Lesson 28. MICROBIOLOGY OF AIR

Module 7. Environmental microbiology

Lesson 28
MICROBIOLOGY OF AIR

28.1 Introduction
The earth's atmosphere is teeming with airborne microorganisms. These organisms are thought to exhibit correlations with air pollution and weather. Most airborne bacteria originate from natural sources such as the soil, lakes, oceans, animals, and humans. Many ‘unnatural’ origins are also known, such as sewage treatment, animal rendering, fermentation processes, and agricultural activities which disturb the soil. Viable airborne microorganisms are not air pollutants, but should be considered as a factor affecting air quality. Air is an unfavourable environment for microorganisms, in which they cannot grow or divide. It is merely a place which they temporarily occupy and use for movement.

There are 3 elementary limiting factors in the air
  • A lack of adequate nutrients
  • Frequent deficit of water (desiccation)
  • Solar radiation
The atmosphere can be occupied for the longest time by those forms which, due to their chemical composition or structure, are resistant to desiccation and solar radiation. They can be subdivided into the following groups:
  • Bacterial resting forms,
  • Bacterial vegetative forms which produce carotenoidal dyes or special protective layers (capsules, special structure of cell wall),
  • Spores of fungi,
  • Viruses with envelopes
28.2 Resting forms of Bacteria
Endospores are the best known resting forms. These structures evolve within cells and are covered by a thick multi-layer casing. Consequently, endospores are unusually resistant to most unfavourable environment conditions and are able to survive virtually endlessly in the conditions provided by the atmospheric air. They are only produced by some bacteria, mainly by Bacillus and Clostridium genera. Because each cell produces only one endospore, these spore forms cannot be used for reproduction.

Another type of resting form is produced by very common soil bacteria, the actinomycetes. Their special vertical, filiform cells, of the so-called air mycelium, undergo fragmentation producing numerous ball-shaped formations. Due to the fact that their production is similar to the formation of fungal, they are also called conidia. Contrary to endospores, the conidia are used for reproduction. There are also other bacterial resting forms, among others, the cysts produced by azotobacters - soil bacteria capable of molecular nitrogen assimilation.

28.3 Resistant Vegetative Cells of Bacteria
The production of carotenoidal dyes ensures cells with solar radiation protection. Carotenoids, due to the presence of numerous double bonds within a molecule (-C=C-), serve a purpose as antioxidants, because, as strong reducing agents, they are oxidizedby free radicals. Consequently, important biological macromolecules are being protected against oxidation (DNA, proteins etc.). Bacteria devoid of these dyes quickly perish due to the photodynamic effect of photooxidation. That explains why the colonies of bacteria, which settle upon open agar plates, are often colored. The ability to produce carotenoids is possessed especially by cocci and rod-shaped actinomycetes. Rod-shaped actinomycetes, e.g. Mycobacterium tuberculosis, besides being resistant to light, also demonstrate significant resistance to drying due to a high content of lipids within their cell wall. High survival rates in air are also a characteristic for the bacteria which possess a capsule, e.g. Klebsiella genus, that cause respiratory system illnesses.

28.4 Fungal Spores
Spores are special reproductive cells used for asexual reproduction. Fungi produce spores in astronomical quantities, for example the giant puffball (Calvatia gigantea) produces 20 billion spores, which get into the air and are dispersed over vast areas. A very common type of spores found in air is that of conidia.

Conidia are a type of spore formed by asexual reproduction. They form in the end-sections of vertical hyphae called conidiophores and are dispersed by wind. The spores of common mould fungi such as Penicillium and Aspergillus are examples of the above. Spore plants such as ferns, horsetails and lycopods also produce spores. Plant pollen is also a kind of spores.

28.5 Resistant Viruses
Besides cells, the air is also occupied by viruses. Among those that demonstrate the highest resistance are those with enveloped nucleocapsids, such as influenza viruses.
Among viruses without enveloped nucleocapsids, enteroviruses demonstrate a relatively high resistance.

Of course, besides the previously mentioned resistant forms, the air is also occupied by more sensitive cells and viruses, but their survival is much shorter. It is believed, that among vegetative forms, gram-positive bacteria demonstrate greater resistance than Gram negative bacteria (especially for desiccation), mainly due to the thickness of their cell wall. Viruses are usually more resistant than bacteria.

