Is a 0.22 μm filter equivalent to a sterilizing-grade filter?
Membrane filters entered the commercial market in the 1960s. At that time, membranes with a 0.45μm rating were regarded as "sterilizing-grade" filters and were successfully applied to the sterilizing filtration of injections. Serratia marcescens was used as the standard test organism for verifying the bacteria-retaining capacity of filter membranes.
Serratia marcescens, also referred to as Bacillus prodigiosus, is a bacterium that produces bright red pigment. It exists in air and water and can grow on both animal-based and plant-based foods. It is a nearly spherical short bacillus with pleomorphic morphology, measuring approximately 0.5 × (0.5–1.0) μm.
Note: Serratia mar was first described and named by Bartolomeo Bizio, an Italian pharmacist, in the early 19th century. At that time, polenta, a food made from cornmeal, was popular in Italy. Bizio found that polenta easily turned red during the warm and humid summer months in Italy. He then isolated and identified a new microorganism by culturing it on fresh polenta, which is the Serratia marces known today.
However, in a paper published in 1960, Dr. Frances Bowman from the U.S. Food and Drug Administration (FDA) discovered that culture media subjected to 0.45μm "sterilizing filtration" could be contaminated by a certain microorganism. A small number of this organism could repeatedly penetrate 0.45μm-grade filter membranes at challenge levels of 10⁴ to 10⁶ CFU per square centimeter or higher.
Following years of industry efforts, a new sterilizing-grade filter membrane was developed, which is the 0.22 μm nominal pore size sterilizing filter membrane still widely used today.
Why is it defined as 0.22 μm?
The nominal pore size of the filter membrane can be calculated according to the following formula:
d: Capillary Diameter (Pore Size)
k: Shape Correction Factor: corrects the deviation between flow through membrane pores and direct flow through a capillary tube.
σ: Surface Tension of the Filter Wetting Liquid
P: The pressure required to overcome surface tension and displace water within the membrane pores
Θ: Contact Angle Between Liquid and Capillary Wall (Filter Membrane)
Therefore, the pore size of a sterilizing filter membrane is not a critical physical dimension; it is a functional definition. In accordance with ASTM F838-15, a filter that can consistently and reproducibly produce sterile filtrate upon proper validation Brevundimonas diminuta at a challenge level of no less than 1×10⁷ CFU per cm² of effective filtration area, is defined as a sterilizing-grade filter.