MICROBIOLOGY OF AIR

Introduction:

Of all environments, air is the simplest one and it occurs in a single phase gas. Various layers can be recognized in the atmosphere up to a height of about 1000km. The layer nearest to the earth is called as troposphere. This troposphere is characterized by a heavy load of microorganisms. The atmosphere as a habitat is characterized by high light intensities, extreme temperature variations, low amount of organic matter and a scarcity of available water making it a non hospitable environment for microorganisms and generally unsuitable habitat for their growth. Nevertheless, substantial numbers of microbes are found in the lower regions of the atmosphere. The study of these microbes in air is called as Aero Microbiology. With respect to environmentalist Microbiology is the study of the composition and physiology of microbial communities in the environment i.e. the soil, water, air and sediments covering the planet. Can also include the microorganisms living on or in the animals and plants that inhabit these areas

Disease caused by air borne microbes:

Bacterial Diseases

·  Brucellosis:

     Brucella suis it is mainly an occupational disease among veterinarian, butcher and slaughter house workers.

·  Pulmonary Anthrax:

    Bacillus anthracis is the causative agent. Transmission is mainly by inhaling the dust contaminated by animal products.

·  Diseases Caused by Streptococcus Pyogenes:

     A number of diseases are caused by Streptococcus pyogenes which is mainly    transmitted through air. Diseases Caused by Streptococcus pyogenes occur in the throat, skin, and systemically. 

·  Rheumatic Fever: This is upper respiratory tract infection by S. pyogenes Characterized by inflammation and degeneration of heart valves.

·  Streptococcal Pneumonia:

    It is of major occurrence among the bacterial pneumonia. Causative agent is

Streptococcus pneumonia

·  Meningitis :

    Haemophilus influenzae causes meningitis in children between 6 weeks and 2 years of age. 

·        Diptheria:

 Diphtheria is mainly contracted by children. Infection of the tonsils, throat and nose and generalized toxemia are the symptoms. The causative agent is

Corynebacterium diphtheria

·        Tuberculosis:

Pulmonary tuberculosis is a severe respiratory disease. Loss of appetite, fatigue, weight loss, night sweats and persistent cough are some of the symptoms. Causative agent is Mycobacterium tuberculosis

·        Legionellosis:

  It is a type of branchopneumonia. Legionella pneumophila is the causative agent. It occurs in natural water. At times it enters and proliferates in cooling tower, air cooler and shower bath. Spraying and splashing of water containing pathogen may produce aerosols which are disseminated in air.

Air Borne Fungal Diseases:

 It consists of many types. They are following

·        Cryptococcosis:

    Leads to mild pneumonitis. Causative agent is the yeast Cryptococcus neoformans. It is a soil saprophyte. Infection is acquired by inhalation of soil particles containing the causative agent.

·        Blastomycosis:

 Formation of suppurative and granulomatous lesions in any part of the body.

 Blastomyces dermatitis is the causative agent. It is a soil borne fungus and     hence inhalation of soil particles containing the fungus produces the infection.

·        Coccidiodomycosis:

Infection may not be apparent but in severe cases it is fatal. Usually infection leads to self-limited influenza fever known as valley fever or desert rheumatism. Causative agent of the disease is a soil fungus, Coccidioides immitis. Inhalation of dust containing arthrospores of the fungus leads to infection.

·        Aspergillosis:

   It is an opportunistic disease of human. Causative agent is Aspergillus fumigatus. Infection occurs through inhalation of spores.

Air Borne Viral Diseases:

 Air borne viral diseases are of different types. They are following,

·        Common Cold:

 It is the most frequent of all human infections. Characteristic symptom includes running noses. Rhinovirus is the causative agent. Droplets with nose and throat discharges from infected persons are the source.

·        Influenza:

 Symptoms of influenza are nasal discharge, head ache, muscle pains, sore throat and general weakness. Causative agents are orthomyxovirus.

