Rapid Semiquantitative Testing of Antibiotic Susceptibility: Use of a Multicell Disk Elution System

Abstract

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.A number of methods have been devised for quantitative measurement of susceptibility of bacteria to various antimicrobial agents. The most widely used procedure is to measure in vitro the minimum inhibitory concentration (MIC) of the agent and then predict its in vivo susceptibility by relating the MIC to the concentration of drug that might be produced at the site of infection (5). This usually entails standardization, requires precise measurements, and involves large investments of time and materials. This is considered impractical by most clinical laboratories in which a large number of tests must be performed daily and results made rapidly available to the clinician.Other procedures which provide a less precise assessment of susceptibility are simpler to perform and to interpret, and the results have been found to correlate with those obtained by some of the more quantitative methods (1, 2, 4). The toxic and effective levels of many antimicrobial agents are widely separated. In tests of susceptibility to these agents, it is possible to sacrifice some degree of precision for ease of performance 'Present address:

Rapid Antimicrobial Susceptibility Test Using Tetrazolium Reduction

“…The Autobac 1 system has also been reported to produce discrepancies with certain organismantimicrobial agent combinations (15,17,20). As with the rapid method, the majority of discrepancies represent false susceptibility.…”

“…Early reading of endpoints has also been reported with the standard broth dilution procedure (10). Tetrazolium reduction has been used previously for rapid minimal inhibitory concentration (MIC) determinations by Gifford and Boring (8) (15,17,20). Autobac 1 has also been shown to be useful for the rapid determination of MICs (6,19 (4,22).…”

“…For each antimicrobial agent concentrations in random wells were assayed by a plate diffusion method (15,23) at the time of preparation and after 2 and 4 weeks of storage.…”

Rapid determination of minimum inhibitory concentrations of antimicrobial agents by regression analysis of light scattering data

“…This basic Autobac system was modified recently to provide rapid measurements of the MICs of a variety of the antimicrobials in current use. This newly developed method is similar to several previously reported methods (7,13,14) These bulk drugs were used to prepare the stock solutions required both for the reference broth dilution method and for impregnating the absorbent paper disks used in the Autobac MIC procedure.…”

“…The 415 bacterial strains used in these studies were clinical isolates acquired by various microbiology laboratories and included 9 strains of Citrobacter diversus, 7 strains of Citrobacter freundii, 7 Inoculum preparation. Vials A standard Autobac incubator-shaker was also employed throughout these studies and was operated at 36 ± 1°C and 220 rpm.…”