Microbiology Procedure

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Settle Plate Method

	Settle Plate Method - The principle behind this method is that the bacteria carrying particles are allowed to settle onto the medium for a given period of time and incubated at the required temperature. A count of colonies formed shows the number of settled bacteria containing particles. In this method petridishes containing an agar medium of known surface area are selected so that the agar surface is dry without any moisture. Choice of the medium depends upon the kind of microorganisms to be enumerated. For an overall count of pathogenic, commensal and saprophytic bacteria in air blood agar can be used.
For detecting a particular pathogen which may be present in only small numbers, an appropriate selective medium may be used. Malt extract agar can be used for molds. The plates are labelled appropriately about the place and time of sampling, duration of exposure etc. Then the plates are uncovered in the selected position for the required period of time. The optimal duration of exposure should give a significant and readily countable number of well isolated colonies, for example about 30-100 colonies.
Usually it depends on the dustiness of air being sampled. In occupied rooms and hospital wards the time would generally be between 10 to 60 'minutes. During sampling it is better to keep the plates about I metre above the ground. Immediately after exposure for the given period of time, the plates are closed with the lids. Then the plates are incubated for 24 hrs at 37°C for aerobic bacteria and for 3 days at 22°C for saprophytic bacteria. For molds incubation temperature varies from 1O-50°C for 1-2 weeks. After incubation the colonies on each plate are counted and recorded as the number of bacteria carrying particles settling on a given area in a given period of time.
Though the method has the advantage of simplicity, it has certain limits. In this method only the rate of deposition of large particles from the air, not the total number of bacteria carrying particles per volume, is measured. Growth of bacteria in the settled particles may be affected by the medium used since not all microorganisms are growing well on all media. Moreover since air currents and any temporary disturbances in the sampling area can affect the count, many plates have to be used.

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