Bioaerosol Chapter Introduction

In the origins of microbiology at the end of the last century and the first half of this century, sophisticated procedures were developed for the classification of micro-organisms. This was largely as a result of recent discoveries demonstrating that micro-organisms are an important cause of human disease. Identification of micro-organisms, in particular bacteria, were principally based on the techniques of culturing using solid and streak plate or pour plate methods, from which the organisms were isolated and identified. Early taxonomists relied mainly on cell size, cell shape, the form of colonies, the growth in various types of broth, histological staining and pathogenicity. Hundreds of such identification methods are now described in the literature. Examples of a few general texts are given in the references. Over the last forty years, it has become generally accepted that the air is an important route for disease transmission. This has led to many advances in the measurement of airborne viable organisms. The majority of sampling techniques are still concerned with counting and identifying bacteria, although methods have been developed specifically for viruses, fungi and yeast as well. More recently problems such as hay fever and asthma have led to further interest in counting and classifying pollen grains, fungal spores and other biological allergenic materials. It is clearly important to sample airborne biogenic material in places such as hospitals, near sewage works, around or inside industrial plants and military establishments, in clean rooms, and in areas where people or their livestock and crops are concentrated (see chapters 12 and 13).

Recent improvements in electronics and immunoassay techniques have lead to the development of fully computer software driven image analysis systems, automated colony counters, and the introduction of fluorescent and specific antibody stains. The latter have proved valuable in the development of rapid bacterial detection systems, such as those using flow cytometers (see also Chapters 12 and 13).

The main methods of bioaerosol sampling are based on impingement in liquids, impaction on solid surfaces, sedimentation, filtration, electrostatic precipitation and thermal precipitation (see Chapters 9 and 10). Frequently, particularly with bacteria or fungi, the collected organisms are isolated in culture medium or on plates. Other chapters within this book are concerned with these methods of bioaerosol sampling and any subsequent culturing of viable particles. This chapter, although mentioning them, will concentrate on the physical methods of counting, morphometry classification (size and shape), and identification techniques presently available (see also chapter 12).

The majority of viable particles within the atmosphere are spores of fungi, myxomycetes, bryophytes and pteridophytes, as well as pollen grains of flowering plants, moss gemmae, propagules of lichen, cells of algae, vegetative cells and endospores of bacteria, cysts of protozoans and virus particles. Particles of biological origin usually vary in size from below 1 um to approximately 50 um or larger. Viruses typically range from 0.005 to 0.05 um, while bacterial cells and spores typically range from 0.2 to 30 um in length. Pollens and plant spores are generally larger with diameters between 10 and 100 um.

Assuming a density of 1.0 g/cm-3, the settling rate of a 0.1 um diameter sphere in still air is 0.3 cm/h, a 1.0 um sphere is 13 cm/h, and a 40 um sphere is 300 cm/min, according to Stokes law. Thus large airborne particles stay airborne for only a short period and are removed by sedimentation, although they may be resuspended by wind or physical disturbance. Submicron particles will stay airborne for days, and are generally removed by rain, diffusion to surfaces or by coagulation with other particles.

Biological particles in the air may consist of single or unattached organisms or may occur in the form of clumps composed of a number of bacteria. The organism may also adhere to dust particles or exist as a free floating particle surrounded by a film of dried organic or inorganic material. Organisms can be associated with liquid droplets, e.g. as splash drops from sewage processing. Some microbes become airborne while in an actively metabolising phase. Vegetative cells are important to health, as they include the primary etiological agents of communicable diseases. However, more commonly bioaerosols contain mostly spores, which are hardier, metabolically less active and often better adapted to dispersal. Many vegetative cells do not ordinarily survive very long in air unless the relative humidity is high and other factors are favourable (see Chapter 6). However some pathogens, such as staphylococci, streptococcus and the tubercle bacillus, will survive for relatively long periods and may be carried considerable distances while still viable, or they may settle on surfaces and be resuspended as an aerosol during activities such as sweeping and bedmaking.