·        Measles:

 Measles is the most common communicable human disease mainly affecting children. Symptoms are fever, cough, and cold and red, blotchy skin rash. Causative virus is morbillivirus. Source of infection is respiratory tract secretions in the form of droplets.

·        Mumps:

 It is a communicable disease and is a common childhood disease. It is characterized by painful swelling of parotid glands and salivary glands. Mumps virus causes the disease. Droplets containing infected saliva are the main source.

·        Adeno Viral Diseases:

 Adenoviruses cause acute self-limiting respiratory and eye infections.  Adenoviruses are transmitted by airborne mode. Diseases include acute febrile pharyngitis, acute respiratory disease and adenovirus pneumonia.

SOURCES OF MICROORGANISM IN AIR:

Although a number of microorganisms are present in air, it doesn't have an indigenous flora. Air is not a natural environment for microorganisms as it doesn't contain enough moisture and nutrients to support their growth and reproduction.
Quite a number of sources have been studied in this connection and almost all of them have been found to be responsible for the air micro flora. One of the most common sources of air micro flora is the soil. Soil microorganisms when disturbed by the wind blow, liberated into the air and remain suspended there for a long period of time. Man made actions like digging or ploughing the soil may also release soil borne microbes into the air. Similarly microorganisms found in water may also be released into the air in the form of water droplets or aerosols. Splashing of water by wind action or tidal action may also produce droplets or aerosols. Air currents may bring the microorganisms from plant or animal surfaces into air. These organisms may be either commensals or plant or animal pathogens. Studies show that plant pathogenic microorganisms are spread over very long distances through air. For example, spores of Puccini a graminis travel over a thousand kilometers. However, the transmission of animal diseases is not usually important in out side air.

The main source of airborne microorganisms is human beings. Their surface flora may be shed at times and may be disseminated into the air. Similarly, the commensal as well as pathogenic flora of the upper respiratory tract and the mouth are constantly discharged into the air by activities like coughing, sneezing, talking and laughing. The microorganisms are discharged out in three different forms which are grouped on the basis of their relative size and moisture content. They are droplets, droplet nuclei and infectious dust. It was Wells, who described the formation of droplet nuclei. This initiated the studies on the significance of airborne transmission.

Droplet:

Droplets are usually formed by sneezing, coughing or talking. Each consists of saliva and mucus. Droplets may also contain hundreds of microorganisms which may be pathogenic if discharged from diseased persons. Pathogens will be mostly of respiratory tract origin. The size of the droplet determines the time period during which they can remain suspended.  Most droplets are relatively large, and they tend to settle rapidly in still air. When inhaled these droplets are trapped on the moist surfaces of the respiratory tract. Thus, the droplets containing pathogenic microorganisms may be a source of infectious disease.

Droplet Nuclei:

 Small droplets in a warm, dry atmosphere tend to evaporate rapidly and become droplet nuclei. Thus, the residue of solid material left after drying up of a droplet is known as droplet nuclei. These are small, 1-4µm, and light. They can remain suspended in air for hours or days, traveling long distances. They may serve as a continuing source of infection if the bacteria remain viable when dry. Viability is determined by a set of complex factors including, the atmospheric conditions like humidity, sunlight and temperature, the size of the particles bearing the organisms, and the degree of susceptibility or resistance of the particular microbial species to the new physical environment. If inhaled droplet nuclei tend to escape the mechanical traps of the upper respiratory tract and enter the lungs. Thus, droplet nuclei may act as more potential agents of infectious diseases than droplets.  Droplets are usually formed by sneezing, coughing and talking. Each droplet consists of saliva and mucus and each may contain thousands of microbes. It has been estimated that the number of bacteria in a single sneeze may be between 10,000 and 100,000. Small droplets in a warm, dry atmosphere are dry before they reach the floor and thus quickly become droplet nuclei.

