The Importance of Detecting Microorganisms in Liquid Samples Using the Membrane Filtration Method Strategies

Clear-looking water is not always free from microorganisms. Even clean water around us may contain various microorganisms that are invisible to the naked eye, including bacteria that have the potential to cause disease.

Therefore, reliable testing methods are needed to accurately detect microorganisms, even when they are present in very small numbers, to ensure that seemingly clear water is truly safe for use. One such method is membrane filtration, which is both interesting and important to understand.

Why is it widely used? Despite its relatively simple principle, this method provides sensitive and dependable results for microbiological analysis. In this article, you will learn how membrane filtration works, the culture media commonly used, as well as its advantages and limitations in detecting microorganisms in liquid samples.

Table of Contents:

• Introduction to Membrane Filtration.
• Basic Principles of the Membrane Filtration Method.
• Advantages and Limitations of the Membrane Filtration Method.

Introduction to Membrane Filtration.

In water quality testing, examining every type of pathogenic microorganism present in a sample can be a complex, costly, and time-consuming process. For this reason, the use of indicator microorganisms as markers of potential pathogenic contamination has become a practical and effective approach.

One of the most widely used indicator microorganisms is Escherichia coli (E. coli). This bacterium naturally inhabits the intestinal tract of humans and animals, so its presence in water is often associated with fecal contamination.

In 1951, Goetz and Tsuneishi introduced a filtration technique that utilized membranes made of cellulose nitrate and cellulose acetate. These membranes were capable of trapping bacteria as liquid samples passed through their surface.

Today, the fundamental principle of this technique is still applied in laboratories around the world. Although the equipment and membrane materials have evolved over time, the concept of capturing microorganisms on a membrane remains the core of the membrane filtration method.

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Basic Principles of the Membrane Filtration Method.

In simple terms, the membrane filtration method works by passing a liquid sample through a membrane with very small pores. As the liquid flows through the membrane, microorganisms that are larger than the pore size are retained on the membrane surface.

In water microbiology testing, the membrane commonly used has a pore size of 0.45 µm and a diameter of 47 mm. This pore size is small enough to capture most bacteria present in the sample.

The filtration process is typically carried out using a filtration unit connected to a vacuum pump. The vacuum helps draw the sample through the membrane, making the filtration process faster and more efficient.

Once filtration is complete, the membrane is not analyzed immediately. Instead, it is transferred onto the surface of a culture medium containing nutrients that allow the trapped microorganisms to grow into visible colonies.

One of the media commonly used is mEndo LES agar, which is designed for the detection of total coliform bacteria. On this medium, coliform colonies typically appear red and may exhibit a characteristic metallic sheen.

Another widely used medium is mFC agar, which is intended for the detection of fecal coliforms. On this medium, target colonies usually appear blue, making them easier to distinguish from other microorganisms.

After incubation, the colonies that develop on the membrane are counted. Each colony is assumed to originate from a single cell or a small group of microorganisms retained during the filtration process.

The test results are generally expressed as CFU (Colony Forming Units) per 100 mL of sample. This value provides an estimate of the level of microbial contamination in the sample and can be used to assess the quality of water or other tested products.

Advantages and Limitations of the Membrane Filtration Method.

One of the main advantages of membrane filtration is its ability to detect microorganisms present in very low numbers. Because relatively large sample volumes can be processed, the likelihood of detecting target microorganisms is higher than with several other microbiological methods.

This method is also known for its good reproducibility, meaning that results tend to be consistent when the procedure is performed correctly. In addition, microorganisms captured on the membrane can be further cultured and examined for identification purposes.

However, membrane filtration is not suitable for every type of sample. Highly turbid samples may clog the membrane pores, slowing down the filtration process or even preventing it from being completed.

Another limitation is the potential interference caused by other microorganisms or substances present in the sample. High levels of background bacteria or the presence of toxic compounds may affect colony growth and lead to less accurate results.

Interestingly, not all bacteria can be completely retained by a membrane with a pore size of 0.45 µm. Some extremely small bacteria may pass through the filter, which is why membranes with a smaller pore size, such as 0.22 µm, are commonly used for sterilizing filtration.

This highlights the importance of selecting an appropriate pore size for successful microbiological testing.

Author: Dherika
Editor: Lina

More Reliable Microbial Detection

For liquid products or samples, the presence of microorganisms needs to be detected using the right method to better ensure quality and safety. Through the membrane filtration method, microorganism detection can be carried out more accurately, especially for samples with specific volumes or low microbial counts.

With IML Testing and Research, microbiological testing can be conducted professionally to help brands ensure product quality, support safety, and strengthen market trust.

References

Forster, B., & Arango Pinedo, C. (2015). Bacteriological examination of waters: Membrane filtration protocol. American Society for Microbiology. https://asm.org/protocols/bacteriological-examination-of-waters-membrane-fi.

Goetz, A., Tsuneishi, N., Kabler, P.W., Streicher, L., & Neumann, H.G. (1951). Application of Molecular Filter Membranes to the Bacteriological Analysis of Water. Journal (American Water Works Association), 43, 943-984.