peculiar small colonies with a dark dense center and a loose edge. These altered forms are known as L-form of bacteria. They occur under the effect of penicillin, immune serum, chemicals and other unfavorable factors on the culture.
Hereditary variations
These are also known as genetic variations, and are due to:
- mutation
- genetic recombinations of genes.
Mutation :
It can be defined as stable inheritable change in the properties of bacteria which results due to change in nucleotide sequences at some point of DNA. Mutations are the principal supplier of elementary evolutionary material.
Mutation may be due to loss (deletion), addition or substitution of one or more bases in DNA molecule.
Bacterial mutations may be:
- Spontaneous mutations
- Induced mutations
Spontaneous mutations:
These occur under the effects of external factors. The role of spontaneous mutations in bacteria ranges from 1.10-12 to 1.10-5. Even though these are rare events, but they may alter many phenotypic characteristics of bacteria.
Induced mutations:
These are caused by certain agents (mutagens) such as radiation, alkylating agents, acriflavine, 5-bromouracil and aminopurine.
Some bacteria particularly enterobacteria have genes which are resistant to effects of mutagens, but undergo changes under natural conditions.
Genetic recombinations:
These occur as a result of:
- Transformation
- Transduction
- Conjugation
Recombination is determined by special genes. In evolutionary process, recombination play a subordinate role.
Transformation:
Process of transferring genetic characters from donor to recipient through the agency of small units of DNA containing 10-15 genes. It was first established by F.Griffith in 1928
Natural transformation has been described with some Gram positive bacteria such as streptococcus, staphylococcus and gram negative bacteria such as moraxella, hemophillus and pseudomonas etc.
This phenomenon does not occur always but only in presence of definite conditions and particular physiological state of cell which are called ‘The state of preparedness ‘or ‘competence’.
Transduction:
It is transfer of genetic material from donor bacterium to recepient through bacteriophage.
Types of transduction:
Three types are distinguished:
- General transduction
- Specific transduction
- Abortive transduction
General transduction:
In this recombination of genetic material carrying one or several character may occur. The frequency of transduction is within the units of 10-4-10-7per phage.
Specific transduction:
It is carried out only by the phage which is obtained as a result of induction of lysogenic strains of bacteria and specific character can only be transferred e.g. galactose positive character.
Abortive transduction:
It is characterized by the fact that the genetic material carried by the phage is not incorporated into genome of recepient bacteria but remain in cytoplasm of bacteria and during cell division of recepient bacterium genetic material of donor bacterium is transferred to one progeny cell. In general it is encountered 10times more frequently than other two types.
Conjugation:
It is the process by which male or donor bacterium make contacts with female or recepient bacterium and transfer genetic material through sexual process.
Donor bacterium is determined by presence of sex or fertility (F) and is designated as F positive. Female lacks fertility and is designated as F negative.
In conjugation process not the whole nucleotide but only definite parts of nucleoid are transferred. This process takes place within 30-90 minutes. During conjugation only DNA material and not RNA or protein is transferred.
Role of extra nucleoid in bacterial variations:
Bacteria possess Genetic properties not only in nucleoid DNA, but also in extra nucleoid DNA material such as plasmid, these are usually less than one twentieth the size of bacterial chromosome, although there is variation in plasmid sizes within a single bacterial cell. The important properties carried by plasmid are fertility factor, ( F- plasmid), drug resistance transfer factor, haemolytic, enterotoxigenic and urease factor etc. These factors can be transferred from donor to recepient bacteria by process of transduction and conjugation.
Author: Samra Idrees
College of Veterinary and Animal sciences
Department of Veterinary medicine
Forms of variation in Bacteria