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Introduction to Bioaerosols - Airborne Micro-organisms
|Bioaerosols are airborne particles, solid or liquid. They can be large molecules or volatile compounds. They contain living organisms. They will vary in size from a fraction of a micron to around 100 microns. As with inert "dust" particles all bioaerosols are governed by the laws of gravity, and will be affected by air movements being transported by turbulence and diffusion.
Air will often contain micro-organisms such as viruses, bacteria, and fungi. None of these actually live in the air, the atmosphere tends to kill off most of them. However they are frequently transported attached to other particles, such as skin flakes, soil, dust, or dried residues from water droplets. Aggregation of cells into clumps can enhance the survival whilst airborne. Bacterial cells when they become airborne normally rapidly die within a few seconds, due to evaporation of water associated with the particle. Thus with higher humidity higher bioaerosol levels can prevail. Airborne fungal cells (yeasts, moulds, spores) can remain viable for much longer periods, even at low relative humidity and high or low temperature extremes.
Sources of Bioaerosols:
Outdoors areas: Wind action on soil, agitation of open water, and raindrop impaction are major sources of bioaerosols. Farming of land, and wastewater/sewage treatment are also significant outdoor sources. Other farming activities, cattle, swine animal houses will generate bioaerosols. Food processing plants, particularly of dairy products can generate higher levels of bioaerosols.
Indoor areas: Many indoor areas are associated with bioaerosol problems. In all food processing plants, hygiene requires that levels of airborne micro-organisms are kept as low as possible. Hospitals and health care facilities are not only sources of a variety of organisms, but require that patients are not exposed to any of them. The presence of undesirable bioaerosols is often associated with sick building syndrome, being one of a number of factors which contribute to building related illness.
|Monitoring for Bioaerosols
Although the use of simple settle plates can be used for collection of bacteria and fungal spores, it can never give a quantitative determination. This passive technique will also fail to enumerate very small particles such as bacteria, which will remain suspended.The simplest quantitative method of monitoring is to use impact samplers such as the MicroBio MB1 or MB2 units. These are single stage impactors, which collect bacteria and fungal spores from air flowing at 100 litres/minute through a series of 1mm diameter air inlets, onto an agar filled 55mm contact plate. The units sample a fixed volume of air , up to 1000 litres ( one cubic metre). The MicroBio samplers are lightweight, battery powered self contained units, and do not require an external vacuum pump.
The agar media used in the 55mm contact plate should be chosen to suit then organsims which are being monitored. For a wide range of bacteria use tryptic soy agar (TSA), casein soy peptone agar (CPSA), and nutrient agar (NA). There are other selective agars for more specific micro-organisms. For fungi (yeasts and moulds) use is made of malt extract agar (MEA) or rose bengal agar (RBA). After sampling with the MicroBio Samplers the agar contact plates are incubated for specified times and temperatures (typically 1-2 days, at 25 37 deg.C) and the colonies which develop are counted. A correction is applied to the colony count to allow for the possibility that two organisms going through one sampling hole will result in only one colony growth being observed (positive hole correction). This is determined from tables or using the MicroBio PC Reporter software supplied with all MicroBio Samplers. From the corrected count, and the sampling volume used, the number of colony forming units per cubic metre (cfu/m3) can be determined.
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