Time-Kill Test for Liquids

Liquid Suspension Time-Kill Test

Welcome to Antimicrobial Test Laboratories' Liquid Suspension Time-Kill Test resource page.

Below, you will find a summary of the Liquid Suspension Time-Kill Test, along with some of its strengths and weaknesses. The Liquid Suspension Time-Kill Test method is an excellent test for use by disinfectant product developers, because it is fast, relatively inexpensive, and very reproducible.

The procedure described below is based on a standardized suspension time-kill test procedure, ASTM E2315 "Standard Guide for Assesment of Antimicrobial Activity Using a Time-Kill Procedure."

Note: The information below is presented for educational purposes only. Current, detailed ASTM Methods can be obtained from ASTM.

Liquid Time-Kill Test Method 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 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 Test Method:

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.

At Antimicrobial Test Laboratories, we are proud of our tradition of collaborative, productive antimicrobial research and development. The suspension-based time-kill test is just one of many tools we use to help companies develop disinfectants and other antimicrobial products.

For more information about liquid time-kill testing or ASTM E2315, Contact the Lab Today!