Demarcation of antigen preparations on object slides in the immunofluorescent antibody test.

this direct assay quality control detect all of these errors. In addition, if the wrong antibiotic is put in a prelabelled plate, the error may be detected if the observed zone size and/or zone edge differ from those that were expected. The spore suspension is particularly convenient, and it had been hoped that it could be used for the assay of all antibiotics except colistin. However, all three strains of B. subtilis tested were found to be resistant to tetracycline as well as colistin and alternative assay organisms were therefore required for these two antibiotics. E. coli was selected for the assay of colistin but it was not suitable for the assay of tetracycline owing to the production of very small hazy zones. Staph. aureus was therefore selected for the assay of tetracycline. Although agar plugs containing trimethoprim and nalidixic acid caused inhibition zones with B. subtilis as indicator organism, E. coli was preferred for these antibiotics because the inhibition zones were larger and clearer. Agar-dilution sensitivity-testing appears to be less subject to error than diffusion methods, and, with the direct assay quality control described here, it achieves a very high degree of reliability.