Rapid determination of bacterial susceptibility to antimicrobial agents by a semi-automated continuous flow method

Abstract

Experiments with a semi-automated susceptibility test system are described. The system was based on a continuous flow apparatus, which was used to estimate extinction in broth cultures (inoculated manually), after 3–4 h incubation. Five media were tested for ability to support bacterial growth; although Todd-Hewitt broth came out best, it was unsuitable for trimethoprim testing and so Iso-sensitest broth was chosen. The system was used to test 167 widely different bacterial strains for susceptibility to ampicillin, nalidixic acid, nitrofurantoin, trimethoprim and tetracycline. These agents were added to the test broth in disks, and similar disks were used to test the same strains by the Stokes modification of the disk-diffusion susceptibility test. One-hundred and nine test strains were freshly isolated from urine specimens, 47 came from a culture collection, and 11 were in fresh urine. Agreement between the semi-automated and disk methods was 83, 68 and 87% for the three groups. Reasons for these discrepancies between the two methods, ways of improving the results and advantages of the semi-automated over current commercial systems are discussed.