The ASTM E2180 method is designed to quantitatively test the antimicrobial effectiveness of incorporated antimicrobial agent(s) in polymeric or hydrophobic materials.
Bacterial inocula (which are aqueous solutions) may bead on hydrophobic test surfaces, creating a domed or nearly spherical shaped droplet, thereby reducing bacterial contact with the treated surface.
This characteristic may thus prevent adequate measurement of antimicrobial activity of a treated hydrophobic surface. The ASTM E2180 test overcomes this obstacle by incorporating the test inoculum into an agar slurry, which provides more uniform contact of the inoculum with the treated surface.
Summary of the ASTM E2180 Test Method
- Inoculation of the agar slurry is performed from a culture transfer as indicated by the method. The agar slurry is what allows full contact between the bacterial cultures and the polymeric and hydrophobic surfaces.
- The agar slurry, now inoculated with the appropriate concentration of bacteria, is added to both antimicrobial (treated) and control test surfaces and allowed to gel before incubation.
- The treated and untreated control surfaces are allowed to incubate at the appropriate temperature for the designated incubation time; 24 hours is commonly used.
- Immediately after inoculation, a separate set of controls are neutralized, sonicated, serially diluted and plated in order to enumerate the starting concentration of the inoculum-agar slurry.
- Enumeration of the surviving organisms is then performed by elution of the inoculated agar slurry via neutralization and adequate removal of the agar slurry from the slides using sonication for a designated time.
- Dilutions are performed and then plated to appropriate agar followed by incubation.
- Agar plates are then removed from the incubator and counted in order to determine the percent reduction via the comparison of the surviving bacteria in the treated carriers to those in the control carriers.
Strengths of the ASTM E2180 Test Method
- The procedure allows companies to quantitatively evaluate the antimicrobial effectiveness of a polymeric or hydrophobic material that would otherwise be difficult with other test methods due to hydrophobic characteristics of the treated surface.
- Hydrophobic surfaces may be repeatedly tested over time to test the presence of any persistent antimicrobial activity.
- The aqueous bacterial inoculum remains in constant contact with the treated surface through a pseudo-biofilm, created with the help of the agar slurry.
Weaknesses of the ASTM E2180 Test Method
- This procedure is not appropriate for determining efficacy of a treated surface against biofilm cells.
Microchem Laboratory recognizes the utility of the ASTM E2180 method for testing the efficacy of hydrophobic surfaces. For testing non-hydrophobic surfaces, the lab often recommends the alternative methods JIS Z 2801 or ISO 22196.