Infectious Dust:

 Large aerosol droplets settle out rapidly from air on to various surfaces and get dried. Nasal and throat discharges from a patient can also contaminate surfaces and become dry. Disturbance of this dried material by bed making, handling a handkerchief having dried secretions or sweeping floors in the patient's room can generate dust particles which add microorganisms to the circulating air.  Most dust particles laden with microorganisms are relatively large and tend to settle rapidly. Droplets expelled during coughing, sneezing, etc consist of sativa and mucus, and each of them may contain thousands of microorganisms. Most droplets are large, and, like dust, tend to settle rapidly. Some droplets are of such size that complete evaporation. Occurs in a warm, dry climate, and before they reach the floor quickly become droplet nuclei. These are small and light, and may float about for a relatively long period. Airborne diseases are transmitted by two types of droplets, depending upon their size.

(1) Droplet infection proper applies to, droplets larger than 100 µm in diameter.

 (2) The other type may be called airborne infection, and applies to dried residues of droplets. Droplet infection remains localized and concentrated, whereas airborne infection may be carried long distances arid is dilute.   Microorganisms can survive for relatively longer periods in dust. This creates a significant hazard, especially in hospital areas. Infective dust can also be produced during laboratory practices like opening the containers of freeze dried cultures or withdrawal of cotton plugs that have dried after being wetted by culture fluids. These pose a threat to the people working in laboratories.

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MICROBES IN ATMOSPHERE:

The atmospheric layers and the airflow pattern are the important forces in determining the distribution and dynamics of viable particles in air. The aero microbiological pathway (AMP) involves the path and pattern of movement of microbial particles in atmosphere. The layer of most interest and significance in aero microbiological is the boundary layer, which extends up to 0.1km form the earth’s surface. However, that airborne transport of microorganisms is by no means limited to this layer and it is not uncommon to have microorganisms associated with layers of the troposphere above the turbulent boundary layer. However, it is the surface boundary layer that is largely responsible for the transport of particles over both short and long distances. The boundary layer consist of three parts: the laminar boundary layer, the turbulent boundary layer and the local eddy layer.

The laminar boundary layer is the layer of still air associated with the earth and all projecting surfaces, weather solid or liquid. This layer can be any where from 1 µm to several meters thick depending upon weather conditions. Still condition cause the thickness of this layer to increase and windy conditions minimize it to a very thin layer that remains in close association with surfaces. The turbulent boundary layer is the layer that is considered to be always in motion and responsible for horizontal transport phenomena (wind dispersion), which occurs whenever micro-organisms associated particles are launched either indoors or outdoors. In the lower level of turbulent layer, the linear flow of air is interrupted by surface projections and their associated laminar boundary layers. The interaction results in the formation of friction against the air flow. This friction, which is apparent in the form of local areas of “swirling: turbulence, determines rate of movement of these particles. The local eddy layer is the actual zone of interaction between the still laminar boundary layer of surface projections and the turbulent boundary layer.

Dispersal of microbes in Atmosphere:

The dispersal of microbes in air begins with the discharge of microbial cells, spores or particle loaded with viable particles (aerosol) to the atmosphere. It is followed by the subsequent transport via diffusion and dispersion of these particles and finally their deposition on any surface. An example of this pathway is that of liquid aerosols containing the influenza virus launched into the air through cough, sneeze or even through the air, inhaled and deposited in the lungs of a near by person, where they may begin a new infection. Traditionally the deposition of viable microorganisms and the resultant infection are given the most attention, but all three processes (launching, transport and deposition) are of equal importance in understanding the aerobiological pathway. While a microbial particle (hypha, cell or spore) germinate and grow, when dispersed on compatible surface, gaining the metabolic efficiency, it perishes on coming in contact with an incompatible surface.

