Evaluation of drug sensitivity tests against bacteria by minimal inhibitory concentration and disc methods

Abstract

The current methods for testing the antimicrobial susceptibiity in general practice are the conventional disk diffusion, broth dilution and agar dilution methods. Each method has the advantage and disadvantages individually to serve the purpose of different laboratories. However, the same distinct disadvantage in all methods is the prolonged (18-20 hrs.) incubation period required. The present study is to evaluate the correlation of the modified broth disk method with other conventional procedures for the antimicrobial susceptibility in aerobic bacteria. A total of 125 strains of the clinically common pathogenic enterobacteria including different genera, which are the major diarrhoel causative microorganisms, isolated during the period of year 1977-1982 were tested for antimicrobial susceptibility by the modified broth disk method uding 12 imported disks of different antibiotics compared with 8 local disks. The results of modified method was recorded at 3,6,8,18 hrs. after incubation. Additionally, the modified broth disk method was then tested in comparison to disk diffusion and MIC dilution methods. The overall results demonstrated that the results of modified broth disk method reading at 6 hrs. showed closer agreement than the 18 hrs. reading, with those from disk diffusion and the MIC methods. This study demonstrated the feasibility of using the modified broth disk method as a rapid antimicrobial susceptibility determination for Salmonella sp.,Shigellasp, E. coli, V. cholera and V. parahaemolyticus tested against Tetrocycline, Chloramphenico, Co-trimoxazole, Peni-cillin, Polymyxin-B also for Shigella sp tested against Kanamycin, and E. coli tested against Gentamicin. Comparing the results obtained testing disk diffusion and modified broth disk method obtained using BBL and local disks. There were 20% and 15% of tests demonstrated the higher percentage susceptibility obtained in local disks as determined by the disk diffusion and modified broth disk respectively. There were 32.5% and 50% showed the equal percentage susceptibility in BBL disk than local disk as determined by the disk d diffusion and modified broth disk respectively. The MIC's obtained testing the agar and broth dilution were quite related. The bacterial percentage susceptibility using the disk diffusion and MIC dilution methods were significantly in agreement (P<0.20) for all tested organism against all antibiotic (except PM). The agreement in percentage suscentibility (n<0.20) obtained using the four methods were demonstrated with Salmonella sp., Shigella sp. E. coli., V. parahaemolyticus tested against Tetracycline, Chloramphenical, Penicillin, Ampicillin and also demonstrated with Shigella sp. tested against KM.