Filtration and Deposition

Microbes and Atmosphere
Part-1
Relationship between Microbes and Atmosphere
Atmosphere as a Habitat
Transport of Spores
Sampling of Air
Launching
Transport of Microorgannisms in Air
Deposition and Adhesion
Types of Deposition
Gravitational Settling
Surface Impaction
Rain and Electrostatic Deposition
Methods of Air Sampling
Impingement
Impaction
Centrifugation
Filtration and Deposition
Sampling Methods and their characteristics

Filtration and Deposition

	Filtration and Deposition Both the methods (Fig. 10.10) are widely used for microbial sampling for cost and portability reasons. Filter sampling requires a vacuum source and involves passage of air through a filter, where the particles are trapped. After collection, the filter is washed to remove the organisms before analysis. Usually membrane filters are used for the purpose with varied pore sizes.
Deposition sampling is by far the easiest and most cost-effective method of sampling. Deposition sampling can be accomplished merely by opening an agar plate and exposing it to the wind, which results in direct impaction, gravity settling, and other depositional forces. There are problems associated with this method of sampling. They have low overall sampling efficiency because it relies on natural deposition. It has no defined sampling rates or particle sizing, and poses an intrinsic difficulty in testing for multiple microorganisms with varied growth conditions. Analysis of microorganisms collected by depositional sampling is similar to impaction sample analysis. There are other simple sampling methods that may be used to supplement volumetric air sampling. Surface samples are taken by tape lift imprint, by swabbing the suspect surface with a culture swab, or by submitting a bulk sample of the suspect surface.
For tape samples, a piece of absolutely clear (not frosted) tape that is one or two inches in length is used. It is handled by the ends only. The adhesive side of the tape is positioned over the suspect area and the tape is gently and firmly pressed. Care is taken that the tape is not rubbed back and forth. The tape is removed from the surface and placed on a clean microscope slide, which is put into a slide box or into plastic bags and further processed. A direct microscopic examination is performed on surface samples. While culturing, a surface sample may help resolve a specific identification problem. Used alone, such a culture may result in an inaccurate characterisation of the surface sampled.
A direct microscopic examination of a surface shows exactly what is there, without any skewing by laboratory procedures. Surface sampling is inexpensive and (for a direct examination) may be analysed immediately. Surface sampling may also reveal indoor reservoirs of spores which have not yet become airborne. The primary purpose of a direct microscopic examination of a surface is to determine whether or not mold is growing on the surface sampled, and if so, what kinds of molds are present. Secondarily, most surfaces collect a mix of spores which are normally present in the environment. At times it is possible to note a skewing of the normal distribution of spore types, and also to note 'marker' genera which may indicate indoor mold growth.
	In addition, when mold growth is present indoors, many more spores of a particular type will be found trapped on surfaces. These spores may be ill forms which indicate recent spore release (close proximity), such as spores in chains or clumps. Marker genera are those spore types which are present normally in very small numbers, but which multiply indoors when conditions are favourable for growth. These would include cellulose digestors such as Chaetomium, Stachybotrys, and Torula.

While a single Stachybotrys spore is occasionally seen as part of the nor al outdoor flora, finding 5 or 6 of these spores on a single scotch tape slide of a duct surface is an indicator that Stachybotrys may be growing indoors.

But the presence of biological materials on a particular surface is not a direct indication of what may be in the air. Health problems related to indoor microbial growth are generally caused by the inhalation of a substantial number of airborne spores, sometimes over a substantial period of time (exceptions being, for example, situations involving small children or immuno-compromised individuals).

The various sampling methods and their characteristics are summarised in Table 10.1.

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