Complexity – another bit of the puzzle in the standardization of testing methods to determine efficacy of natural products

Both EUCAST and CLSI specify Mueller Hinton agar for evaluating antimicrobial susceptibility using the Kirby-Bauer disc-diffusion method [1], [2]. While a number of factors can affect the results obtained (for example, how the bacteria is applied to the agar or depth of agar) neither organization recommends a particular manufacturer of the Mueller Hinton agar (M-H). Antimicrobial susceptibility is evaluated in terms of specific breakpoints (MIC and inhibition diameter) for antimicrobial agents (antibiotics) at specified dosages.

This research investigated any variability in results of antimicrobial susceptibility testing of natural products. M-H from three different manufacturers was investigated using two control strains of Staphylococcus aureus, namely ATCC 29213 and ATCC 25923 using the well-diffusion method.

The results showed that the source of the M-H had significant effects on the antimicrobial bioassay.

In conclusion, the source of the Mueller Hinton agar used in antimicrobial susceptibility testing would appear to be another variable that needs to be considered when investigating the antimicrobial properties of natural products.

• References

• 1 European Committee on Antimicrobial Susceptibility Testing (EUCAST). Frequently Asked Questions (FAQ), 2018 http://www.eucast.org/fileadmin/src/media/PDFs/EUCAST_files/General_documents/FAQ/FAQ_EUCAST_20180216.pdf
• 2 Clinical and Laboratory Standards Institute. Performance Standards for Antimicrobial Susceptibility Testing; Twentieth Informational Supplement (M100-S20). Wayne: CLSI; 2010