Air sampling by open plate method

 Aim 

To compare air flora of various locations by plate exposure technique. 

Principle 

Air is not a natural environment for the growth and reproduction of microorganisms. It doesn’t contain the necessary amount of moisture and utilizable form of nutrients. Air is mainly a transport and/or dispersal medium for microorganisms which occur in relatively small numbers in air when compared with soil or water. Microorganisms are still found in air and their presence is of considerable importance economically and to public health. 

Microorganisms in the air include mainly vegetative cells and spores of bacteria, fungi and algae, viruses and protozoan cysts. The species vary depending on the locality. However, certain forms are quite uniformly present. Molds and yeasts are commonly present in the air and in some localities, they outnumber bacteria. They produce spores which are capable of resisting unfavourable conditions for long periods of time. The air microflora consists of molds such as Aspergillus, Penicillium, Alternaria, their spores etc. and of bacteria including spore forming and non-spore forming Gram positive bacilli, Gram positive cocci and Gram negative rods. The aerobic spore forming Bacilli from soil are quite frequently seen in air. Bacillus subtilis, known as Hay Bacillus, is one such common bacteria. Microorganisms are introduced into the air by various sources, most important being the dust particles which contain vegetative cells and spores. 

The microbiological quality of air is dependent upon factors such as temperature, relative humidity, amount of dust stirred up and exposure to ultraviolet or electromagnetic radiation. 

Air sampling is undertaken in areas such as operation theatres, pharmacy sterile units and sterile supply units to assess air quality. It may be used as a continuous monitoring system e.g. in laboratories, in order to assess fluctuations in background counts which may contaminate cultures. Sampling may be either active or passive. Active sampling involves the use of mechanical equipment which draws in known volumes of air which then impinge on culture media or filters. Numbers of microbes present per unit volume of air can then be calculated accurately. Passive sampling needs no special equipment – agar plates containing a suitable medium are horizontally exposed in the area for a defined period of time. The plates are then incubated at a suitable temperature and the number of colonies which develop are then counted. Several plates would usually be exposed at the same time in order to assess the average microbial count. This method is time-consuming and needs careful interpretation as air movements and activity may lead to wide fluctuations in results. 

Settle-plate technique has been widely used to study the air quality outdoor and indoor. The number of organisms present in the air at a given time is dependent upon the activity in the environment and the amount of dust stirred up. An active environment shows a higher microbial count than an inactive one. The numbers in a dusty, untidy room are greater than in a clean room. 

Materials Required 

Media - Nutrient Agar and Sabouraud’s Dextrose agar 

Equipment - Bunsen burner, Petri dishes, incubator 

Procedure 

1. Nutrient agar and Sabouraud’s Dextrose agar were prepared and poured into petri dishes 

2. The petriplates were exposed to air in flow, at different locations. In each location, two petriplates with medium were exposed, for different time intervals such as 5 and 10 minutes. 

3. The petriplates were then closed and nutrient agar plates incubated at 37^oC for 24 hours and Sabouraud’s Dextrose agar plates at room temperature for 3-5 days. 

4. After incubation, the plates were observed and colony count noted. 

Observation and Result 

The number of colonies developed in Nutrient Agar and Sabouraud’s Dextrose agar plates exposed at various locations were determined

                Air sampling by plate exposure method (Left hand side) 

	Number of colonies on Nutrient agar plate
Location	After 5 minutes exposure	After 10 minutes exposure
Lab	14	20
Library	12	29
Laminar air flow chamber	4	10
Outdoor	27	40

 

 

 

 

 

 

	Number of colonies on SDA plate
Location	After 5 minutes exposure	After 10 minutes exposure
Lab	10	16
Library	11	21
Laminar air flow chamber	5	8
Outdoor	18	31