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Sampling of Bacteria From Air

The process of collecting bacteria from the air for analysis involves several techniques, one of the simplest being the utilization of open Petri dishes filled with a growth medium. These dishes are exposed to the air, allowing airborne bacteria to settle and subsequently grow on the medium, which can then be examined to identify the types and concentrations of bacteria present.

For more precise measurements, advanced tools like slit-sampler devices have become popular. These devices are designed to capture air samples more efficiently and with greater accuracy. The core component of a slit-sampler is a mechanical pump connected to a chamber where a Petri dish containing nutrient agar is placed. As the dish rotates, air is drawn through a narrow slit directly above the dish. The length of this slit is carefully designed to match the radius of the Petri dish, ensuring a uniform exposure of the agar surface to the air sample. This method is particularly useful in environments like industrial facilities, where monitoring airborne particulates is crucial for maintaining air quality and safety standards.

Another method involves the use of bacterial filters with pore sizes typically around 0.45 or 0.8 micrometers. Air is passed through these filters, trapping bacteria, which are then transferred to a nutrient-rich surface to allow for the growth and counting of colony-forming units (CFUs). However, this technique has a limitation; the process of filtration and subsequent exposure to conditions on the filter can lead to the desiccation and death of some bacterial cells. This can result in an underestimation of the actual viable bacteria present in the air sample.

Each of these methods offers a unique approach to sampling bacteria from the air, with applications ranging from environmental monitoring to ensuring the sterility of healthcare and food production environments. Understanding these techniques is essential for professionals in fields related to microbiology, public health, and industrial hygiene, enabling them to assess and manage the microbial content of the air effectively.

Requirement for Sampling of Bacteria From Air

  • Air Sampler: This device is essential for capturing airborne microorganisms in a controlled manner. It allows for the precise measurement of the volume of air sampled, ensuring that the data collected is quantitative and can be used for comparative analysis.
  • Nutrient Agar Plates: These are used for the growth and isolation of bacteria from air samples. Nutrient agar provides a rich environment that supports the growth of a wide range of bacterial species, making it an ideal medium for general bacterial culture.
  • Czapek Dox Agar Plates: Specifically formulated for fungi, these agar plates contain nutrients that cater to the growth requirements of fungal species. They are used alongside nutrient agar plates to differentiate and cultivate fungal organisms that may be present in the air sample.
  • Colony Counter: After incubation, this device aids in the accurate counting of microbial colonies that have grown on the agar plates. It ensures that the quantification of microorganisms is precise, facilitating the assessment of air quality and microbial load.


  1. Preparation of Agar Plates:
    • Begin by preparing plates with nutrient agar and Czapek Dox agar media. Nutrient agar is a versatile medium suitable for growing a wide range of bacteria, while Czapek Dox agar is formulated specifically for the cultivation of fungi. These media provide the necessary nutrients to support microbial growth.
  2. Placement in Air Sampler:
    • Once the agar plates are prepared, place them inside the chamber of an air sampler. The air sampler is a device designed to capture airborne microorganisms by allowing air to pass over the surface of the agar plates.
  3. Exposure to Air:
    • Expose the agar plates to the air for a duration of 5-10 minutes. This exposure time allows sufficient air to come into contact with the agar surface, enabling airborne microorganisms to settle on the medium. The exact duration can be adjusted based on the expected level of microbial contamination in the air.
  4. Incubation:
    • After exposure, remove the agar plates from the air sampler and incubate them at a temperature of 27°C (approximately 80.6°F) for 24 hours. Incubation at this temperature creates an ideal environment for the growth of most bacteria and fungi captured from the air.


Upon completing the incubation period for the agar plates used in air sampling, the next crucial step is to analyze the outcomes. This involves a careful examination of the plates to identify and quantify the growth of microbial colonies. The process yields valuable insights into the microbial composition of the air sample, specifically in terms of bacteria and fungi. Here’s a breakdown of how the results are obtained and interpreted:

  • Observation of Microbial Growth:
    • The first step is to visually inspect the agar plates for the presence of microbial colonies. These colonies appear as distinct clusters of microorganisms that have grown from individual cells or spores that were deposited on the agar surface during the sampling process.
  • Counting Colony Forming Units (CFUs):
    • The quantification of microorganisms is done by counting the colony-forming units (CFUs) on each plate. A CFU represents a colony that has arisen from a single microorganism or a group of identical microorganisms. Counting CFUs provides an estimate of the viable (living and capable of growth) number of bacteria and fungi present in the air sample.
  • Interpreting the Results:
    • The CFU count is a direct indicator of the microbial load in the air. A higher number of CFUs signifies a greater concentration of viable microorganisms. By comparing the CFU counts on nutrient agar (for bacteria) and Czapek Dox agar (for fungi), it is possible to assess the levels of bacterial and fungal contamination separately.

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