antibiotics

Mechanisms of Antibiotics Resistance

Ampicilin - binds to and inhibits a number of enzymes in the bacterial membrane that are involved in the synthesis of the cell wall. The amp^r gene on the plasmid codes for an enzyme that is secreted into the peiplasmic space of the bacterium, where it catalyzes hydrolysis of the B-lactam ring, with concomitant detoxification of the drug.

Chloramphenicol - binds to the ribosomal 50S subunit and inhibits protein synthesis. The Cam^r gene codes for a tetrameric , cytosolic protein (MW of each subunit 23,000) that in the presence of acetyl coenzyme A, catalyzes the formation of hydroxyl acetoxy derivatives of chloraphenicol that are unable to bind to the ribosomes.

Kanamycin - binds to ribosomal components and inhibits protein synthesis.

Tetracyclin - binds to a protein of the 30S subunit of the ribosome and inhibits ribosomal translocation. The tet^r gene encodes a membrane associated protein that prevents the antibiotics from entering the cell.

The tutorial Antibiotics Attack gives detailed information on antibiotics, their function, and their targets. It includes animations & quizzes.

**Stock Preparation**
**(Concentrated X 1000)**
Antibiotics	Stock Concentration	Storage	Comments
Ampicillin	50-100 mg/ml in H₂O	-20	High level expression of ß-lactamase can destroy ampicillin in the medium surrounding the Amp resistant colonies,allowing Amp sensitive satellite colonies to grow.
Chloramphenicol	34 mg/ml in ethanol	-20
Kanamycin	10-30 mg/ml in H₂O	-20	High levels of Kan resistance are not expressed if the pH of the medium is less than 7.2.
Tetracycline	12.5 mg/ml in ethanol	-20	Light sensitive.Wrap Tet vials and plates in alu foil. Use media without magnesium salts (e.g., LB), because the magnesium ions antagonise tetracycline action.

Different concentrations should be used for maintenance of low- or high-copy number plasmids, respectively.