Analyzing Novel Methods to Test Antimicrobial Drugs

Cara N. Wilder, Ph.D.



Since the discovery of penicillin, the first medically-employed antibiotic, scientists have strived to further unearth and synthesize novel antibiotics to treat infections. However, in recent years, the overuse of antibiotics has led to the emergence of numerous antibiotic-resistant microbial strains. This complication has further spurred scientists to develop novel therapeutic agents, such as antimicrobial peptides, to complement current antibiotic treatments. With the development of these novel antimicrobial therapies, traditional drug analysis methods must now be re-evaluated to determine their applicability.  

Currently, a standardized approach used to evaluate the efficacy of novel antimicrobial therapies is the time-kill study. This method, which was proposed by the Clinical and Laboratory Standards Institute (CLSI), is used to examine dynamically the bactericidal rate of an antimicrobial agent at varying concentrations and time intervals. Unfortunately, this contemporary time-kill testing procedure requires subcultures from broth macrodilutions prepared in flasks, thus necessitating the excessive use of the antimicrobial test compound. This is problematic as most antimicrobial therapies are expensive to produce.

In light of high production costs associated with manufacturing new antimicrobial drugs, a more cost-effective and convenient method for analyzing drug efficacy is desired. The use of a 96-well microplate approach has been of recent interest as it was successfully employed in the examination of minimum inhibitory concentration (MIC). When used in these assays, the microplate procedure proved to be quick, reproducible, and cost-effective. To determine if a microplate-based tactic was also applicable in a time-kill assay, Zhou et al. compared this method to the conventional CLSI macrodilution approach.

To analyze the validity of the microplate time-kill assay, Zhou et al. tested the bactericidal activity of conventional antibiotics, antimicrobial peptides, and antisense peptide nucleic acids against Escherichia coli (ATCC^® 25922™) and Staphylococcus epidermidis (ATCC^® 14990™). The bacterial growth inhibition rates of each antimicrobial therapy were analyzed to compare directly the microplate and macrodilution time-kill assays. From these assays, they found that results from the microplate-based method were directly comparable to the contemporary macrodilution method. Additionally, the microplate-based method provided a high-throughput screening approach that required significantly smaller volumes of drugs, conveniently allowing for direct measurements of microbial turbidity, and provided reproducibility among replicate experiments. Thus, this technique provides a more economical, effective, and expedient approach to test the antimicrobial efficacy over current standardized protocols.

Overall, with the synthesis of novel antimicrobial agents, it is imperative that accurate and applicable antimicrobial efficacy tests are available. As demonstrated by Zhou et al., this can be possible through the use of standardized reference materials and the adaptation of currently used methods that have demonstrated past success.

Happy culturing!