Challenge testing: principles and practice

It is difficult to predict accurately the ultimate effectiveness of a preservative in any but the simplest cosmetic or pharmaceutical formulation. It is thus necessary to obtain some assurance of its likely in-use performance in a formulated product. A challenge test is a procedure in which a product is challenged by exposure to specified types of bacteria and fungi to determine whether it is adequately preserved. Assessment of preservative efficacy is needed over the intended shelf-life of that product. Test organisms should be representative of those likely to occur as contaminants during use and should consist of Gram-positive and Gram-negative bacteria, mould and yeast. In-house factory isolates obtained as a result of contamination of earlier batches of a product may also be included. The organisms, as single or mixed inocula, are inoculated (usually as a single challenge, although some advocate multiple challenges) into samples of the product and aliquots removed at appropriate intervals for the determination of survivors. Interpretation of data is normally based on pharmacopoeial or other official protocols. Challenge testing should be undertaken at the beginning, during and at the end of the shelf-life of the product. An alternative, the D-value, approach is open to criticism and further studies are required that utilize rapid methods, e.g. impedance, for the detection of survivors before these can be considered to be a suitable replacement for the more traditional but very time-consuming, viable counting procedures.