Bacterial cell size and surface charge characteristics relevant to filter validation studies

There are two recognized mechanisms whereby organisms are retained by liquid filters; namely, sieve-retention and adsorption. The efficiency of each may be influenced by the organism, suspending milieu, and by the filtration conditions. Validations of sterilizing filtrations require the use of organism suspensions in product-specific media. However, where the product is bactericidal to the challenge organism(s), surrogate solutions may be required. The ideal surrogate solution would minimize adsorptive retention, ensuring that the sterilizing action of the filter under consideration is the consequence of sieve-retention. This review explores the impact that various physicochemical factors may have on bacterial cell size and cell surface characteristics. An understanding of interactions among challenge bacteria, suspending fluid, and filter medium is essential for the development of surrogate solutions that provide a "worst case" mileu for filter validation studies or a "placebo," non-inhibitory challenge solution.