Bioaerosol:

The bioaerosol are the atmospheric particles, mist of dust of µm range, associated with metabolically active or inactive viable particles. Bioaerosols vary considerably in size and composition depends on a variety of factors including the type of microorganism or toxin, the type of particles they are associated with such a mist or dust and gases in which the bioaerosol is suspended. Bioaerosol in general range from 0.02-100 µm in diameter and are classified on the basis of their size. The smaller the particle <0.1 µm in diameter are considered to be in the nuclei mode, those ranging from 0.1-2 µm are in the accumulation mode and the larger particles are considered to be in the coarse mode, which undergo rapid sedimentation. The particles in nuclei or accumulation ode are considered to be fine particles and have the capacity to move long distances. These particles have also a long residence time in the environment. The particle in coarse mode are considered coarse particles a they settle within few meters to few kilometers from the source. The composition of bioaerosol can be liquid or solid or the mixture of the two and should b thought of as microorganism associated with air borne particles containing microorganism. This is because it is rare to have microorganism (toxins) that are not associated with other airborne particles such as dust or mist.

Launching :

The process whereby microbes loaded particles become suspended within the earth’s atmosphere is termed launching. Because bioaerosol must be launched in to the atmosphere to be transported. The launching of bioaerosol is mainly from terrestrial and aquatic sources, with greater airborne concentrations or atmospheric loading being associated with terrestrial sources than with aquatic sources. The contribution of aerial source is considered minimum.  This phenomenon is related to the limited potential for microorganisms to reproduce with airborne. This however an area of aeromicrobiology for which there is little available information is. In addition, a significant contribution of viable particles to the atmosphere is also made from surfaces of plants and animals.

Launching in to surface boundary layers is influenced by a number of factors such as: (a) air turbulence created by the movement of humans, animals and machines; (b) the generation, storage, treatment and disposal of waste material; (c) natural mechanical processes such as the action of water and wind on contaminated solid or liquid surfaces; and (d) the release of fungal spores as a result of natural fungal life cycles. Airborne particles can be launched from either point, linear, or area sources. A point source is an isolated and well defined site of launching such as a pile of biosoild material, before it is applied over the field or an infected leaf of a plant launching the spores of a pathogen to air. Point sources tend to play general conical-type dispersion. Point sources can be  further defined on the basis of launching phenomenon: (1) instantaneous point sources, for example, a single event such as a sneeze, or (2) continuous point sources,  from which launching occurs over extended period of time, such as the biosolid pile. In contrast to point sources, linear sources and area sources involve larger, less well defined areas. When considered on same size scale, linear and area sources display more particulate wave dispersion as opposed to the conical type of dispersion displayed by point sources. Linear and area sources can also be divided into instantaneous and continuous launching points of origin.

For example, an instantaneous linear source might be a passing aircraft releasing a biological warfare agent or a passenger jet releasing the unburnt carbon particles source

Bioasrosol transport:

 Transport or dispersion is the process by which a viable particle moves from one point to another with the speed of wind or when it is launched in to air with a force. The force of airborne particle is dependent on the kinetic energy gained by it from the force at which it is launches to the atmosphere and the wind speed. Transport of bioaerosols can be defined in terms of time and distance. Submicroscale transport involves short periods of time, under 10  minutes, as well as short distances, under 100m. this type of transport is common within buildings or other confined spaces. Micro transport ranges from 10 minutes to 1 hour and from 100 m to 1 km and is most common type of transport phenomenon

Because most microorganism have limited ability to survive with suspended in atmosphere, the most common scales considered are the submicroscale and micro scale . Some macro scale transport can be global in nature importantly on pathogenic point of view like spores of wheat rust fungus.

As bioaerosol travel through time and space, different forces act upon them such as diffusion, inactivation and ultimately deposition. The relative amount of diffusion that may occur in association with particulates such as bioaerosols can be estimated by using the method of Osbert Reynolds. He said that factors associated with wind could provide an indication of the amount of turbulence associated with linear flow.

Re = velocity × dimension

    Viscosity

The limiting value for the renoylds equation is usually considered to be 2000 (for an object with 5cm diameter, the non turbulent wind speed is 2 kmlhr), with values above this number indicating turbulent conditions. The higher the value, greater the relative turbulence of airflow and micro-organism associated particles diffusion that occurs per unit time. Thus the rate of diffusion and transport is directly proportional to the value of Re.

