The Dried Disc Technique with Bacitracin, Carbomycin, Erythromycin, Polymyxin B, and Tetracycline

The bacteriological control of antibiotic therapy is becoming increasingly important in view of the steady production in the U.S.A. of potent new antibiotics, some of which have a limited range of activity.It is therefore necessary to have a rapid and reliable method of carrying out the sensitivity tests in order to make this a practical routine procedure.The dried disc technique gives satisfactory results with penicillin, streptomycin, chloramphenicol, chlortetracycline (aureomycin), and oxytetracycline (terramycin) and is now widely used.It is therefore desirable that the range of discs should be extended to include bacitracin, carbomycin (magnamycin), erythromycin, polymyxin, and tetracycline. TechniqueThe preparation of the dried discs followed closely the technique with the established antibiotics (Fairbrother and Martyn, 1951).Discs (8 mm.diameter) were punched from coloured blotting paper (Ford) and sterilized for one hour in the hot air oven at 1500 C. The discs were then carefully impregnated with one drop of freshly prepared solutions of the different antibiotics, using a " 50-dropper" pipette, and then dried for one and a half to two hours in the incubator, after which they were stored in sterile screw-cap bottles (I oz.) at 00 to 40 C. (Table I).