Answer to Question #325267 in Genetics for Dds

Question

• How are antibiotic-resistance cassettes acquired by integrons? Once acquired, can they be lost?
• What role do antibiotic-resistance genes, trans- posable elements, and transmissible plasmids play in relation to certain pathogenic bacteria that are simultaneously resistant to multiple, chemically unrelated antibiotics?
• How could you distinguish between a bacterial strain that has the phenotype Lac1 and one that has the phenotype Lac2?
• How could you distinguish a Lac1 Amp-r bacterial strain from one that is Lac2 Amp-s? (Amp-r denotes resistance to the antibiotic ampicillin, and Amp-s denotes sensitivity.)
• How does the physical state of the F factor differ between an F1 bacterial cell and an Hfr bacterial cell?
• Is the F1 state of a bacterial cell infectious? Is the Hfr state infectious? Explain why or why not.

Answer 1

• If bacteria are in an environment with one or another antibiotic, the integrase gene is activated in their integron - the gene of an enzyme that excises and integrates cassettes into the right places in the integron, thus facilitating the acquisition of a cassette that is resistant to this type of antibiotic. These cassettes can be lost by the participation of same integrase if these bacteria are cultivated too long in a medium devoid of this type of antibiotic.
• Antibiotic-resistance genes are usually assembled into operons and encode all types of proteins that help to destruct this or that type of antibiotic. In pathogenic bacteria that are resistant to multiple and chemically unrelated antibiotics, transposable elements and transmissible plasmids contain many different operons for simultaneous destruction of many types of antibiotics.
• A bacterial strain that has the phenotype Lac1 synthesizes an enzymes for metabolism of lactose, unlike the strain having the Lac2 phenotype. This means that bacteria with the Lac1 phenotype can grow on a medium supplemented with lactose as the sole source of carbohydrates, in contrast to the bacterial strain with the Lac2 phenotype, which cannot grow on such an environment.
• A Lac1 Amp-r bacterial strain will grow on a medium supplemented with the antibiotic ampicillin and lactose as the sole carbohydrate source, and a Lac2 Amp-s bacterial strain will not grow on such an environment.
• An Hfr bacterial cell has a complete F-episome that is integrated into the genome, and an F1 bacterial cell has the F-plasmid that exists outside the genome.
• The F1 state, as well as the Hfr state of a bacterial cell are infectious, because due to the presence of the F-factor in them, they can equally carry out the process of conjugation with cells that have the F2 state.