Sources of Microorganisms in Air

Aero microbiology is the study of intramural or indoor and extramural or outdoor microorganisms in air.  In other words, aero microbiology deals with the distribution, transmission and existence of microorganisms like bacteria, fungi, viruses, yeast and protozoans in air. Air is not a natural environment for microorganisms.  The physical and chemical parameters prevailing in air do not support the growth and reproduction of microorganisms. Microorganisms in air are exposed to sunlight, UV radiation, desiccation, less nutrients etc which will affect the existence of microorganisms. Hence the amount of microorganisms in air is less than soil and water.  

Vertical distribution of microorganisms in air is controlled by air currents, wind flow etc. whereas their horizontal distribution is affected by various physical and chemical factors.  The distribution of microorganisms considerably reduces as the altitude increases. At Higher altitudes, microorganisms are exposed to decrease in temperature, less oxygen content, low atmospheric pressure, low water availability, less organic carbon etc. which will limit their distribution. These conditions limit the distribution of microorganisms above the troposphere layer of Earth. Only resistant spores are found above the troposphere layer, but in low concentrations. 

Microorganisms present in air are liberated from various other sources. These various sources include soil, water, plant and animal surfaces and human beings.  Microorganisms remain in the air for a varying time period depending upon speed of air currents, size of particles to which they are attached and humidity of the atmosphere. In still air microbes settle easily whereas a gentle air current can keep the microbes suspended indefinitely in air. Also organisms attached to dust particles or droplets settle out faster than the free organisms which are only slightly heavier than the air. A humid atmosphere contains less amount of organisms than a dry one as organisms are carried down by the droplets of moisture.  This explains why the microbial load is more in summer than in winter.

 Sources of microorganisms in air include

1.      Soil

2.      Water

3.      Plant and animal surfaces

4.      Human beings

1.   Soil

Soil is the most common source of microorganisms in air. From soil microorganisms are liberated to air by various environmental activities as well as by human activities. Environmental activities like wind blow, air currents liberate soil microorganisms into air and these organisms will remain in the air suspended for longer periods. Human activities like digging, ploughing etc. will also liberate microorganisms from soil to air. An active soil environment liberates more microorganisms to air than less active soil environment.  Also air above rich, fertile and cultivated soil shows a higher viable count than sandy and uncultivated soil.  Similarly, soil covered with vegetation liberates less amount of microorganisms into air than bare surfaces as bare surfaces can be easily acted upon by wind and air currents.

2.  Water

Microorganisms are liberated from water into air as droplets or aerosols.  Here also environmental activities like splashing of water by wind and tidal action as well as human activities like swimming, water sports etc liberate microorganisms into air. 

3.  Plant and Animal Surfaces

Microorganisms on plant and animal surfaces are liberated to air. These organisms can be commensals or pathogens.   70% of plant diseases are transmitted through air. Plant pathogens spread over long distances. Eg: Spores of Puccinia graminis.  Animal diseases are less frequently transmitted through air.

4. Human beings

Human beings are the main source of microorganisms in the air. Surface flora of the human body is shed at intervals. In addition to that human beings also produce bioaerosols which may contain commensals as well as pathogenic microflora of mouth and upper respiratory tract by the activities like coughing, sneezing, talking, laughing, singing etc. Particles suspended in air are called aerosols. Bioaerosols contain biological contaminants like pathogenic bacteria, virus, microbial toxins etc. which on ingestion or inhalation cause infectious diseases in human beings.  Bioaerosols vary considerably in their size and composition. Their size ranges between 0.02  to    100 μm.  Based on size bioaerosols are divided into droplets, droplet nuclei and infectious dust. Composition of bioaerosols depends on the type of microorganisms or toxins they are attached with and also the type of particles they are attached to like mucus, dust etc. 

4.1. Droplets

Droplets are formed by human activities like coughing, sneezing, talking, laughing etc and also during disease diagnosis procedures like suctioning and bronchoscopy. Droplets consist of saliva or mucus, epithelial cells, cells of the immune system and various microorganisms. Hundreds of microorganisms can be seen in such droplets and these organisms can be pathogenic, if discharged from infected persons.  Usually pathogens of the respiratory tract are liberated as droplets. The size of the droplet determines its period of suspension in air.  Droplets are usually having a larger size, greater than 10μm or more and hence, they will settle rapidly in still air.True aerosolization does not occur in the case of droplets. They travel less than 1m through the air and are immediately deposited on the nasal or oral mucosa of the new host or in their immediate environment. If they are inhaled, they are usually trapped on the moist surfaces of the respiratory tract and cause the upper respiratory tract infections. They cannot move to the lower parts of the respiratory tract because of their size and hence cannot cause lower respiratory tract infections.

4.2. Droplet nuclei

Droplet nuclei are airborne particles originating from droplets by the evaporation of large droplets. Droplets in a warm and dry atmosphere evaporate rapidly and the solid material left after drying up of the droplet is called droplet nuclei. They are less than 5 μm in size (usually between 1-4μm) and contain microorganisms, dust particles, skin cells and other debris. Here aerosolization takes place and droplet nuclei remain suspended in the air for longer periods of time. Droplet nuclei are widely dispersed by air currents, remain for hours or days and are inhaled by susceptible hosts. Once inhaled, they can escape the mechanical traps of the upper respiratory tract and enter the lungs to cause lower respiratory tract infections.  They are more potential agents of infection than droplets and play an important role in  transmission of airborne diseases, particularly respiratory infections. The role of droplet nuclei in transmission of  airborne diseases was first studied by Wells in 1955. 

To cause an infection, microorganisms present in droplet nuclei should be viable.  Viability of organisms in droplet nuclei is determined by atmospheric conditions like humidity, sunlight, temperature, also by the size of particles bearing the organisms and degree of susceptibility or resistance of microbial species to the new physical environment. 

4.3. Infectious dust

Large droplets settle out rapidly from air onto various surfaces including cloths, floor, wall, table tops and other exposed surfaces and get dried. These droplets may include nasal and throat discharges of patients containing infectious pathogens.   Disturbance of these dried materials during bed making, sweeping of the floor, handling of contaminated handkerchief etc. will liberate infectious dust into the air. These infectious dust remain suspended in air for longer periods of time and are a serious hazard in hospital areas. Also laboratory practices like opening of frozen bacterial cultures or the cotton plugs dried after being wetted by the culture broth will liberate infectious dust in laboratories. 

View my Video: " Sources of Microorganisms in Air"

Youtube Link: https://www.youtube.com/watch?v=dlzdUd_yLq0&feature=youtu.be