28.6 Factors Affecting Growth of Microorganism in Air

There are several factors which influence the ability of a bioaerosol to survive in air:
  • Particular resistance for a given microorganism (morphological characteristics)
  • Meteorological conditions (inter alia, air humidity, solar radiation),
  • Air pollution,
  • The length of time in air.
28.6.1 Resistance of microorganisms

It is a species dependent feature, which relies on the microorganism’s morphology and physiology.

28.6.2 Relative humidity
The content of water in air is one of the major factors determining the ability to survive. At a very low humidity and high temperature cells face dehydration, whereas high humidity may give cells protection against the solar radiation. Microorganisms react differently to humidity variations in air, but nevertheless most of them prefer high humidity. The morphology and biochemistry of cell-surrounding structures, which may change its conformation depending on the amount of water in air, are crucial. Actually, an exact mechanism of this is not known. Forms of resting spores with thick envelopes (e.g. bacterial endospores) are not particularly susceptible to humidity variations. Gram-negative bacteria and enveloped viruses (e.g. influenza virus, myxo) deal better with low air humidity which is contrary to gram-positive bacteria and non-enveloped viruses (e.g. enteroviruses) that have higher survival rates in high air humidity.

28.6.3 Temperature
Temperature can indirectly affect cells by changing the relative-air humidity (the higher the temperature, the lower the relative humidity) or a direct affect, causing, in some extreme situations, cell dehydration and protein denaturation (high temperatures) or crystallization of water contained within cells (temperatures below 0°C). Therefore, it can be concluded that low temperatures (but above 0°C) are optimal for the bioaerosol. According to some researchers the optimal temperatures are above 15°C.

28.6.4 Solar radiation

Solar radiation has a negative affect on the survival rate of the bioaerosol, both visible as well as ultraviolet (UV) and infrared radiation due to the following factors:
  • Causes mutation,
  • Leads to the formation of free radicals, which damage important macromolecules.
  • Creates a danger of dehydration.
Visible light rays of about 400-700 nm wavelength, create the so-called photodynamic effect, which produces free radicals within cells, especially compounds such as peroxy and hydroxyl radicals. These radicals demonstrate strong oxidizing activities and may cause damage to crucial macromolecules, e.g. DNA or proteins.

UV radiation has a much larger affect on cells than visible light does, especially the rays of 230-275 nm wavelengths. The mechanism of this effect is based on changes to DNA, both directly (e.g. by creating thymine dimer and consequently causing mutation), as well as indirectly, by creating free radicals as in the case of the visible light.

In addition, infrared (IR) radiation may have a negative effect upon cells contained in air - heating up and consequently dehydration.

28.6.5 Biological aerosols
Microorganisms in air occur in a form of colloidal system or the so-called bioaerosol. Every colloid is a system where, inside its dispersion medium, particles of dispersed phase occur whose size is halfway between molecules and particles visible with the naked eye. In the case of biological aerosols, it's the air (or other gases) that has the function of the dispersion medium, whereas microorganisms are its dispersed phase. However, it is quite rare to have microbes independently occurring in air. Usually, they are bound with dust particles or liquid droplets (water, saliva etc.), thus the particles of the bioaerosol often exceed microorganisms in size and may occur in two phases:
  • Dust phase (e.g. bacterial dust) or
  • Droplet phase (e.g. formed as the result of water-vapour condensation or uring sneezing).
The dust particles are usually larger than the droplets and they settle faster. The difference in their ability to penetrate the respiratory tract is dependent on the size of the particles; particles of the droplet phase can reach the alveoli, but dust particles are usually retained in the upper respiratory tract. The number of microorganisms associated with one dust particle is greater than in the droplet phase.

The average size of bioaerosols ranges from about 0.02 μm to 100 μm. The sizes of certain particles may change under the influence of outside factors (mainly humidity and temperature) or as a result of larger aggregates forming. By using size criterion, the biological aerosol can be subdivided into the following:
  • Fine particles (less than 1μm) and
  • Coarse particles (more than 1μm)
Fine particles are mainly viruses, endospores and cell fragments. They possess hygroscopic properties and make-up the so-called nucleus of condensation of water vapour. At high humidity water collects around these particles creating a droplet phase.

Then, the diameter of the particles increases. Coarse particles consist mainly of bacteria and fungi, usually associated with dust particles or with water droplets.

