Here are some things you still don’t know about microbial testing!

When we talk about microorganisms, we naturally think of bacteria, fungi, and the like. "Micro" simply means "small," so microorganisms are a collective term used to describe all tiny living organisms—organisms so minuscule that they can only be seen clearly with the aid of a microscope. While some microorganisms are beneficial, others can trigger a variety of health issues and infectious diseases, negatively impacting human well-being. In fact, certain microorganisms even cause food and raw materials to spoil or deteriorate. That’s why strict quality control is essential when it comes to microbial testing.

So, what exactly are the various aspects of the microorganisms we often talk about?

First and foremost, we must talk about food. Since food contains nutrients that microorganisms can rely on for growth, it is inevitably subject to varying degrees of microbial contamination. Food safety and quality are directly linked to the health, well-being, and even the very lives of people, as well as to the stability of our society and economy. To ensure food safety and protect public health, microbial testing is absolutely essential.

Additionally, hospitals cannot be overlooked. Hospital-acquired infections are all closely linked to microorganisms. Strengthening microbial testing in hospitals plays a crucial role in preventing and controlling these infections.

Additionally, there’s the air—something none of us can do without. Indoor air serves as a critical pathway for the transmission of microbial contamination. Moreover, comfortable indoor temperature and humidity levels, combined with relatively enclosed spaces, create ideal conditions for the proliferation of various harmful microorganisms. Only by conducting thorough microbial testing to accurately determine their presence can we implement effective measures, ensuring that people’s health remains safe from microbial threats.

What are the various methods for microbial detection?

  1 • Counter Assay

The method involves counting directly using a hemocytometer. A specific volume of the cell suspension sample is placed into the counting chamber of the hemocytometer, and the cells are counted under a microscope. Since the volume of the counting chamber is fixed at 0.1 mm³, the bacterial concentration in the sample can be calculated based on the number of bacteria observed within the grid lines of the hemocytometer. This technique is simple, easy to perform, and provides immediate results, making it suitable not only for bacterial counts but also for counting yeast cells and mold spores.

  2 • Electronic counter counting method

The electronic counter works by detecting changes in the electrical resistance of a liquid as it passes through a tiny aperture—small enough for only one cell at a time. When a single cell moves through the aperture, the resistance increases significantly, generating a pulse that is automatically recorded by the electronic device. This method provides highly accurate results, though it can only measure particle size and cannot distinguish whether the particles are bacteria or not. Therefore, the bacterial suspension must be free of any debris.

3. Live Cell Counting Method

A commonly used method is the plate count method, which is designed based on the principle that each viable bacterium can form a single colony. A specific volume of bacterial suspension is taken, followed by a series of serial dilutions. Then, a measured amount of the diluted solution is spread onto agar plates for incubation. By counting the resulting colonies, the total number of viable bacteria can be calculated. This method is highly sensitive and serves as an effective way to detect the number of viable contaminating bacteria—indeed, it’s a microbial testing technique widely adopted by many countries around the world today. When using this method, please note the following:

① Typically, plates with colony counts between 30 and 300 are selected for counting—counts that are too high or too low tend to be inaccurate.

② To prevent colony spread and ensure accurate counting, 0.001% of 2,3,5-triphenyltetrazolium chloride (TTC) can be added to the culture medium.

③ This method is specifically designed for microorganisms that form colonies. It is widely used in bacterial testing of various materials, such as water, milk, food, and pharmaceuticals, making it a commonly employed microbial detection technique.

  4 • Turbidimetry

The turbidimetric method indirectly determines bacterial counts by measuring the amount of light transmitted through a bacterial suspension. Within a certain range, the concentration of the bacterial suspension is inversely proportional to its transmittance and directly proportional to its optical density. Therefore, a spectrophotometer can be used to measure the bacterial solution, with optical density (OD value) serving as an indicator of the sample’s bacterial concentration. This method is simple and quick, but it’s only suitable for suspensions containing high bacterial concentrations, providing relative rather than absolute cell counts. Additionally, it cannot be applied to samples with excessively dark colors.

  5 • Cell weight assay

This method is divided into the wet-weight method and the dry-weight method. In the wet-weight method, the wet biomass of a given volume of culture is weighed after centrifugation. In the dry-weight method, the same volume of culture is centrifuged, rinsed thoroughly with distilled water, transferred to a desiccator, and then heated and dried to remove all moisture before being weighed. This method is particularly suitable for samples with high cell concentrations and is a commonly used approach in microbial analysis.

  6 • Determining total nitrogen or total carbon content in cells

Nitrogen and carbon are major cellular components with relatively stable concentrations, making it possible to estimate cell mass by measuring their levels. This method is particularly well-suited for detecting microorganisms at higher concentrations.