Zone of Inhibition

This Zone of Inhibition (ZOI) test is a preliminary antimicrobial screening method used to assess the inhibitory activity of cosmetic ingredients and finished formulations against selected microorganisms. The test provides an early indication of antimicrobial performance by measuring clear, visible zones of growth inhibition on agar plates following incubation.

One of the key advantages of this method is its highly visual output. The formation of distinct inhibition zones allows for easy interpretation and direct comparison between samples, making it an effective tool for demonstrating antimicrobial activity to technical teams, stakeholders, and clients. High-resolution plate images can be captured to provide clear visual evidence of performance, supporting data interpretation and reporting.

While not a regulatory requirement, the ZOI test is widely used within research and development to support preservative selection, compare prototype formulations, and generate proof-of-concept data prior to formal preservative efficacy testing. The method is recognised internationally as a rapid screening tool and should be used alongside, rather than in place of, full challenge testing required for cosmetic safety assessment.