TABLE OF CONTENTS A variety of international standard methods are available to test the susceptibility of various plastics to microbial growth and to check and demonstrate the effectiveness of the biocide to prevent colonisation and growth of microorganisms. A distinction should be made between those added as a biostabiliser (to protect the plastic itself) or those intended to provide an anti-microbial (hygienic) effect. This must also be disassociated from a disinfectant which has quick cleansing but relatively short-term effect (see Figure 13).
Many plastics are non-polar and hydrophobic in character, causing water droplets to run away or pool. 'Wetting' of the plastic surface may be enhanced by the presence of soiling or a surfactant. Agar plate tests attempt to mitigate the problem of surface wetting by creating a high humidity environment in a petri dish.
It is appropriate for plastics that are used outdoors or in wet environments to be pre-treated or conditioned by artificial weathering (exposure to UV light, condensation and water spray) or leaching before testing. This also alleviates the hydrophobic effect.
Most laboratory tests are used routinely to provide relatively rapid results. They serve as a model to mimic conditions found in practice. However many procedures are not reliable in this respect, since they do not take account of factors such as: the degree of soiling, the potential level of contamination, presence of cleaning residues, increased surface area caused by abrasion, the age, exposure to weather and so on.
Agar plate...