Biological aerosols as a human hazard source .
  • What types of dangers are connected to the presence of microorganisms in air?
  • Infectious diseases (viral, bacterial, fungal and protozoan),
  • Allergic diseases,
  • Poisoning (exotoxins, endotoxins, mycotoxins).
Bioaerosols may carry microorganisms other than those which evoke respiratory system diseases. The intestinal microorganisms contained in aerosols may, after settling down, get into the digestive system (e.g. by hands) causing various intestinal illnesses.

28.7 Infectious Airborne Diseases
The mucous membrane of the respiratory system is a specific type of a 'gateway' for most airborne pathogenic microorganisms. Susceptibility to infections is increased by dust and gaseous air-pollution, e.g. SO2 reacts with water that is present in the respiratory system, creating H2SO4, which irritates the layer of mucous. Consequently, in areas of heavy air pollution, especially during smog, there is an increased rate of respiratory diseases.

Bioaerosols may, among other things, carry microbes that penetrate organs via the respiratory system. After settling, microbes from the air may find their way onto the skin or, carried by hands, get into the digestive system (from there, carried by blood, to other systems, e.g. the nervous system). Fungi that cause skin infections, intestinal bacteria that cause digestive system diseases or nervous system attacking enteroviruses are all examples of the above.

28.7.1 Viral diseases
After penetrating the respiratory system with inhaled air, particles of viruses reproduce inside the cuticle cells of both the upper and lower respiratory system. After reproduction some of the viruses stay inside the respiratory system causing various ailments (runny nose, colds, bronchitis, pneumonia), whereas others leave the respiratory system to attack other organs (e.g. chickenpox viruses attack the skin). The most noteworthy viruses are:

Influenza (orthomyxoviruses) Influenza, measles, bronchitis, mumps and pneumonia among newborns (paramyxoviruses)
  • German measles (similar to paramyxoviruses)
  • Colds (rhinoviruses and koronaviruses)
  • Cowpox and true pox (pox type viruses)
  • Chickenpox (cold sore group of viruses)
  • Foot-and-mouth disease (picorna type viruses)
  • Meningitis, pleurodynia (enteroviruses)
  • Sore throat, pneumonia (adenoviruses)
28.7.2 Bacterial diseases

Similarly to viruses, some bacteria that find their way to the respiratory system may also cause ailments of other systems. Especially staphylococcus infections assume various clinical forms (bone marrow inflammation, skin necrosis, intestinal inflammation, pneumonia). Often, a susceptible base for development of various bacterial diseases is first prepared by viral diseases, e.g. staphylococcus pneumonia is usually preceded by a flu or mumps. Bacterial airborne diseases include:
  • Tuberculosis (Mycobacterium tuberculosis),
  • Pneumonia (Staphylococcus, Pneumococci, Streptococcus pneumoniae, less frequently chromatobars of Klebsiella pneumoniae),
  • Angina, scarlet fever, laryngitis (Streptococcus),
  • Inflammation of upper and lower respiratory system and meningitis (Haemophilus influenzae),
  • Whooping cough (chromatobars of Bordetella pertussis),
  • Diphtheria (Corynebacterium diphtheriae),
  • Legionnaires disease (chromatobars of Legionella genus, among others L. pneumophila),
  • Nocardiosis (oxygen actinomycetes of Nocardia genus).
28.7.3 Fungal diseases
Many potentially pathogenic airborne fungi or the so-called saprophytes live in soil. They usually have an ability to break down keratin (keratinolysis) - difficult to decompose proteins found in horny skin formations, e.g. human or animal hair, feathers, claws. Some of the keratinolytic fungi, the so-called dermatophytes, cause mycosis of the outer skin (dermatosis), as the break down of keratin enables them to penetrate the epidermis. Other fungi, after penetrating the respiratory system, cause deep mycosis (organ), e.g. attacking lungs. The following are examples of airborne fungi diseases:
  • Mycosis (Microsporum racemosum),
  • Deep mycosis: aspergillosis (Aspergillus fumigatus), cryptococcus (Cryptococcus neoformans).
28.7.4 Protozoan diseases
Some protozoa, which are able to produce cysts that are resistant to dehydration and solar radiation, may also infect humans by inhalation. The most common example of the above is Pneumocystis carinii which causes pneumonia. Dangers connected with pathogenic bioaerosols do not concern only human diseases. Other significant diseases are those that attack cultivated plants or farm animals. The following are examples of the above:
  • Blight - grain disease caused by Puccinia graminis, and
  • Aphthous fever - very infectious disease that attacks artiodactylous animals.
28.8 Basic Sources of Bioaerosol Emission

