Introduction to ASTM E2315, Liquid Suspension Time-Kill Testing
The ASTM E2315 liquid suspension time-kill test, sometimes called a kill-rate test, is excellent for disinfectant product developers because it is a fast, relatively inexpensive, and reproducible way to measure the biocidal potential of a liquid antimicrobial formulation. It consists of directly inoculating a liquid test substance with a high concentration of test microorganisms and then determining the percentage killed over time.
The suspension-based time-kill test has been standardized by ASTM as ASTM E2315 “Standard Guide for Assessment of Antimicrobial Activity Using a Time-Kill Procedure.”
ASTM E2315 Liquid Time-Kill Test Summary
- A microbial culture is prepared. For most bacteria, a 24 hour culture in nutrient broth works well. For most fungi, a spore preparation from a saline wash works well.
- Equal volumes of the test product are placed in sufficient sterile test vessels.
- A volume of microbial culture (usually 1/10 of the product volume) is placed in the test vessel and then immediately mixed.
- After the predetermined contact times, small aliquots of the mixture of bacteria and product are removed and microorganisms are enumerated.
- To measure initial microbial concentrations, a saline control vessel is spiked with the same microbial culture and then enumerated.
- Numbers of microorganisms in the reaction vessel are plotted over time.
- Neutralization controls are run as appropriate.
Strengths of the ASTM E2315 Liquid Time-Kill Test
- The impact of a disinfectant product on microorganisms over time (death curve) can be studied with relative ease using the suspension-based test method.
- Suspension-based time-kill tests are relatively inexpensive.
- The test parameters for suspension-based time-kill tests are easy to control in the laboratory setting, so comparisons can be made fairly easily between various products tested under the same conditions.
- The suspension-based time-kill test involves exposing microorganisms to excess disinfectant in a liquid setting, so the test is a fitting model system for instances in which disinfectants will be used to kill microorganisms in liquid settings (such as in a disinfectant rinse for drinking glasses in a restaurant).
- Very brief contact times can be studied with relative ease.
Weaknesses of the ASTM E2315 Liquid Time-Kill Test
- The test method is difficult to relate to disinfection of microorganisms on a surface (such as results from the AOAC use-dilution test method).
- It is somewhat of a “best case” method meaning that good percent reductions are likely to be seen if the test chemical is indeed antimicrobial and the contact times are sufficient.
- The way in which the microbial inoculum is prepared can have an impact on the test outcome, since different broths/suspensions will interact with disinfectants differently.
Application of the ASTM E2315 Liquid Time-Kill test to antimicrobial product development
ASTM E2315 is a great starting point for the evaluation of germicidal products. It is cost-effective and tells formulators the response of various microorganisms to the antimicrobial suspension over time. This test is often followed by standardized tests required by EPA or FDA for product registration or pre-market approval.
The suspension-based time-kill test is just one of many tools we use to help companies develop disinfectants and other antimicrobial products. Contact us today for a quote for this relatively inexpensive test.
