Background：Antimicrobial susceptibility testing of Helicobacter pylori(H. pylori) is not yet standardized but broth dilution or agar dilution are considered as standard methods. In the broth microdilution method, antibiotic dilutions of different concentrations are made each time, but most of it is discarded because only small volumes of dilutions are used. To improve this tedious procedure and the waste of reagents, antibiotic solutions in 96-well microplates were frozen at -20℃ to evaluate their useful storage periods.

Methods：Various concentrations of metronidazole(MTZ) and clarithromycin(CLR) solutions were divided into ten plates of 96-well microplates, sealed and stored at -20℃. The broth microdilution susceptibility test was done with fresh and preserved antibiotic dilutions each month on 5 occasions for 4 strains(initial minimum inhibitory concentration(MIC) for MTZ 1, 4, 16, 64 ug/mL, initial MIC for CLR <0.125, <0.125, <0.125, 32 ug/mL) of H. pylori. The difference of MIC values of more than ±2 log2 dilution was considered significant.

Results：For both MTZ and CLR, the difference of MIC values of fresh and frozen antibiotic solutions was within ±1 log2 dilution and the results of susceptibility test were the same for 7 months.

Conclusions：Various concentrations of frozen MTZ and CLR solutions could be used for at least 7 months for the antimicrobial susceptibility testing of H. pylori. (Korean J Clin Microbiol 1999;2:49~53)