Rapid semiquantitative testing of antibiotic susceptibility: use of a multicell disk elution system

Semiquantitative data regarding antibiotic susceptibility of bacteria were obtained by measuring the growth of bacteria in several different compartments ("cells") of a plastic module. Each compartment contained an inoculum of the organism and various concentrations of antibiotics in broth, achieved by elution of antibiotic from paper disks placed into the individual cells. Growth of the organisms was measured using an automated monitor to detect the scattering of light. Susceptibility of 300 strains of gram-negative bacilli to multiple concentrations of nine antibiotics was determined by this disk elution system, and the results (expressed in terms of four clinical susceptibility groupings) were compared with those obtained by a quantitative agar dilution method. Results obtained by the two methods agreed completely in 78% of the 2,700 determinations. In evaluating whether individual strains would be susceptible to systemic therapy or not, results obtained by the two systems agreed in all except 149 of the tests. Results of testing by the disk elution method were available on the same day that testing was begun. A system of this type may prove useful, for it provides information of a semiquantitative nature and decreases the time between isolation of a bacterial pathogen and availability of susceptibility data to the clinician.