Maximizing Bacterial Retention
Following the categorization of low surface tension test fluids, we analyzed events of bacterial penetration in each category
and observed that the risk, qualitatively defined here as the likelihood of an occurrence of bacterial penetration during
retention validation challenge, was the highest for liposome solutions, followed by lipid and lipid-like solutions and then
by surfactant-containing solutions.
Figure 1. Risk of Brevundimonas diminuta penetration events with 0.2 μm sterilizing grade membranes in low surface tension
solutions as a function of total challenge bacteria load (CFU/cm2), relative to baseline, i.e., risk at lowest bacterial load.
With these higher penetration risk test fluids, challenge parameters associated with bacterial penetration were identified
as the total Brevundimonas diminuta bacteria load (CFU/cm2 EFA) and the rate of load (CFU/min) during the B. diminuta challenge test (Figures 1 and 2). Although the minimum total load cannot be changed (because of the regulatory requirement
for achieving ≥1 x 107 CFU B. diminuta/cm2 EFA during bacterial retention validation), our data suggest that B. diminuta challenge greater than 1 x 108 CFU/cm2 may significantly increase the risk of validation issues (either premature plugging or bacterial penetration) when conducting
bacterial challenges of sterilizing grade membranes with these potentially higher-risk solutions. If possible, the B. diminuta bacteria challenge loading rate should be minimized.
Figure 2. Risk of Brevundimonas diminuta penetration events with 0.2 μm sterilizing grade membranes in low surface tension
solutions as a function of bacterial load rate (CFU/min), relative to baseline, i.e., risk at lowest bacterial load rate.
Bacterial challenge studies are conducted either under constant flow or constant inlet pressure conditions, so the categories
were analyzed with regard to these challenge conditions (Figure 3). The data suggest that with higher risk solutions containing
liposomes, lipids, or surfactants, a constant flow challenge condition may have a higher risk of a penetration event than
constant pressure conditions. In a situation of constant flow, the pressure inevitably varies in an attempt to maintain a
constant flow rate. This changing (generally increasing) pressure may result in increased potential for penetration. Similar
observations have been previously reported.5
Figure 3. Risk of Brevundimonas diminuta penetration events with 0.2 μm sterilizing grade membranes in categories of low surface
tension solutions according to challenge method, relative to baseline, i.e., risk for surfactants solutions and constant pressure
These categories were further compared with regard to the flux across the filter. To determine flux (flow per unit area),
the maximum recorded flow rate (if available) or the average flow rate (if available) was divided by the effective filter
area (EFA) of the test filter. For each category, two histograms of flux were generated; one for validated sterilizing filtrations
and one for filter challenges where B. diminuta bacteria penetration was reported. The peak values of these histograms provide insight into the differences between the retentive
and penetrative filtration populations, as seen in Table 2. The data suggest that increased flux (or flow rate) does not necessarily
increase the risk of penetration. In fact, when all data are considered, higher flux appears to suggest a correlation with
a reduced risk of penetration, and particularly in the case of lipid solutions. This however, may only be relative to the
positive association between rapid plugging of the membrane and bacterial penetration.
Table 2. The peak value of flux histograms (mL/min/cm2) within a category (the shape of the histograms are skewed such that most of the test fluids have flux rates less than or
equal to the peak).