Bioaerosol deposition:

Bioaerosol is regarded as the last step in AMB pathway. Depending on the size and the kinetic energy gained by it during launching and transport. Under standard conditions, however, the rate of deposition of a particle is directly proportional to its mass, volume and mass/volume ratio.

(a)               gravitational settling:

The main mechanism associated with deposition is the action of gravity on particles. Force acts on the particles heavier then air, pulling them down. Larger particles will have higher velocities and will settle down of the AMB pathway faster.  It should be however noted that for particles of microbiological relevance that are exposed to winds above 8 ×10³ m/hr, gravitational deposition may be negligible unless the particles cross out of the laminar flow via processes such as downward molecular diffusion or increase in density because of condensation reaction such as air deposition.

(b)   Downward molecular diffusion:

It is a randomly occurring process caused by natural air currents eddies that promote and enhance the downward movement of air borne particulate matters. Molecular diffusion is also influenced by the force of the wind and deposition rate increases with increasing wind speed and turbulence of air.

( c)  Surface impaction:

 It is a process in which particles make contact with surfaces, such as leaves, trees, wall and furniture, with impaction there is an associated loss of kinetic energy. In nature, it is rare to find flat, smooth surfaces on which wind currents are unobstructed. Thus, surface impaction is a very critical factor influencing the rate of deposition of aerosols. The impaction potential causes the the collision of a particle to the surface and facilitates its attachment to the same. However, depending on the nature of the surface of a particle can bounce after collision. Bouncing off a surface causes the particle to reenter the air current at a lower rate, which can have one of the two effects: (1) it can allow subsequent downward molecular diffusion and gravitational settling to occur, resulting in deposition on another nearby surface, or (2) it can allow the particle to escape the surface and once again reenter the air current. Studies have shown that impaction is influenced by the velocity and size of the particle as ell as the size, shape and nature of surface it is approaching.

D) rain and electrostatic deposition:

Rainfall and electrostatic charge can also affect deposition. Rainfall deposition occurs a s a condensation reaction between two particles, which combine and create a bioaerosol with a greater mass, making it to settle faster. The overall efficiency of rain deposition also depends upon the spread area of the particle plume. Larger, more diffuse plumes undergo stronger impaction than smaller, more concentrated plumes. Rain deposition is also affected by the intensity of rain fall. Electrostatic deposition also condenses bioaerosols, but it is based on electrovalent particle attraction. All particles tend to have some type of associated charge. Microorganisms typically have an overall negative charge associated with their surfaces at neutrals pH.these negatively charged particles can associate with other positively charged airborne particles, resulting in electrostatic condensation.

Outdoor aero microbiology:

In outdoor or extramural environment, the expanse of space and the presence of air turbulence are the two controlling factors in the movement of bioaerosols. Brief account of these areas are given below

Airborne crop pathogen:

Bioaerosol are of direct relevance to agriculture. Air borne microbial pathogens are responsible for a large range of important disease of crop plants. Bioaerosols contaminate the crops and thus have significant economic impact worldwide.not only in crop, but in vegetable plants like potato also, airborne pathogens are responsible for outbreaks of late blight disease .bioaerosol are also important in animal husbandry. The occurrence of foot -and –mouth disease is an example of the role of bioaerosols in the spread of air borne disease. These also transmit gastrointestinal pathogens.

Waste disposal:

A range of pathogenic microbes, viruses-bacteria, protozoa and helminthes associates with waste effluents bring about health hazard during their treatment and disposal handling. Aerosols containing pathogenic microbes are also generated during other treatment processes, such as composting and land disposal etc.

