MATERIALS AND METHODS Bacteria

ROUTINE antibiotic sensitivity tests require a simple technique in which the influences of its several components have been thoroughly investigated and taken into account. Antibiotic diffusion tests come into this category, and the paper-disk method in particular has been favoured by many microbiologists. By means of this system it is possible to calculate a value for the minimal inhibitory concentration (MIC) of an antibiotic (Ericsson and Sherris, 1971) on the basis of the linear relation between the logarithm of the MIC and the diameter of the zone of inhibition (Cooper, 1963). We describe a method that can be standardised with a strain of known sensitivity to give results expressed in terms of MIC. We have evaluated the method for determining the sensitivity of certain enterobacteria and of Pseudomonas aeruginosa to the aminoglycosides gentamicin, kanamycin, streptomycin and tobramycin, and have compared the results with those obtained by a method involving titration in agar plates and the use of a multi-point inoculation (MPI) technique