Air Microbiology
The atmosphere consists of a mixture of permanent gases and variable quantities of water and solid particles. Its gaseous content, vapour pressure and suspended matter are not constant in composition. Most of the water vapour is concentrated in the lower atmosphere. The amount is usually about 1.2 percent by volume, but in cold weather this quantity may fall almost to zero. Unlike the gaseous content, the suspended matter shows considerable variation.
Air is not a natural environment for the growth and reproduction of microorganisms. It does not contain the necessary amount of moisture and nutrients in a form utilizable by bacteria and other microbes. Therefore, air does not possess a flora. In other words, no organisms are indigenous to the air. Yet organisms are found in air and their presence is of considerable importance economically and to public health. Microorganisms of the air are mostly organisms of soil that have become attached to dried leaves, or dust particles.
The number of organisms in the air depends on the activity in the environment and on the amount of dust stirred up. An active environment shows a higher bacterial count than a less active one. Similarly, the count is. more in dirty rooms than clean ones. Microorganisms are thus introduced into the air by various forces.
The species vary depending on the locality. However, certain forms are quite uniformly present molds and yeart are commonly found in the air. These organisms produce spores that are able to resist unfavourable conditions for a long time. The aerobic spore forming bacteria from soil are found freuently in the air. The best known member of this group is Bacillus subtilis. The spherical, saprophytic, chromogenic organisms (Sarcinae and micrococci) are also observed.
The spores of many pathogenic fungi, plant pollens and seeds & some animal pathogens can be transmitted from continent to continent by air currents. Consequently, transportation of pathogenic microorganisms via air currents is of prime concern to epidemiologists.
Techniques for Microbiological Examination
Biological contaminants suspended in air are referred to as aerosols. Each particle of an aerosol may vary in size from less than 1 to 50, or more. The particles may consists of a single organism in the free state or mostly a clump composed of many microorganisms. Sometimes an organism is not free-floating but adhering to a dust particle. Since conditions in the atmosphere are not favourable to the existence of vegetative cells, most of the viable organisms found in air are spores. Some vegetative cells such as staphylococci, streptococci, and mycobacteria are able to resist drying. They may survive for relatively longer periods and be carried by air currents. If they are permitted to settle in a quiet atmosphere, they may again become airborne during sweeping or bed making like activities.
A large number and variety of methods have been proposed for the enumeration of organisms in air. On the basis of their mode of action, aerosol samplers may be classified as - Impingement in liquids, Impaction on solid surfaces, Filtration, Sedimentation, Centrifugation, Electrostatic precipitation and thermal precipitation.
We shall discuss impingement in liquids and solid impaction in details.
Impingement in Liquids
In this type of sampler, the air is drawn through very small openings or a capillary tube an bubbled through the liquid. The airborne particles are bacteriological analysis. Air enters at (A) and depos its its dust on the sand, (B) which is supported by a cotton plug (C) The air leaves at (D). The sand is latter washed with broth from which a plate count is made .
wetted and entrapped in the collecting medium. Since the air is bubbled through the liquid, fragmentation of clumps of organisms will occur.
Kluyver and Visser (1950) found the capillary impinger type of sampler to be the most efficient and satisfactory. The air is brought in contact with water through a capillary tube to break up any clumps of bacteria. Small amount of this water is then mixed with melted nutrient agar in a petriplate. After incubation, the colonies are counted. This device recovers at least 99 percent of the spores of B. cereus from artificially contaminated air.
Dust and microorganisms may also be collected by drawing air through a tube containing a filter of wet sand or cotton. The cotton or sand is then shaken in broth. This is one of the oldest and simplest devices.
Impaction on Solid Surfaces
Funnel device used for sampling air borne bacteria
This method is simple, portable and efficient. All testing is carried out in one operation and hence widely accepted. Samplers of this type deposit suspended particles lirectly on solid surfaces. The organisms are generally deposited on agar medium. It is then incubated until viable colonies appear and counted.
