Microflora of Air


The presence of microorganisms in the air is inconstant. It ensues from many factors: the locality of the area, air physical characteristics (temperature, humidity and air movement), the degree of air pollution with industrial and agricultural wastes, air contamination from the soil and water, the amounts of rainfalls, etc. Aerosol particles (dust, smoke, water droplets) adsorb many microorganisms.

Microflora of Air
Microflora of Air
  • Air microflora is composed of a vast number of species entered there from the soil, plants, animal or human bodies. Numerous saprophytic bacteria like micrococci, sarcinae, various bacilli (e.g., B. cereus, B. subtilis) and fungi (moulds, yeasts), actinomycetes are often determined in the air.
  • The total number of microbes in the air is greatly variable in the range from single cells to many thousands per 1 m3. As an example, the air of polar regions harbors only several bacteria per 1 m3. On the contrary, in the large cities, the air might be highly polluted.
  • In the air of coniferous forests, there are only a few microbes mainly because of the production of volatile phytoncides that are the potent biocides with high antimicrobial activity. Also, the bacteria render poor growth in the air upon the oceans, upon snow-covered lands, in the high mountains, etc.
  • Actually, the air is not a favorable medium for microbial habitaion. The lack of nutrients, desiccation, the microbicidal activity of sunlight create deleterious effects against bacteria, and most of them lose their viability.
  • Nevertheless, despite the rather short time of microbial presence in the air, pathogenic microorganisms are able to infect susceptible persons. They spread by air-borne route, thereby causing outbreaks and epidemics of respiratory diseases.
  • Airflows transfer microbes by aerosol with dust particles and droplets. A patient can discharge a droplet aerosol with pathogenic bacteria into the surrounding environment within a radius of 1.0-1.5 m and even more.
  • The density of microbial aerosol is related to the viscosity of mucosal secretions produced by the respiratory tract.
  • A less viscous liquid secretion is spread in the smallest droplets (1-10 μm) and may stay suspended in the air for hours or even days. Larger droplets of 100-2000 μm in size can be expelled to distances up to 2-3 m but rapidly undergo sedimentation.
  • The causative agents of influenza, measles, rubella, and other viral acute respiratory infections; bacterial respiratory illnesses, e.g., tuberculosis, diphtheria, meningococcal infections, whooping cough, scarlet fever and many other diseases can be spread by microbial aerosol generated from sputum and other discharges after speaking, coughing, or sneezing.
  • The total amount of microbes is strictly controlled in the air of industrial sites such as manufacturing plants with their multiple production lines, especially in the fields of electronics, food industry, biotechnology, and pharmaceutical industry.
  • In the air of living rooms, the number of microbes is strongly dependent on sanitary hygienic conditions of the house.
  • In the case of poor ventilation, insufficient cleaning, or overcrowding the total microbial load of the air rises sharply.
  • The microbial contents of the air of health care facilities (hospitals, clinics, ambulatory centers) are also the subjects of strict sanitary control. For instance, in the surgical operating rooms (operation theatres) the total airborne microbial count before the operation must be less than 500 cells per 1 m3 of the air, and after the operations not more than 1000. In addition, pathogenic hemolytic staphylococci and streptococci should be not detected there.
  • For patients with severe immunosuppression (post-chemotherapy cancer patients or allograft recipients) the cleanrooms are organized, where the number of microbes is greatly reduced by air filtration.

Microbiological testing of air is performed to control the number and quality of air microflora.

The laboratory determination of airborne total microbial count comprises two main groups of methods – aspiration and sedimentation tests.

Simplest is the sedimentation method, where sterile opened Petri dishes with MPA are placed in different points of the room. After complete sedimentation of air microbes within 5-30 minutes depending on method modifications, the dishes are closed and placed for incubation into thermostat at 37oC for 24 h.

The grown colonies are counted and total microbial quantity is calculated by special formulas.

For a more accurate assessment of air microbial contents, a number of special instruments and tools is used. In the aspiration method, the air is pumped through the apparatus containing opened Petri dish with nutrient medium.

Sanitary indicator microorganisms of air comprise hemolytic and viridans streptococci and pathogenic staphylococci (S. aureus). They are tested by special microbiological methods for their identification.

For the purpose of prophylaxis of air microbial pollution, a number of protective methods is used that diminishes the number of airborne dust particles with microorganisms. The air of wards, operating theatres or laboratory rooms is decontaminated by UV-irradiation, the sputum and other discharges are disinfected, bacterial filters are installed into ventilation systems.