Certification of probability of sterilization of liquid by filtration

Our plain-English summary of Certification of probability of sterilization of liquid by filtration, by D.M. Portner, C.R. Phillips, R.K. Hoffman (Applied Microbiology, 1967).

The study

• Authors: D.M. Portner, C.R. Phillips, R.K. Hoffman
• Journal: Applied Microbiology
• Published: 1967;15(4):800-7
• Cited by: 2 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Four types of hydrosol filters, two reusable (diatomaceous cylinder and fritted-glass funnel) and two disposable (asbestos pad and membrane filter) were challenged with a heavy bacterial suspension to assess their ability to produce sterile filtrates. Two of the four diatomaceous earth filters, the four fritted-glass funnels, and all of the asbestos pads tested generally gave sterile filtrates. However, only one type of filter, one of the membranes in its manufacturer's own holder, consistently gave sterile filtrates. The two other types of membranes usually gave sterile filtrates if tested in one manufacturer's holder, but all types invariably gave contaminated filtrates when tested in another manufacturer's holder. Contaminated filtrates were generally attributed to a poor reusable filter or to a faulty holder used with a disposable filter. If a high degree of certainty is required for sterile heat-labile filtrate, it is suggested that the liquid be passed through two or more filters in a previously tested and proven system.

In plain English

This was a laboratory study, done at Fort Detrick for the National Aeronautics and Space Administration, of whether filtering a liquid once can reliably make it sterile. It is not a study of asbestos and health. Asbestos appears only as one filter material, a disposable asbestos pad. The other filters were reusable diatomaceous cylinders, reusable fritted-glass funnels and disposable membrane filters from three makers. In 288 tests, the filters were challenged with a heavy suspension of the bacterium Serratia marcescens. The fritted-glass funnels and asbestos pads generally gave sterile filtrates. Two contaminated asbestos-pad results came from a holder later found to have a minute hole that let air in. Two of the four diatomaceous cylinders mostly let bacteria through. For membrane filters, the holder appeared at least as critical as the filter. Only membrane A used in manufacturer A's holders never gave a contaminated filtrate. The authors found no clear evidence that any one technique was much more efficient. They recommended that, for critical experiments, liquids be passed through two or more filters in a tested and proven system.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory experiment comparing filtration units, run in random order in repeated runs
• Who was studied: No people or animals. Filtration units were tested: 4 diatomaceous cylinders, 4 fritted-glass funnels, 48 asbestos pads in 4 bronze holders, and three makes of membrane filter (A, B, C) in 4 glass holders from manufacturer A and 4 from manufacturer B. In total there were 288 tests in 24 runs at the Department of the Army, Fort Detrick, Maryland.
• Exposure: Not a health exposure. Each filter, including the asbestos pads (fiber type not stated), was challenged with a suspension of Serratia marcescens. Two 25-ml samples, each containing about 10 million cells, were filtered per test.
• Main results:
  • Two of the four diatomaceous cylinders (nos. 1 and 4) mostly gave contaminated filtrates. Nos. 2 and 3 gave sterile filtrates in all but two instances.
  • The four fritted-glass funnels and all the asbestos pads generally gave sterile filtrates.
  • Asbestos pad holder no. 1 had a minute hole that let air in behind the pad. Contamination did not occur after the hole was repaired.
  • Contaminated filtrates occurred with every type of filter unit except membrane A used in the four holders from manufacturer A.
  • For membrane filters, the holder appeared at least as critical as the filter. All three membranes performed poorly at first in manufacturer B's holders, and performance improved when extra binder clips were added.
  • Where bacteria got through, the measurable leakages were all under 0.01% of the cells in the suspension.
  • The two ways of checking filtrates, broth culture and pour plate, agreed in most tests: both negative in 197 tests and both positive in 69.
  • For the better reusable filters, which were sterile in all 12 trials, the authors inferred that the true mean number of organisms passing for 240,000,000 cells was not over 3.
• The authors' conclusion: The authors found no clear evidence that any one filtration technique was much more efficient than the others. They recommended that, for critical exobiological experiments, a tested and proven system using two or more filtrations be preselected.
• Limitations the authors state:
  • A few unexplained contaminants appeared. The authors suspected these arose after filtration, possibly from faulty equipment, an air leak or a break in aseptic technique.
  • The test conditions used far more microorganisms than the fluids in actual space use would probably contain.
  • More testing would be needed to be sure that an individual disposable filter is as efficient as others from a lot shown to be good.
  • True pore size of the filters could not be determined.

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• Toxicity of cell culture medium due to filtration through asbestos pads — W. House (1964)
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