Microbial Retention Testing Under Liquid and Aerosol Conditions: 3 Techniques

Microbial retention is the single most important performance attribute of sterilizing-grade air and gas filters. In the food and beverage industry, the sterility of air or gas is crucial to the success of filtration processes to ensure the end product is contaminant-free.

Understanding how sterilizing-grade cartridge air filters are validated for microbial removal from air or gas assists the end user in effective filter selection. According to guidance from FDA, the validation work carried out by filter manufacturers demonstrates the microbial retention and removal efficiency of hydrophobic sterilizing-grade cartridge air filters when challenged with a defined minimum concentration of model microorganisms, namely, Brevundimonas diminuta (B. diminuta) (ATCC 19146) at 10⁷ cfu/cm² of effective filtration area, either from a liquid suspension (“liquid bacterial challenge”) or from a gas (“aerosol bacterial challenge”), or both.

In addition to these basic bacterial challenge tests, which sufficiently demonstrate the sterilizing ability of cartridge air filters, filter manufacturers may do more validation work demonstrating filter retention of bacterial spores, long-term aerosol bacterial challenges, and aerosol bacteriophage removal, to illustrate the filters’ fit for purpose in various applications.

Our team at Pall leverages three ways to validate microbial retention performance of filters. Below are three methods used by filter manufacturers to validate microbial retention performance.

Filters perform differently based on whether they are removing contaminants from a liquid or a gas. Filters have both a liquid and a gas removal rating. Due to the mechanisms at play in liquid or gas filtration, it is more “difficult” to remove contaminants from liquids than from gases, i.e., it requires a “tighter” filter to remove contaminants from a liquid than from a gas. Further, a liquid-rated 0.2 µm filter, which removes the described contaminants from a liquid, is much tighter than a gas-rated 0.2 µm filter, which removes these contaminants from a gas.

The bacterial challenge test is conducted under normal (gas) and worst-case (liquid) lab conditions simulating production conditions.

Comparison of Liquid Bacterial Challenge and Aerosol Bacterial Challenge Validation

Both microbial validation tests demonstrate the gas sterilizing capability of a filter according to the definition for sterilizing-grade cartridge gas filters, however under different conditions. Both methods provide extremely high sensitivity by evaluating the totality of filter effluent, they can detect even very low numbers of viable bacterial cells which may have penetrated the filter.

Validating sterile filtration of gases that come into contact with products or packaging is a critical component to ensuring the filtration process is fit for purpose so that the end product is contaminant-free. You can learn more about the details of each validation test in our full article.