Development of the agar disk method for the rapid selection of cephalosporin producers with improved yields

To screen the abilities of mutant strains of Cephalosporium to produce cephalosporin C, colonies of the organism were grown on the surface of small (4-mm diameter) disks of agar medium. After incubation of the disks for periods of up to 5 days, the antibiotic contents of the disks were assayed by placing them on agar plates of the assay organism and determining the diameters of the inhibition zones. The amount of nitrogen source in the agar disk medium was used to control the amount of antibiotic produced in the disk and, thus, the sensitivity of screening. The relation of agar disk inhibition zone diameters to log shake-flask titers was linear with short incubation times (2 to 3 days) of the disks, but shifted towards a higher order with prolonged incubation (4 to 5 days). The optimum incubation time for the disks was 4 to 5 days, and then a 15% difference in zone diameters was significant with 10 disks per sample. The minimum difference between the shake-flask titers, which could be detected by the agar disk method with 10 disks per sample, was about 30% with 5 days of incubation for the disks. The results suggest that the shake-flask culture underestimated the degree of improvement in strain productivity.