Germ warfare:

 Biological warfare has become the most dangerous hidden, inhuman weapon these days. However, such a strategy is wars in very old days. These aerosols were released into air circulation of a subway system and into the air off the coast. an accident at a biological warfare research institute in Russia caused the widespread exposure of nearby populations to genetically modified strain of Bacillus anthracis. Detection of biological warfare agents is an area that requires intensive training and sophisticated equipment to develop an advanced antibiological warfare defense.

Indoor aero microbiology:

It involves home and work place environments in which air borne microbes create major public health concerns. Microbes can survive for extended period in indoors as they have relatively less exposure to radiations. Some of the indoor environments are described as following

Private homes and office building:

Extent of bioaerosols development determines the health of any building. These include several factors that influence the formation of bioaerosols. this include the presence of air filtering systems designed and fitted in the building , the health and hygiene of the occupants, the amount of clean outdoor air circulated through the building, the type of lightning, the ambient temperature in the building and the relative humidity. In spite of all precautions some microbes may develop mechanism for survival and transmission.

Hospital and laboratories:

These two indoor environments have such potential for the aerosolisation of pathogenic microbes. Microbiological laboratories are also a breeding center for pathogenic microbes.

Space flight:

Microbes have been detected even from harsh environments. They are associated with every aspect of life even space craft. Microbes are also beneficial for us. Air purification is an example of a beneficial use of microbes in association with AMB pathway. Biological air filtration (BAF) is a method currently being investigated for use during aircraft. This method reduces more than 99 % of toluene, chlorobenzene and dichloromethane in the air stream.

Public health:

AMP pathway is used for immunization against some disease like they are currently being used for influenza vaccines. However they are not widely used because they are painful.

Bioaerosol control in laboratory:

Bioaerosol containing airborne microbes can be controlled at every point by using different mechanism which includes:

Ventilation:

 It is the most common method to check build up of airborne particles. This can be achieved by open windows or use of air conditioning and heating units that pump outside air into the room. This is cost effective and this will at least reduce the amount of microbes inside room.

Filtration:

Unidirectional air flow filtration is also simple and effective for bioaerosol control. HEPA is used for this purpose and it removes virtually all infectious particles. Bag house filtration has also become common in building

Biocidal agents:

These are used for super heating, super dehydration, ozonation and UV irradiation to eradicate the microorganisms. The most commonly used method is ultraviolet germicidal radiation (UVGI).

Isolation:

Is the enclosure of an environment through the use of positive or negative pressurized air gradients and air tight seals. Isolation chamber in TB wards in hospitals provide protection to other present inside the are air from these rooms is exhausted in to the atmosphere passing through a HEPA filter and biocidal control chamber. This system work on negative pressurized air. Positive –pressure isolation chambers, working on the opposite principle force air out of the room thus protects occupants of the room from outside contamination.

Factors affecting microbial survival in air:

Many environmental factors have been shown to influence the ability of microorganism to survive the most important of them are given below:

Atmospheric humidity:

The relative as well as the absolute humidity content of the air play a major role in the survival of the air borne microorganism. In general it has been reported that most gram-negative bacteria associated with aerosols tend to survive for longr periods at relative low humidity by regulating their metabolic activities. This tends to be opposite for gram- positive bacteria. However at 100% relative humidity, longer exposure decreases the viability vis-à-vis survival. One mechanism that explains loss of viability in association with very low relative humidity is structural change in the lipid bilayers of the cell membrane. Intracellular ionic imbalance and loss of cellular metabolites occur when the cell is exposed to unfavorable humidity level. Viruses with enveloped nucleocapsids tend to have better survival in aerosols than without.

Temperature:

Temperature is the major factor in the inactivation of microbes. High temperature promotes inactivation, mainly associated with desiccation and protein denaturation and lower temperature promotes longer survival times. When temperatue approaches freezing, however, some organisms lose viability because of formation of ice crystals on their surfaces. The metabolic activities of microbes in air show a diurnal fluctuations in proportion to temperature fluctuations.

