Bacteriology of Air
The load of microorganisms present in the air depends on whether the air is indoors or outdoors. The number of bacteria at any time is dependent on many factors, the most important of which are the number of persons present, the amount of their body movements, and the amount of disturbance of their clothing.
Observations of the number of bacteria carrying particles in the air may be required in premises where safe working depends on the air’s content of bacteria being kept at a very low level, e.g., operation theaters. Bacteriological examination of air is also necessary for monitoring air quality in hospital wards, store house of food and pharmacy, etc.
Ideally, bacterial count should not exceed 1 per cubic foot of air in operation theater for neurosurgery, 10 per cubic foot in operation theater for other surgery, and 50 per cubic foot in homes, offices, and factories.
Various methods have been devised for the measurement of bacterial content of air. A primary distinction must be drawn between the methods that measure the rate at which bacteria carrying particles are settling by gravity on to exposed surfaces and those that count the number of bacteria carrying particles in a given volume of the air. Two types of methods used for bacteriological examination of air are as follows:
Settle plate method
Slit sampler method
Settle plate method is used for testing bacteriological quality of air in surgical operation theaters and hospital wards. In this method, Petri dishes containing nutrient agar and blood agar (for detecting pathogenic staphylococci and streptococci) are left open for half an hour to one hour. During this period of exposure, large bacteria carrying dust particles settle on to the media. The plates are then incubated at 37°C for 24 hours, fol-lowing which the colonies formed on the media are counted.
Slit sampler method is most efficient and convenient method used for estimation of the number of bacteria present in a mea-sured volume of air. In this method, one cubic foot volume of air is directed onto a plate containing culture medium through a slit 0.25-mm wide. The plate is then rotated so as to allow the microorganisms present in the air to spread out evenly on the medium. The culture medium is incubated and the num-ber of colonies formed on the medium indicates the number of bacteria present in the air.