An Hfr strain with the genotype cys⁺ lue⁺ met⁺ str^S is mated with an F^- strain carrying the genotype cys⁻ lue⁻ met⁻ strᴿ. In an interrupted mating experiment, small samples of the conjugating bacteria are withdrawn every 3 minutes for 30 minutes. The withdrawn cells are shaken vigorously to stop conjugation and then placed on three different selection media, composed as follows:


Medium 1: Minimal medium plus leucine, methionine, and streptomycin
Medium 2: Minimal medium plus cysteine, methionine, and streptomycin
Medium 3: Minimal medium plus cysteine, leucine, and streptomycin


What is the purpose of adding streptomycin to each selection medium?

Verified step by step guidance

1. Understand the context: Streptomycin is an antibiotic that inhibits protein synthesis in bacteria. The Hfr strain is streptomycin-sensitive (strˢ), while the recipient strain is streptomycin-resistant (strᴿ). This difference is key to understanding the purpose of adding streptomycin to the media.
2. Recognize the goal of the experiment: The interrupted mating experiment is designed to track the transfer of specific genes (cys⁺, lue⁺, met⁺) from the Hfr strain to the recipient strain over time.
3. Identify the role of streptomycin: Adding streptomycin to the media ensures that only the recipient strain (strᴿ) can grow. This prevents the growth of the Hfr strain (strˢ), which would otherwise interfere with the analysis of gene transfer.
4. Relate streptomycin to selection: By killing the Hfr strain, streptomycin ensures that any colonies growing on the media must have received the necessary genes (e.g., cys⁺, lue⁺, met⁺) from the Hfr strain via conjugation.
5. Conclude the purpose: Streptomycin acts as a selective agent to differentiate between the donor (Hfr) and recipient strains, allowing researchers to focus on the genetic transfer events in the recipient strain.

Key Concepts

Conjugation in Bacteria

Conjugation is a process of genetic transfer in bacteria where genetic material is exchanged between two bacterial cells through direct contact. In this case, an Hfr strain (high frequency of recombination) transfers genes to a recipient strain during mating. Understanding this process is crucial for analyzing how traits are inherited and expressed in the offspring.

Selection Media

Selection media are specially formulated growth environments that allow only certain organisms to thrive based on their genetic traits. In this experiment, the media are designed to select for bacteria that have successfully received specific genes from the Hfr strain, allowing researchers to determine which genes were transferred during conjugation.

Role of Antibiotics in Selection

Antibiotics like streptomycin are used in selection media to inhibit the growth of unwanted bacteria, allowing only those that have acquired resistance or specific traits to grow. In this context, streptomycin helps to ensure that only the bacteria that have successfully taken up the genes from the Hfr strain can survive and be analyzed, thus facilitating the study of gene transfer and expression.