Enumeration of Microorganisms in Air:

 There are several methods, which require special devices, designed for the enumeration of microorganisms in air. The most important ones are solid and liquid impingement devices, filtration, sedimentation, centrifugation, electrostatic precipitation, etc. However, none of these devices collects and counts all the microorganisms in the air sample tested. Some microbial cells are destroyed and some entirely pass through in all the processes. Some of the methods are described below.

Impingement in liquids: In this method, the air drawn is through a very small opening or a capillary tube and bubbled through the liquid. The organisms get trapped in the liquid medium. Aliquots of the liquid are then plated to determine its microbial content. Aliquots of the broth are then plated to determine microbial content.

Impingement on solids: In this method, the microorganisms are collected, or impinged directly on the solid surface of agar medium. Colonies develop on the medium where the organism impinges. Several devices are used, of which the settling-plate technique is the simplest, in this method the cover of the pertri dish containing an. agar medium is removed, and the agar surface is exposed to the air for several minutes. A certain number of colonies develop on incubation of the petridish.
Each colony represents particle carrying microorganisms. Since the technique does not record the volume of air actually sampled, it gives only a rough estimate. However, it does give information about the kind of microorganisms in a particular area. Techniques wherein a measured. Volume of air is sampled have also been developed. These are sieve and slit type devices. A sieve device has a large number of small holes in a metal cover, under which is located a Petri dish containing an agar medium A measured volume of air is drawn, through these small holes. Airborne particles impinge upon the agar surface. The plates are incubated and the colonies counted. In a slit device the air is drawn through a very narrow slit onto a Petri dish containing agar medium. The slit is approximately the length of the Petri dish. The Petri dish is rotated at a particular speed under the slit one complete turn is made during the sampling operation.

Centrifugation:

Air is sucked into a conical tube to create a vortex of sufficient velocity that particles are sedimented into a liquid trap at the base. In this figure air is drawn into the sampler at an angle (tangential) to the walls of the device so that it circulates around and down the wall. As it circulates decrease in the diameter of the sampling body causes a dramatic increase in the velocity of the air and subsequently on the particle’s terminal velocity. This increase in gravitational settling potential causes the particles to be trapped in the lower chamber because their ‘centrifugally increased’ mass prevents them from exiting with the return air flow.

Air-sampling methods:

Air sampling is used routinely to monitor the populations of airborne particles, and to inform the public about air quality and pollen/spore counts through public broadcasting (weather reports, etc.). It is used by major hospitals to monitor the populations of specific allergenic particles (fungal spores, etc.), so that the causes of patients' allergies can be determined. And it is used in crop pathology for disease-forecasting, so that growers can apply fungicides as and when required.

Here we will consider three major types of sampling device for detecting fungal spore loads in air:

• the rotorod sampler
• the Burkard sampler
• the Anderson sampler

The Rotorod spore sampler

The rotorod sampler (Figure C ) is a cheap, simple and portable air sampler. It consists of a U-shaped metal rod attached by a spindle to a battery-powered electric motor. The motor causes the upright arms of the metal rod to rotate at high speed. To use the sampler, the upright arms are covered with narrow strips of sticky tape, so that any spores in the air will impact onto the tapes. Then the tapes are removed and examined microscopically to identify the spores and other particles such as pollen grains in the air.One of the advantages of the rotorod sampler is that it can be used to precisely locate a source of spores of a particular fungus. The famous aerobiologist, PH Gregory, did this in the 1950s by placing rotorod samplers at different positions in a field and "homing in" on a source of spores of the fungus Pithomyces chartarum, which causes a condition known as facial eczema of sheep.

The Burkard spore trap

The Burkard spore sampler acts on the same principle as the rotorod sampler, but is used to give a continuous record of particles in the air over a period of 24 hours or up to 7 days. The apparatus (Figures J, K) consists of an air-sealed drum that contains a clockwork rotating disc (arrowhead in Fig. K) which makes a single revolution in 7 days. The surface of this disc is covered with adhesive tape, to trap spores that impact onto it. When the apparatus is assembled, air is sucked into the drum at high speed through a slit orifice (arrowhead in Fig J) by means of a motor at the base of the apparatus.

