Sampling of Microorganisms From Soil

Exploring the microscopic world within soil requires precise techniques for the effective sampling of microorganisms, such as bacteria. These organisms play crucial roles in various ecological processes, including nutrient cycling, and understanding their presence and diversity is vital for both scientific research and environmental monitoring. Two widely recognized methods facilitate the direct observation of these microorganisms.

The first method involves the use of specialized glass capillaries, which are essentially micro-slides with a unique rectangular cross-section. One of the walls of these capillaries is designed to be exceptionally thin, measuring approximately 0.17 millimeters in thickness. These micro-slides are typically grouped and connected, forming a cluster of five, and are subsequently filled with sterile distilled water to prepare them for the sampling process.


The second approach employs a more straightforward setup, utilizing a watch glass to hold the soil sample. The process begins by placing the soil at the bottom of the glass, followed by the careful addition of water. The water is added until it forms a layer above the soil sample, creating a suitable environment for the microorganisms to be more easily observed. A floating cover slip is then gently placed on the water’s surface, which allows for the collection of microorganisms from the sample for detailed examination.

Both methods are instrumental in the study of soil microorganisms, offering a window into the complex and often unseen world beneath our feet. Through these techniques, researchers can gain insights into the microbial composition of soil, which has implications for agriculture, ecology, and our broader understanding of environmental health and sustainability.


Requirement for Sampling of Microorganisms From Soil

  1. Glass Capillaries (Micro-Slides): These specialized glass slides are fundamental in the sampling process. They are designed with a specific rectangular cross-section, with one of the walls being exceptionally thin, usually around 0.17 millimeters. This thinness is crucial as it allows for optimal viewing of the microorganisms under a microscope. Glass capillaries serve as the medium where the soil sample mixed with water is placed for observation.
  2. Distilled Water: The use of distilled water is vital in this process to avoid introducing any external microorganisms or chemicals that might be present in tap or non-distilled water. Distilled water ensures that the environment within the glass capillary is sterile, which is critical for obtaining an accurate representation of the soil’s microbial community without any contamination.
  3. Microscope: A high-quality microscope is an indispensable tool for viewing and analyzing the microorganisms extracted from the soil samples. Microscopes enable researchers to observe the microorganisms at a cellular level, allowing for detailed examination of their structures, movements, and interactions. This detailed observation is essential for identifying different types of microorganisms and understanding their roles within the soil ecosystem.


  1. Positioning the Glass Capillaries: Begin by inserting the glass capillaries vertically into the soil. This positioning ensures that the capillaries can penetrate the soil effectively and come into direct contact with the microorganisms residing within. The vertical alignment is crucial for ensuring an even distribution of soil particles and microorganisms along the length of the capillary.
  2. Retrieval of the Capillaries: Carefully remove the glass capillaries from the soil immediately after insertion. This step requires precision to avoid disrupting the soil and microorganisms collected within the capillary. Prompt removal is essential to preserve the integrity of the sample and prevent any external contamination.
  3. Cleaning the Capillary: Once removed, it is necessary to wipe the outer surface of the capillary gently. This step is performed to remove any excess water or soil particles that may have adhered to the outside of the glass. The objective is to create a thin, clean layer on the capillary, which will facilitate clearer observation under the microscope. Care must be taken to avoid disturbing the soil sample within the capillary.
  4. Microscopic Examination: Finally, place the cleaned capillary onto the stage of a microscope in such a manner that the microorganisms contained within can be easily observed. The placement should allow for optimal lighting and focus, enabling the detailed examination of the microorganisms growing inside the capillaries. This step is crucial for the identification and analysis of the microorganisms, providing insights into their types, behaviors, and interactions within the soil ecosystem.

1. Direct Total Cell Counting

Direct total cell counting is a critical technique in microbiology for quantifying the number of cells in a sample, particularly bacteria. This method involves direct observation and enumeration of cells using advanced microscopy techniques, such as epifluorescence microscopy, which enhances the visibility of the microorganisms. The process is vital for various applications, including environmental monitoring, food safety testing, and clinical diagnostics.

