Comparison of microplate and macrodilution methods in time-kill study of new antimicrobial drugs

In consideration of high production costs of new antimicrobial drugs, a more convenient and economical method for time-kill study is urgently required. In the present experiment, we attempted to demonstrate the feasibility of microplate method as an alternative measure of macrodilution method for time-kill study. Three conventional antibiotics (ciprofloxacin, ceftazidime, and levofloxacin) and two antimicrobial peptides [A-thanatin and K(4)-S4(1-16)a] were used to determine time-kill curves against Escherichia coli ATCC 25922 and Staphylococcus epidermidis ATCC 14990. Meanwhile, both methods were also performed with three antisense peptide nucleic acids (PNA3, PNA4, and PNA5) targeting ropD gene of Staphylococcus aureus ATCC 29213 and MRSA WHO-2. In order to study the correlation between the two methods, the growth inhibition rate of PNAs, antimicrobial peptides, and antibiotics for the tested strains were evaluated. A strong agreement between the results obtained from the two methods has been demonstrated. Although microplate method required longer incubation time for a significant result than macrodilution method, the former provides a more convenient, economical, and stable way to perform time-kill test for these agents. Thus, we concluded that microplate method was an available measure for time-kill study of new antimicrobial drugs.