List of four air-borne microorganisms:- 1. Bacteria 2. Protozoa 3. Microalgae 4. Micro-Fungi.
1. Bacteria:
Bacteria comprise a heterogeneous group varying in size from 0.3 to 10-15 µm and those bacteria that are capable of withstanding prolonged desication only can persist in air for long periods. Fortunately, such forms are saprophytic.
Among these are sporulating species of bacilli, cocci like Sarcina, and non-sporulating rods like Serratia. However, it is common to find throat and mouth inhabiting bacteria exemplified by Streptococcus, Staphylococcus, Pneumococcus, etc. in air.
Without any active dispersal mechanism the bacteria are carried in air via mechanical disturbances of particulate matter, and on clothing and surgical dressings, etc. Bacteria-laden minute droplets are continuously thrown into air by rain-splashes, breakers or sea sprays, sneezing or coughing. Indoor source of airborne bacteria are mainly the man and animals.
2. Protozoa:
Most of the protozoans are transported as cysts; the cysts range from 2-50 µm in diameter. When protozoa are not in cyst condition, they vary in shape from irregular, plasmodial to rigid flattened or ovoid.
3. Microalgae:
Some common airborne microalgae are Chlamydomonas, Chlorococcum, Gongrosira, Phormidium, Protococcus, Diatoms, Spirogyra, Oscillatoria, Chlorella, Amphora. Some cyanobacteria (former blue green algae) like Aulosira, Lyngbya, Nostoc have also been reported in air.
The microalgae are airborne mainly due to air currents even in the absence of adaptation for take off into air. A regular constituent of airborne microalgae comprises of algal cysts or other resting stages. The microalgae in the range of 1-90µm are significant in respiratory diseases and allergies as these are retained by the mucous membrane of respiratory tract.
4. Micro-Fungi:
Fungal spores are the propagative phase of micro-fungi and many of which are adapted to air transport. The density of fungal spores in still air is approximately 1.0. However, they are able to remain suspended in the atmosphere and become disseminated through the action of turbulent air movement even though they are subjected to gravity. Their fall rate is governed by Stroke’s Law.
Hyphal Fragments of micro-fungi constitute a significant population of air spora; these are primarily the conidiophores of asexual forms. Culturing of these hyphal fragments revealed that they mostly belong to Cladosporium, Alternaria, and Penicillium.
These usually inhabit leaves of many plants and are dispersed due to scrubbing action of the infected plants which occurs during air currents. Hyphal fragments constitute an important airborne micro-flora. Tilak and Bhalke (1981) have described and classified various airborne hyphal fragments of fungi.
Impact of Air-borne Microorganisms on Living Beings:
1. Crop Plant Diseases:
Majority of plant pathogens which cause diseases are air-borne; the dominant ones are bacteria, viruses, mycoplasma, and fungi. The air-borne microorganisms thus have a significant and vital impact on plant productivity and damage caused by them is enormous resulting in far reaching social effects which are difficult to assess.
The more we know about these organisms the more we would be in a position to deal with the consequences. However, one can consult a book on ‘Plant Pathology’ for details.
Some plant pathogenic fungi form slimy spores which are generally covered by a film fluid with dew or rain. When a rain drop with terminal fall velocity crashes into this suspension, several secondary droplets laden with fungal spores splash to some distances and are easily carried by wind. Most plant pathogenic bacteria produce slimy exudate. The important bacteria- dispersal-mechanism is through insect and rain-splash dispersal.
2. Human Diseases:
Air-borne microorganisms cause various dangerous diseases in human beings.
However, a chart representing airborne diseases is given below for ready reference:
Air-borne microorganisms cause two types of hypersensitivity:
1. Immediate allergic reactions and
2. Delayed allergic reactions.
The immediate allergy causing microorganisms include large fungal spores such as those of Puccinia and Alternaria spp. which would get deposited in the nose, and Cladosporium sp. which can reach the larger bronchi.
Contrary to it, the microorganisms causing delayed allergic reactions are generally smaller than 5 pm and consist of actionomycetes, Aspergillus and Penicillium. The air-borne fungal spores get dehydrated thus reducing their size and density while they are in air. On inhalation, these spores quickly absorb moisture from the saturated air of the nose, increase in their size and modify the site of their disposition inside the human body.