 Any particles in the air impact onto the sticky tape near the slit orifice, giving a record of the particles in the atmosphere at a specific time of day. At the end of a 7-day run, the tape is removed, cut into sections representing hourly or daily periods, then examined microscopically.

In this way, it is possible to distinguish clearly between night-released and day-released spores or other particles, and also to relate the types of particle to different weather conditions (e.g. humid or dry periods) while the apparatus was running. The Burkard spore trap is commonly used for continuous monitoring of spore or pollen loads in the air. For example, these traps are commonly sited on hospital roofs, meteorological stations, and other public buildings, and provide public information through TV and radio broadcasts.  The principle is exactly the same as in the rotorod sampler because the trapping of particles is based on impaction. The limitations also are the same: only the larger particles with sufficient mass will impact on the tapes at the air speeds generated by this type of sampler.

The Anderson sampler

The Anderson sampler (Figure L) is an ingenious device for selectively trapping different sizes of particles according to their size (momentum). This sampler consists of a stack of 8 metal sections that fit together with ring seals to form an air-tight cylinder. Each metal section has a perforated base (Figure N), and the number of perforations is the same in each section, but the size of these perforations is progressively reduced from the top of the column to the bottom. To use this sampler, open agar plates are placed between each metal section, resting on three studs (Shown as

 arrowheads in Figure N).

When fully assembled (with an open agar plate between each unit) an electric motor sucks air from the bottom of the unit, causing spore-laden air to enter at the top (arrowhead in Figure L) and to pass down through the cylinder. The path taken by this air is shown in Figure O.

One of the interesting features of the Anderson sampler is that it mimics the deposition of spores (or other airborne particles) in the human respiratory tract (Figure O). For example, relatively large fungal spores and pollen grains tend to be trapped on the mucus-covered hairs of our nostrils, where they can cause "hay fever" symptoms in sensitized individuals. Smaller particles are not trapped in the nostrils but instead are carried down into the bronchioles and alveoli. Here the air speed is very low, because the successive branching of the respiratory tract has reduced the air speed to a minimum. But spores of about 2-4 micrometers diameter can settle onto the mucosal surfaces of the alveoli. Some of these spores are important in initiating infections of the lungs.

       

However, it is important to note that the underlying mechanisms of spore deposition in the Anderson sampler are entirely different from those in the human respiratory tract - the Anderson sampler traps spores by impaction, whereas spores are deposited in human respiratory tract mainly by sedimentation.

Significance of Microorganisms in Air:

As long as microorganisms remain in the air they are of little importance. When they come to rest they may develop and become beneficial or harmful. Knowledge of the microorganisms in air is of importance in several aspects.

Food manufacture:

Microorganisms that have been transported through the air and have settled on, or in, the material are involved in various fermentation products. Productions of alcoholic beverages, vinegar, sauerkraut, ensilage, dairy products, etc., are often due to microbial activity.

Spoilage of foods and fermentation products:

Microorganisms are often troublesome in the home and in industry where foods and other fermentation products are prepared. In industrial   processes, where particular organisms are to be grown, to supply sterile air free from contaminating organisms is a considerable problem.

References:

·        http://www.sylvane.com/learning-center/iqair-pathogens.html

·        http://www.textbookofbacteriology.net/normalflora.html

·        http://www.scribd.com/doc/19079015/Air-Microbiology-2009

·     http://www.horizonpress.com/gateway/micro.html\

·     Pradipta K. mohapatra. 2008. Environmental microbiology. I.k international publications pvt. Ltd

·    http://www.uta.edu/biology/chrzanowski/classnotes/3328/Aeromicrobiology.pdf

·     sharma P D.2005. Environmental microbiology.

·     www.biology.ed.ac.uk/.../microbes/airborne.htm

·     www.microbiologyprocedure.com