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 In an interrupted mating experiment, small samples of the conjugating bacteria are withdrawn every 3 minutes for 30 minutes. The withdrawn cells are shaken vigorously to stop conjugation and then placed on three different selection media, composed as follows: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

Key Concepts

Conjugation in Bacteria

Conjugation is a process of genetic transfer in bacteria where one bacterium transfers genetic material to another through direct contact. This often involves the transfer of plasmids or chromosomal DNA, allowing for the exchange of traits such as antibiotic resistance. Understanding this process is crucial for interpreting the results of the mating experiment described in the question.

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 acquired specific genes from the Hfr strain, enabling researchers to determine which genes were successfully transferred during conjugation.

Antibiotic Resistance

Antibiotic resistance refers to the ability of bacteria to survive and grow in the presence of antibiotics that would normally inhibit or kill them. In this experiment, streptomycin is included in the media to select for bacteria that have acquired resistance genes, allowing researchers to identify successful gene transfer events from the Hfr strain to the recipient strain.