There are two basic sources of bioaerosol:
Natural sources: These are mainly soil and water, from which microorganisms are being lifted up by the movement of air, and from organisms such as fungi, that produce gigantic amounts of spores that are dispersed by the wind. Therefore, there are always a given number of microorganisms in the air, as a natural background. It is estimated, that the air is considered to be clean, if the concentration of bacteria and fungi cells does not exceed 1000/m3 and 3000/m3 respectively. This latter statement is only true when the concentration of microorganisms consists of saprophytic organisms, not pathogenic organisms. If the concentration of microorganisms in the air exceeds the above values, or contains microorganisms dangerous to humans, then such air is considered to be microbiologically polluted.
Human activities: From the hygienic point of view, living sources of bioaerosols related to human activity, are more important than the natural sources. The emissions from these sources are dangerous due to the following two reasons
  • They may distribute pathogenic microorganisms,
  • They often cause a high increase of microorganisms in the air, significantly exceeding the natural background.
The emission sources of biological aerosols can have a localized character (e.g. aeration tank) or a surface character (e.g. sewage-irrigated field).

The most important sources of bioaerosol emission are:
  • Agriculture and farming-food industry,
  • Sewage treatment plants,
  • Waste management.
28.9 Microbiology of Inside Air
Bacteria are microscopic organisms found in indoor environments typically come from human sources (skin and respiration) or from the outdoors. Like mold, most of the bacteria found in the air in buildings are saprobes meaning they grow on dead organic matter. As far as building envelopes are concerned the primary concern is about bacteria colonies that may grow in damp areas. Most of the bacteria are shed from human skin surfaces. It is not surprising to find hundreds of thousands of bacteria per gram of dust in carpets. As long as the bacterial types are a mixture of those listed below, there is generally no cause for concern. Bacteria may also enter with outdoor air or floodwater and grow in indoor environmental reservoirs. Common indoor reservoirs are water systems, air handling unit and wet organic material. Inadequately maintained air handling system is an important source for bacterial exposure that may lead to allergic type disease. Air handling system must be check for the contaminated water where chest tightness, cough, and fever are associated with a particular indoor environment.

28.9.1 The most abundant bacteria present include

28.9.1.1 Micrococcus sp
Micrococcus species are human shed bacteria and are caused by the normal shed of skin. It is found in areas of higher occupant density and/or inadequate ventilation. Micrococcus species are generally regarded as being harmless bacteria. Normally, these bacteria are removed through ventilation systems or cleaning procedures such as mopping or vacuuming.

28.9.1.2 Bacillus sp

Bacillus sp mainly associated with soil and dust. Appropriate temperature and moisture with deposited dust on hard surfaces allow for ideal growing conditions. Most are not serious pathogens.

28.9.1.3 Staphylococcus sp

Staphylococcus sp is an inhabitant and shed from of the skin surfaces. Among the Staphylococcus species that are commonly found indoors is Staphylococcus aureus, which is an important pathogen in hospital environments. It shouldn’t be a matter of concern unless it is the predominating colony found on air or surface samples in indoor environment.

28.9.1.4 Gram positive rod

Gram positive rod bacteria mainly associated with soil and dust. Appropriate temperature and moisture allow for ideal growing conditions on carpet, wall, furniture’s etc. Most are not serious pathogens. These bacteria can be removed by good house keeping practice and adequate ventilation systems.

28.9.1.5 Gram negative rod
These organisms are rare in indoor environments, if they found in higher concentration may be related to the bio aerosol of contaminated water or other contamination of wet/moist surfaces or materials, or possibly air handling units systems in which they are proliferating. Some Gram negative bacteria (or endotoxin extracted from their walls) have been shown to provoke symptoms of fever. Occasionally, growth in air handling units has been great enough for aerosols to be generated which contained sufficient allergenic cells to have caused the acute pneumonia like symptoms. If there has been a sewage spill or flood, then Gram negative bacteria are to be expected and such environments should be thoroughly cleaned with disinfectant.

Identification of bacteria by cultural analysis is based on morphology (e.g., spherical, rod-shaped, etc.), by staining reactions (e.g. Gram positive or negative) and by the pattern of results from a series of biochemical tests.
Last modified: Tuesday, 25 September 2012, 8:39 AM