Proactive companies from all sectors; pharmaceutical, food and beverage, biotechnology, hospitals and environmental protection, are realizing the importance of an active air sampling program. There are several options in sampling methods, some more efficient than others.
Explore this issueJune/July 2005
Sedimentation or settle plates are the most primitive method for sampling airborne microorganisms. A Petri dish containing a suitable agar is exposed to the atmosphere and the agar medium will collect bacteria-laden particles that eventually settle by gravity. It is a passive, non-volumetric method and imprecise by over-representing larger particles due to their rapid settling rate.
This method is inefficient for collecting small particles because air turbulence around a plate can affect the results and small particles may never settle. In order to counteract this air turbulence effect, the plate has to be left out longer, which can cause desiccation. Agar drying out leads to poor bacterial growth and reduces the viable count of stress-sensitive microorganisms. Settle plates are also impossible to validate because there is no way to measure the volume of air sampled.
The impactor is essentially a jet that draws air into the sampler, which then directs the air stream at the collection agar. The different types of impactors are listed and discussed below.
In a slit-to-agar impactor, a known volume of air is drawn by vacuum through a slit opening and then accelerated and directed toward the surface of a Petri dish containing agar media. The plate rotates on a turntable at a selected rate of speed and the impacted microorganisms are separated spatially by the plate’s rotation, providing an analysis based on time. The microorganisms, because of their higher mass, become impacted on the agar surface, while the rest of the air mass flows around the plate and exits the air sampler.
While this type of impactor offers the unique benefit of time analysis, it also presents several efficiency flaws. A specific, large plate is necessary for the rotating turntable, which could conflict with evaluation methods designed for standard (90mm) Petri dishes, such as colony counters. Also, between the vacuum action and the rotating plate, the media often dehydrates; which as mentioned above, can lead to poor bacterial growth and reduce the viable count of stress sensitive microorganisms. Because air enters into the sampler device and is then directed toward the collection medium, not all particles will impact on the intended, rotating plate. Particle deposits onto surfaces other than the impaction medium can account for up to 10 percent of total particles, skewing the selected sample from a true representation of the air quality. Perhaps the greatest drawback, however, is the slit-to-agar sampler’s inefficiency of collecting smaller particles. While larger particles with sufficient inertia will deviate from the streamlines and impact on to the agar surface, smaller particles, less than .5 µm will follow the gas streamlines, missing the agar surface altogether and being exhausted back into the atmosphere.
There are several types of sieve impactors. One-stage impactors have only one perforated plate set in front of on agar plate, while stacked sieve samplers can have up to six stages of perforated plates and agar plates. In stacked sieve, or cascade samplers, each perforated plate is held above an agar plate with successive plates having smaller holes. At a constant flow, larger particles impact on the first stage, whereas smaller particles impact on the last impaction stage. The major advantage of a stacked sieve impactor is that it can provide data on particle size.
Sieve impactors are not without their own set of specific problems. Unless sampling times are short or the relative humidity high, the areas of nutrient agar directly under each hole of a stage can rapidly dry out, adversely affecting the growth of fastidious microorganisms. Using plastic dishes may cause electrostatic effect, deflecting particles away from the dish to deposit on other internal parts of the sampler. While these problems are significant, much like the slit-to-agar sampler, the major problem with sieve impactors is their inefficiency at collecting smaller particles. In order to impact smaller particles, the holes need to be small and the air flow rate accelerated, however, if the flow rate is too fast, the smaller microorganisms will be killed due to the shear force of impact.
Centrifugal samplers create a vortex in which particles with sufficient inertia leave the airstreams to be impacted upon a collection medium. Air is drawn into the sampler by an impeller housed inside an open shallow drum. The air is then accelerated by centrifugal force toward the inner wall of the drum. Lining the inner wall is a plastic strip supporting a thin layer of agar medium, onto which airborne particles are impacted. The major advantage of centrifugal samplers used to be that they were battery operated and small enough to be hand-held. However, other samplers are now comparative in portability to the centrifugal samplers.