Comparison of Single-disc and Tube-dilution Techniques in Determining Antibiotic Sensitivities of Gram-negative Pathogens

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Abstract
This study compares antibiotic sensitivity tests employing a single paper disc with the standard tube-dilution assay. Simultaneous tests were performed with 486 gram-negative pathogens, including Escherichia coli, Klebsiella, Paracolon, Proteus mirabilis, indole-positive Proteus and Pseudomonas aeruginosa. Antibiotics tested were tetracycline, chloramphenicol, streptomycin, kanamycin and polymyxin B. Except for Proteus species, there was good agreement between results of antibacterial susceptibility as determined by measurements of the inhibitory zones around the disc and by tube-dilution tests. The zone sizes indicating antimicrobial sensitivity in this study were greater than 17 mm for tetracycline, greater than 17 mm for chloramphenicol, greater than 13 mm for streptomycin, greater than 14 mm for kanamycin, and greater than 11 mm for polymyxin B. The single-disc technique provides a reliable measurement of antimicrobial sensitivity, which obviates the routine performance of tube-dilution tests or the use of multiple paper-discs.

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