MICROBIOLOGY OF AIR
Introduction:
Air is the simplest one of the main source of microbes 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.
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.
2.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.
4 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.
.
5 .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.
1.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.
2.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.
3. The local eddy layer is the actual
zone of interaction between the still laminar boundary layer of surface projections and the
turbulent boundary layer.
1 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.
2 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 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.
3 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 terrestrialsources 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) nTo 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 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 ×10
3 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-Ã -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
the rotorod sampler
the Burkard sampler
the Anderson sampler
*The Rotorod spore sampler
The rotorod sampler 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 consists of an air-sealed drum that contains a clockwork rotating disc
(arrowhead in 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
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
TTo 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.
*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
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
and to pass down through the cylinder.
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 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 consider
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