U.S. Micro-Solutions, Inc. Environmental Laboratory
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Swab - Surface Sampling Protocols
Surface samples
provide information similar to that obtained by bulk samples regarding
whether environmental materials are contaminated beyond background
levels. Often, surface sampling is preferred over
bulk sampling when a less destructive method of sample collection is
desired. Surface samples are collected by washing, rubbing, or brushing
a swab over an area.
Swab samples are suspended in liquid
and diluted as necessary. Aliquots of the sample are inoculated on appropriate agar media. The selection of agar depends on the sampling
objective and the incubation temperature of the agar depends upon the
microbe under investigation. Once the media is selected and the
incubation temperature is determined, the colonies are allowed to
grow. As they mature, they are quantified and identified to the genus
level for fungal and bacterial isolates and speciated when indicated.
The concentrations are expressed in terms of colony forming units per
swab or colony forming units per area sampled. A few guidelines when swab surface sampling:
Aseptic techniques must be strictly applied.
Do not use “Q-tips” or other cotton swabs intended for personal use. These swabs are not appropriate for environmental sampling.
Prior to collection, swabs should be moistened with sterile water or in the transport fluid in the swab container to enhance particle collection.
Swabs should be delivered to the laboratory as soon as possible and protected from temperature extremes during transport.
Note: A measured
area should always be swabbed when a culture is ordered. This way, the
lab can report in units of CFU per area. Otherwise,
the results will be difficult to interpret and will be reported as
CFU/swab, a fairly meaningless unit. An area between 1 and 4 square
inches is recommended.
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