Method for Qualifying Microbial Removal Performance of 0.1 Micron Rated Filters Part II: Preliminary Characterization of Hydrogenophaga (formerly Pseudomonas) pseudoflava for Use as a Standard Challenge Organism to Qualify 0.1 Micron Rated Filters

Abstract

In this article, we report on the preliminary characterization of Hydrogenophaga (formerly Pseudomonas) pseudoflava for potential use as a standard challenge organism to qualify 0.1 μm rated filters. Filter-cloned H. pseudoflava (ATCC 700892) was easily cultured in a low nutrient broth (R2A broth) under standard laboratory conditions, reaching high titers of 10⁸-10⁹ cfu/mL within 48-65 hours of incubation at 25±5°C. Under these conditions, H. pseudoflava is a rod-shaped bacterium, averaging 0.25±0.03 μm by 1.65±0.35 μm, and appears to be smaller than Brevundimonas diminuta in width (0.31±0.03 μm), but somewhat longer in length (0.88±0.19 μm), which may partly explain the observed penetration. In total, thirty-five 0.2/0.22 μm rated filter discs, spanning five different “sterilizing grade” filter types from two different filter manufacturers were challenged with H. pseudoflava. In all cases, H. pseudoflava was shown to consistently penetrate every 0.2/0.22 μm rated filter disc tested. These tests also spanned three different challenge durations, including short-term challenges (30-40 minutes), and two different challenge fluids. The use of serial (double) 0.22 mm rated filters, which is a common industry practice to reduce the prefiltration bioburden to the final “sterilizing” filter, was also shown to be inadequate to fully retain H. pseudoflava under the challenge condition used. In contrast, two different 0.1 μm rated filter types functionally qualified with a specified high titer reduction claim for Acholeplasma laidlawii, were shown to consistently and fully retain H. pseudoflava, and retention by these two filter types was shown to be robust and independent of the challenge duration.