Perhaps one of the most hostile environments for many micro-organisms is the atmosphere. Suspended in the air, the tiny microbial propagule may be subjected to desiccation, to the damaging effects of radiant energy from the sun, and the chemical activity of elemental gaseous oxygen (O2) to which it will be intimately exposed. Many micro-organisms, especially Gram-negative bacteria, do indeed die very rapidly when suspended in air and yet, although none is able to grow and multiply in the atmosphere, a significant number of microbes are able to survive and use the turbulence of the air as a means of dispersal.
The quantitative determination of the numbers of viable microbial propagules in the atmosphere is not a simple job, requiring specialized sampling equipment, but a qualitative estimate can be obtained by simply exposing a Petri dish of an appropriate medium solidified with agar to the air for a measured period of time. Such air exposure plates frequently show a diverse range of colonies including a significant number which are pigmented (Figure 2.2).
The bacterial flora can be shown to be dominated by Gram-positive rods and cocci unless there has been a very recent contamination of the air by an aerosol generated from an animal or human source, or from water.
The pigmented colonies will often be of micrococci or corynebacteria and the large white-to-cream coloured colonies will frequently be of aerobic sporeforming rods of the genus Bacillus. There may also be small raised, tough colonies of the filamentous bacteria belonging to Streptomyces or a related genus of actinomycetes.
The possession of pigments may protect micro-organisms from damage by both visible and ultraviolet radiation of sunlight and the relatively simple, thick cell walls of Gram-positive bacteria may afford protection from desiccation.
The endospores of Bacillus and the conidiospores of Streptomyces are especially resistant to the potentially damaging effects of suspension in the air. The effects of radiation and desiccation are enhanced by another phenomenon,