The Hollaender and Dalla Valle (1939) sampler consists of a brass container with a removable bottom. The container is fitted with an inverted glass funnel which sits approximately 1/2 in from the bottom of a petri dish. The latter is placed in the lower portion of the container before use and screwed tightly against the gasket. The inside of the funnel and rim are swabbed with alcohol before use. The air sample passes through the funnel stem, and the air-borne organisms are impinged upon the agar medium. The air is drawn by means of a pump in series with a flowmeter. A sampling rate of 28 liters per min. was found to be the most effective.
Another device of this type is the Andersen air sampler. This sampler automatically separates air particles into six aerodynamic sizes which indicate respiratory tract penetration and hazard. The viable organisms are collected on agar plates, incubated and counted. The acrodynamic size of air borne particles is important in studies concerned with lung penetration.
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| Andersen air sampler |
The Andersen sampler has 400 drilled small jets in each stage instead of one large jet. Due to this, the device is extremely sensitive and efficient.
Airborne Transmission of Pathogens
Many microbial pathogens have an airborne mode of transmission and cause infections of the respiratory tract. The infections caused by such airborne organisms tend to occur in epidemic form; attacking large number of borne organisms. people within a short time. Their incidence usually increases during the fall and winter when people are more likely to occupy crowded quarters
Droplet Nuclei Droplets
Most airborne diseases are transmitted by either infectious dust or droplet nuclei via the respiratory route. When a person with a respiratory infection sneezes and coughs, he is likely to exhale millions of microdroplets of saliva which contain varying numbers of infectious microorganisms. The moisture surrounding such droplets quickly evaporates, leaving in air the mucus-coated living bacteria which contaminate every person who inhales them.
The size of bacteria-containing droplets expelled into the air by coughing and sneezing determines the time period during which they can remain suspended and also determines their ability to be trapped on the moist surfaces of the respiratory tract during inhalation.
Air infections are said to be produced by two types of droplets, depending upon their sizes. Droplet infection proper applies to droplets larger than 0.1 mm in diameter. These droplets rapidly settle out a short distance from their source and before drying occurs. If inhaled, most become trapped in the nasal baffle and nasopharynx from where they can reach the oropharynx by downward flow of mucus.
The other type may be called airborne infection and applies to the dried residues of droplets derived from droplets less than 0.1 mm in diameter. These are the droplet nuclei. Such nuclei can remain suspended in air for hours or days, travel long distances and serve as a continuing source of infection, if the bacteria remain viable when dry. Viability is governed by (a) the atmospheric conditions such as humidity, sunlight and temperature, (b) the size of particles bearing the organisms, and (c) the degree of susceptibility or resistance of the particular microbial species to the new physical environment.
Droplet infection remains localized and concentrated whereas airbome infection may be carried long distance and is dilute.
Infectious Dust
Large aerosol droplets settle out rapidly from the air on various surfaces, such as textiles used by a patient, where they dry. Nasal and throat discharges from a patient also can contaminate surfaces and become dry. Bedmaking, handling a handkerchief having dried secretions on it or sweeping a floor in the patients room can generate dust particles which add microorganisms to the circulating air. The survival of microorganisms for relatively long periods in dust creates a significant hazard, particularly in hospital areas. Such dust particles bearing pathogenic microorganisms is termed as infectious dust. Tubercle bacilli have been isolated from the dust of sanitoria; diphtheria bacilli and hemolytic streptococci have been found in floor dust near patients or carriers harboring these organisms. (Aerosol Fine sprays producing droplets that remain suspended in air for a time).
Some of of the important airborne diseases transmitted by aerosols includes brucellosis, tuberculosis, pneumonia, pneumonic plague, influenza, mumps, diphtheria, meningitis, scarlet fever, chicken pox, Q fever, typhus fever, whooping cough and streptococcal, staphylococcal infections.