Requirements for Direct Total Cell Counting:

  • Fluorescence Microscope: A key instrument that uses fluorescence to illuminate the bio-samples, making the cells visible and distinguishable under high magnification. This microscope is essential for identifying and counting the stained microorganisms on the filter or slide.
  • Stains: Acridine orange (0.01%) and Fluorescein isothiocyanate are commonly used fluorescent dyes. Acridine orange is a versatile nucleic acid stain that binds to DNA and RNA, making the cells fluoresce under the microscope. Fluorescein isothiocyanate, another fluorescent stain, binds to proteins and other cellular components, providing a different mechanism for cell visualization.
  • Sterile Distilled Water: Used to rinse the samples after staining to remove excess dye and prepare the slides for observation, ensuring that the observations are not hindered by background staining.

Procedure for Direct Total Cell Counting:

  1. Staining the Sample: Begin by staining the bacteria present in the sample. This can be done by adding a 0.01% solution of acridine orange or a similar concentration of fluorescein isothiocyanate to the sample before or after it has been filtered. The staining process, which typically lasts for about one minute, is crucial for making the cells visible under fluorescence microscopy.
  2. Rinsing: After staining, the sample must be thoroughly rinsed with sterile distilled water. This step is essential to remove any unbound stain, reducing background fluorescence and improving the clarity of the cell images under the microscope.
  3. Slide Preparation and Observation: Prepare a smear of the rinsed sample on a microscope slide. The prepared slide is then observed under a fluorescence microscope. During observation, cells will appear brightly lit against a dark background, making them easier to count. The counting can be done across the entire field of view provided by the microscope’s objective or by using an eyepiece graticule, which is a special type of microscope eyepiece that contains a precisely etched scale, allowing for more accurate enumeration of the cells.

The Acridine Orange Direct Count (AODC) method is among the most popular techniques for direct total cell counting due to its sensitivity and effectiveness in highlighting both live and dead cells. By employing fluorescence microscopy, researchers can achieve a detailed and accurate count of microbial populations in a given sample, providing invaluable data for a wide range of scientific and industrial applications.


2. Direct Viable Cell Counting

Direct viable cell counting is a refined method used to quantify the number of living cells in a sample, particularly bacteria. Unlike total cell counting, which includes both living and dead cells, viable cell counting focuses exclusively on those cells capable of growth and reproduction. This technique is crucial in microbiology, environmental biology, and food safety, as it provides insights into the health and functional status of microbial communities.

Techniques for Direct Viable Cell Counting:

  1. Dilution of Samples: Due to the potentially high concentration of bacteria in natural samples, it’s often necessary to dilute these samples before analysis. This step ensures that the resulting colonies are sparse enough to be individually counted following incubation.
  2. Use of Selective Agents: The Nalidixic Acid Method is one technique used in viable cell counting. This method involves the addition of a growth substrate, such as 0.025% yeast extract, to support cellular metabolism and growth. Concurrently, 0.002% nalidixic acid, a DNA synthesis inhibitor, is added to prevent cell division. This allows cells to enlarge without dividing, making them more easily observable under a microscope.
  3. Incubation: The treated sample is incubated, typically at 30°C, for a period, often around 6 hours. This period allows viable cells to metabolize the yeast extract and react to the nalidixic acid by increasing in size.
  4. Filtering and Staining: After incubation, the sample is filtered and stained with 0.01% acridine orange or fluorescein isothiocyanate. These fluorescent dyes are used for epifluorescence microscopy, a technique that enables the visualization of fluorescently labeled cells.
  5. Fluorescence Microscopy: The stained sample is observed under a fluorescence microscope. Viable cells, enlarged and stained, become easily identifiable, allowing for an accurate count.

Alternative Approach with Vital Fluorogenic Dyes:

Another method involves the use of vital fluorogenic dyes, such as fluorescein diacetate (FDA), at concentrations ranging from 0.02% to 0.05%. FDA is non-fluorescent until it penetrates living cells, where intracellular esterases cleave it to release fluorescein, a brightly fluorescent compound. This fluorescence is readily detected by fluorescence microscopy, providing a direct measure of cell viability.


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