Isolation of airborne microbes by settle plate method

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 37^oC 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).