Principle:
The
concentration and quality of micro organisms in atmosphere can have a direct
bearing on human health and environment. In recent years air microbiology has
gained a lot of attention. Microbiological quality of air can be considered as
a mirror of hygienic conditions of any place.
Air
samples can be collected in two ways: by active air samplers or by passive air
sampling (the settle plates).Passive air sampling is performed using settle
plates. Petri dishes containing solid nutrient medium or PDA medium are left
open to air for a given period of time. Microbes carried by inert particles
fall onto the surface of the nutrient, with an average deposition rate. After
incubation they grow into colonies in a number proportional to the level of
microbial contamination of the air. Despite a few limitations, the settle plate
method is still widely used as a simple and inexpensive way to qualitatively
assess the environments over prolonged exposure times. They are sterile,
economical and readily available. Many sites can be checked at the same time.
Materials
Required:
Petri dish, Nutrient
agar, Potato Dextrose Agar, Incubator, colony counter
Procedure:
1. Prepared sterilized pert
plates of Nutrient agar and PDA
2. The plates were
transported to the sites or area by using sealed bags.
3. The plates were
labeled with sample number and site number.
4. Petri dishes left open
to the air according to the 1/1/1 scheme (for 1 hr, 1 m from the floor, at
least 1 m away from the walls or any obstacles).
5. After the exposure
the plates were covered with lids and taken to the laboratory in sealed plastic
bags and incubated at 37oC for 24 hours.
6. The
culture plates that showed discrete macroscopic colonies were counted using
plate colony counter.
7. The concentration of
airborne bacteria /fungi was expressed as colony forming units per cubic meter
(cfu/m3).
