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KMID : 0377619640060040401
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Korean Jungang Medical Journal
1964 Volume.6 No. 4 p.401 ~ p.418
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Studies on the Methods for Antibiotic Sensitivity Test
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Abstract
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In order to know some aspects for the standardization of antibiotic sensitivity test under the simplified conditions, some basic pr blems for tests were studied on penicillin (P), streptomycin (SM), tetracycline (TC), aureomycin (AM), terramycin (TM), chloramphenicol (CM), neomycin(NM), erythromycin (EM), kanamycin (KM) and albamycin (ALM).
The comparative evaluation of well-known tube dilution, plate dilution and paper disc methods was also carried out with the most commonly used antibiotics against at random selected staphylococci, salmonellae and shigellae,
1) The effect of media on the results of sensitivity tests was studied with brain heart infusion, trypticase soy broth and nutrient broth. Among the antibiotics tested, all except SM, NM and KM were almost equally active in three media, SM, NM and KM showed stronger activity in nutrient broth than other media.
2) The effect of pH (range 6. 0 to 8. 0) of media on the activity of antibioitcs was investigated, SM, NM and EM showed stronger activity in alkaline media than aciduric, while AM showed stronger activity in aciduric media. The activities of P, TC, TM, CM, KM and ALM were not influenced by the pH of media.
3) The effect of the preservation of antibiotic solutions was studied on the changes in the activity, The decrease in the activity of p solution was marked even in refrigerator. P was rapidly inactivated according to the elapse of time, and the decrease in the activity was more rapid when preserved at low concentrations, The activity of P at the concentration of 40, 000 u/ant decreased to one fourth of the original solution within 5 days at 4°C. However, the activity was not decreased within 15 days when P was contained in nutrient agar at the test concentrations as low as O. 1 u/mi and preserved at 4°C. The other antibiotic solutions tested were fairly stable and most of them can be stored in a refrigerator for more than 20 days without significant loss in the activity.
4) The minimum inhibitory concentration (MIC) of antibiotics against at random selected staphylococci, salmonellae and shigellae was compared at neutral pH of media by the plate dilution method with nutrient agar and the tube dilution method with nutrient broth. The MIC of SM, TC, TM, EM and AM to the most of staphylococci were higher by the plate method than, the tube method, and the differences of MIC between two methods for the same strains were one to . three dilutions by doubling dilution procedure. There were some strains which were inhibited at the same concentrations Jay two methods. However the MIC of P and AM against saphylococci was variable by two methods. About one half of staphylococci was inhibited in higher concentrations by the tube method than the plate method, and the other half in higher concentrations by the tube method than the plate method,
5) The MIC of salmonellae and shigellae was higher in the plate method by one to three dilutions than the tube method. Some strains were inhibited at the same concentrations in two methods or at higher concentrations by the tube method than plate method, but MIC of these strains were very low in general.
6) The results of the plate method and the paper disc method were compared, and it was found that no good agreement was observed between the results of two methods. The inhibitory zones of paper discs were markedly differ among the strains of same MIC by the plate method, and the MIC of strains can not be estimated by diameters of inhibitory zones. Higher concentration discs showed wider zones than low concentration discs, but zones of low concentration discs sometimes showed wider zones than high concentration discs when several strains of same MIC were compared. The limit of MIC of antibiotics for which strains were not inhibited by the paper disc method was clearly indicated.
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