THE MICROBIOLOGY OF THE ATMOSPHERE 



The death-rate of a microbial pure culture normally proceeds expon- 

 entially with time, the same fraction of surviving individuals dying in 

 each successive equal time-interval — a process analogous to radioactive 

 decay. But natural populations are often heterogeneous, and these mixtures 

 of individuals or species with different propensities for hfe may deviate 

 from the exponential die-away curve. Measurements of survival-time are 

 often expressed as the time taken for most or all of the organisms to die 

 under a particular set of conditions. Yarwood & Sylvester (1959) rightly 

 point out that this limit is difficult to measure accurately, and that a more 

 useful concept is the half-life of a population (as already used for decay of 

 radioactivity). Apart from being easier to measure accurately than the 

 end-point, the half-life is more logical than an arbitrarily selected value 

 such as 90 per cent or 99 per cent death of the population, because it is 

 the time at which all the individuals in a population which were alive 

 at the start have an equal chance of being alive or dead. As an example, 

 Yarwood & Sylvester give the half-life of basidiospores of Cronartium 

 ribicola as 5 hours. It is not always easy to determine the status of a par- 

 ticular spore or cell. If it can be grown it is clearly viable, but failure to 

 grow may merely reflect failure to provide suitable conditions. 



Standard texts on microbial physiology deal with the effects of external 

 conditions on the longevity of micro-organisms. Most of the experimental 

 work is in the laboratory, with organisms in a liquid or at a solid/gas 

 interface, and it is evident that the main factors of the aerial environment 

 which affect survival (not always in the direction expected) are : humidity, 

 temperature, and the visible or ultra-violet radiation. For instance, in 

 laboratory tests, Whisler (1940) found the common airborne Sarcina 

 lutea to be one hundred times more resistant to ultra-violet radiation than 

 was the intestinal Escherichia coli. 



At first sight, conditions in the atmosphere might appear to be very 

 unfavourable for the survival of isolated microbial cells (or even of resting 

 spores) which, because of their minute size, have a high surface/volume 

 ratio giving great exposure to external conditions. Endospores of bacteria 

 are highly resistant to unfavourable environments; but this is not true 

 of all plant spores, many of which are more delicate structures than the 

 parent. 



Desiccation is a hazard already mentioned; it is greatest in day-time 

 and in air-layers near the ground. At higher altitudes, and throughout 

 the atmosphere at night, conditions are less favourable for evaporation, 

 and spores may even be found germinating in clouds — a phenomenon 

 occasionally reported for the uredospores of rust fungi. We are still not 

 clear how to relate meteorological observations to conditions for viability. 

 Evaporation in still air is a function of the absolute dr^Tiess of the air, 

 but in moving air evaporation may be more nearly related to relative 

 humidity. Possibly it is best to regard an airborne spore as 'still' in relation 

 to the air in which it is suspended. A complicating factor is that, because of 



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