Question

Does the streptomycin in the medium cause the mutations? explain

Answer 1

ANSWER

Mutation is a heritable change in the nucleotide sequence of DNA. Mutations may be characterized according to either the kind of genotypic change that has occurred or their phenotypic consequences. Mutations can alter the phenotype of a microorganism in several different ways. Morphological mutations change the microorganism’s colonial or cellular morphology. Nutritional or biochemical variation may occur in a gene that encodes an enzyme involved in a metabolic pathway of amino acid synthesis. Changes in gene regulation occurs when mutation occur in a gene encoding a transcription factor. Lethal mutations prevent the reproducing capability of the organism, and when expressed, it results in the death of the microorganism.Mutations often inactivate a biosynthetic pathway of the microorganism, and frequently make a microorganism unable to grow on a medium lacking an adequate supply of the pathway’s end product. Based on this principle microorganism are classified as Prototrophic and Auxotrophic. Prototrophic organisms (wild type) have the same nutritional requirements as that of their ancestors. They need only inorganic salts, an organic energy source such as sugar, fat, protein and water to survive and grow. That is, the Prototroph’s need only "Minimal medium" for their growth and survival. Auxotrophic mutants are unable to grow without one or more essential nutrients. Auxotrophs are mutant for particular nutrient synthesis pathway enzymes. Such an error is known as an inborn error of metabolism, whether it occurs in a bacterium or a eukaryote. An auxotroph can be grown only on an enriched medium that provides the particular nutrient that the mutant cannot metabolize on its own.If an organism has the ability to produce mutant strains resistant to antibiotics, the nature of mutation, whether it is spontaneous or induced have to be tested. It would be a difficult task to identify a few mutant colonies from a vast population of 100-500 colonies. This can be accomplished by a replica plating technique. The technique was developed by Joshua and Esther Lederberg in 1952 for providing the direct evidence for the existence of pre-existing mutations. This technique isolates both nutritional mutants and antibiotic resistant mutants. Their actual experiment concerned with replicating master plates of sensitive cells to two or more plates containing either streptomycin or T1 phage.
Replica plating allows the observation of microbes under a series of growth conditions. The bacteria are grown in an environment that is not selective for given mutation. This technique is used to transfer the members of each colony to a selective environment. A simple velveteen covered colony transfer device is used to transfer the colonies in nutrient agar medium supplemented with or without a particular antibiotic or nutrient. The fibers of velvet act as fine inoculating needles, picking up the bacterial cells from the surface of this master plate. The velvet with its attached microbes is then touched to the surface of a sterile agar plate, inoculating it. In this manner, microbes can be repeatedly stamped onto media of differing composition. By comparing the presence of colonies following incubation we can indirectly determine the mutant colonies by their absence in the selective environment. A colony that develops on a complete medium fail to develop on a minimal medium that lacks a specific growth factor, the occurrence of a nutritional mutant is indicated. The microbes that do not grow on the minimal medium represent auxotrophic strains. A simple velveteen covered colony transfer device is used to transfer the colonies in nutrient agar medium supplemented with or without a particular antibiotic or nutrient. A colony that develops on a complete medium fail to develop on a minimal medium that lacks a specific growth factor, the occurrence of a nutritional mutant is indicated. The microbes that do not grow on the minimal medium represent auxotrophic strains. This method has been applied in numerous experiments to identify the occurrence of mutations. Many of the biochemical pathways in microbes were elucidated in this way by using nutritional mutants