Air sampling techniques- Part 2 - Based on gravitation

 

Air samplers based on Settling under Gravity include Hesse’s tube and Settle plate method.

1.         Hesse’s Tube

 

Hesse’s tube is a horizontal glass tube with a layer of solid media at the bottom. Particles would settle on the media based on gravity as air entered  into the tube through inlet. Larger particles settle faster than the smaller ones. Upon incubation, colonies develop on the medium. If the tube is long enough or the flow is sufficiently low, all particles will settle out before reaching the outlet end.

2.         Settle Plate Method

 In this method, Petri dishes containing sterile agar medium or any suitable medium of known surface area are used.  Media surface should be dry. Bacteria carrying particles are allowed to settle on to the medium based on gravitational force for a given period of time and incubate at required temperature. Count of colonies show the number of viable bacteria containing particles.

Choice of the media depends on the type of organism to be enumerated. Nutrient agar is commonly used to enumerate most of the pathogenic, commensal and saprophytic bacteria. Selective media like Blood agar, Malt extract for molds also can be used for particular pathogens.

First,  the agar plates are labelled with place, date, time of sampling, duration of exposure etc.  Then the plates are kept open in the selected area for the required time period.  Optimal duration of exposure would provide well developed, readily countable, isolated colonies, approximately between 30-100. Number of colonies depends on the dustiness of air. In occupied rooms and in hospitals, duration of sampling should be less as there is a chance for more organisms. Plates should always be kept 1 m above ground level to avoid settling of particles from ground. Immediately after sampling, the petri plates   are closed with a lid and incubated for a required time period. For aerobic bacteria, incubation at 37^o  C for 24 hrs and for saprophytic bacteria incubation at 22^o  C for 3 days are usually done. Fungus require incubation at room temperature for 1-2 weeks.

Advantages

❖      Simple, economical and easy to perform method

Limitations

❖      Larger particles settle easily and hence  total number of bacteria carrying particles per volume cannot be measured

❖      Growth of settled organisms will depend on the type of media used

❖      Air current and temporary disturbances in the sampling area would affect the count. So multiple plates have to be used.