Testing Microbial Resistance of Liquid Paints

These tests are used to determine whether a fully formulated water-borne paint (preserved or un-preserved) will allow survival of a bacterial or fungal inoculum added to the paint.

Bacterial Resistance Testing: PRA test

Scope and Field of Application

This test can be used to determine whether a fully formulated water-borne paint (preserved or un-preserved) will allow survival of a mixed bacterial inoculum.

This work is not carried out to any international standard, we use our own in-house method.

Summary of Method

Normally a total of 11 bacterial cultures are used to prepare a mixed inoculum suspension but this can be varied according to the customer’s requirements. After measuring the suspension concentration by nephelometry, a measured volume of the mixed inoculum is used to infect the test paint. A susceptible control paint is also infected in order to check that the inoculum is viable.

The paints are then incubated at 30 °C and samples plated out periodically to check for the presence of viable bacteria. The method allows for re-inoculation after an agreed period.

There are a number of ways of increasing the severity of the test. The paints may be stored under adverse conditions prior to infection in order to check whether any preservative present will degrade or separate out. The paints may also be diluted prior to infection in order to reduce the concentration of any preservative.

The method is semi-quantitative. The presence of viable bacteria in the test paint is taken as evidence that any preservative present has failed to protect the paint against infection. If the number of bacteria increases with time, this is taken as evidence that growth is occurring.

Sample Requirements

We require a 500 ml sample.