Optimizing Suitable Antibiotics for Bacterium Control in Micropropagation of Cherry Rootstock Using a Modified Leaf Disk Diffusion Method and E Test

Bacterial contamination is a major and constant threat to the establishment and subculture of in vitro plant culture. In this study, we used a slightly modified qualitative disk diffusion method to screen optimal antibiotics to control the growth of bacterial contaminants isolated from explants of cherry rootstock ‘Gisela 6’. Bacterial susceptibility to eight different antibiotics was tested. The results showed that tetracycline was the most effective antibiotic for controlling bacterial growth; cefotaxime, carbenicillin, kanamycin, and streptomycin were less effective, whereas ampicillin, penicillin, and cefazolin did not inhibit growth of the isolated bacteria. Using the quantitative E test, the minimal inhibitory concentration (MIC) of tetracycline was determined to be 1.0 µg mL−1. We also measured the Fv/Fm values, chlorophyll content, and enzymatic activity of superoxide dismutase and peroxidase to explore the effect of different tetracycline concentrations, 0, 0.064, 0.5, 1.0, 16, and 256 µg mL−1, on the growth of bacteria and explants over 30 days. Results indicated that 1.0 µg mL−1 tetracycline was effective in restricting bacterial growth, with non-significant negative effects on explants at low concentrations, but were enhanced negative effects at high concentrations. The application of the disk diffusion method and E test enabled the identification of an antibiotic and its MIC value effective for eliminating bacterial contaminants while causing minimal damage to explants, indicating a high potential of these methods to control bacterial contaminants in in vitro plant culture.