Comparison of antibiotic discs from different sources

Antibiotic discs from Oxoid, Mast, AB Biodisk, Difco, and Baltimore Biological Laboratories were compared, where discs of similar antibiotic content were available, in diffusion-sensitivity tests against organisms of known sensitivity. Discs from Oxoid and Mast gave zones 1-5 mm larger than discs from other manufacturers with penicillin 2 units, ampicillin 10 mug, cephalothin 30 mug, methicillin 10 mug, carbenicillin 100 mug, erythromycin 15 mug, chloramphenicol 50 mug, and trimethoprim 1-25 mug, while discs containing aminoglycoside antibiotics, lincomycin, fusidic acid, tetracycline, nalidixic acid, penicillin 10 units, and polymyxin B gave similar zone sizes whatever the source. Where tested, different batches of single discs from the same source did not vary significantly in antibiotic content as indicated by variation in zone size; but with some antibiotics Multodisks gave larger zones than single discs from the same source. The implications of these differences are discussed.