Sampling of bacteria from air
Air can be sampled for the presence of bacteria by their collection on culture medium using open Petri dishes containing a suitable medium. Now-a-days more accurate sampling can be done by use of a slit-sampler device that impringes air on agar surface. The apparatus consists of a mechanical pump, a chamber in which a Petri dish of a nutrient agar is mechanically rotated under a slit of equal in length with the radius of the plate. This slit allows the definite volume of air to enter and contact
with the surface as the plate turns. This device is common to sample particulates of industrial units. In this case bacterial filters of 0.45 or 0.8 µm are used. The filter is transferred on a nutrient surface for a count of colony forming unit (CFU) after incubation. In this method many bacterial cells may die due to dessication on filter, hence viable count is not accurate.
Nutrient agar plates (for bacteria)
Czapek Dox agar plates (for fungi)
(i) Prepare plates of nutrient agar and Czapek Dox agar media.
(ii) Keep the plates in the chamber of air sampler.
(iii) Expose the plates for 5-10 minutes to allow the air to expose on the surface of medium on the plates.
(iv) Take out the plates from the chamber and incubate at 27 C for 24 hours.
Observe the growth of bacterial and fungal colonies. Count the colony forming units (CFUs) of bacteria and fungi which indicate the viable number of bacteria and fungi present in air.
Dr. R. C. Dubey – Practical Microbiology