Harmful microbial interactions
Harmful microbial interaction is otherwise described as negative interaction or antagonistic interaction. The composition of the microf-lora microfauna of any habitat is governed by the biological balance created through interactions and associations of all individuals present present in a community. Any inhibitory effect of an organism created by any means to the other organisms is known as harmful interactions or antagonistic interaction and the phenomenon of this activity is called antagonism. Harmful interactions have three types. They are amensalism, competition and parasitism.
Amensalism is the phenomenon where one microbial species is affected by the other species, where as other species is unaffected by first one. Amensalism is accomplished by secretion of inhibitory sub-stances such as antibiotics. Certain organisms may be of great practical importance, since they often produce antibiotics or other inhibitory sub-stances, which affect the normal growth of other organisms. Antago-nistic relationships are quite common in nature. For example,
Pseudomonas aeruginosa is antagonistic towards Aspergillus terreus.
A negative association may result from competition among spe-cies for essential nutrients. In such situations the best adapted micro-bial species will predominate or eliminate other species which are de-pendent upon the same limited nutrient substance.
Parasitism is defined as a relationship between organisms in which one organism lives in or on another organism. The parasites feed on the cells, tissues or fluids of another organisms, the host, which is harmed in this process. The parasite depends on the host and lives in intimate physical and metabolic contact with the host. All types of plants and animals are susceptible to attack by microbial parasites.
The beneficial interactions such as symbiosis (mutualism), proto cooperation, and commensalism are found to operate among the soil inhabitants.
Mutualism is an example of symbiotic relationship in which each organism benefits from the association. One type of mutualistic asso-ciation is that involving the exchange of nutrients, between two species, a phenomenon called syntrophisms. Many microorganisms synthesis vitamins and aminoacids in excess of their nutritional requirements. Others have a requirement for one or more of these nutrients.
Symbiosis is an obligatory relationship between two populations that benefit both the populations. Both populations are living together for mutual benefit. The relationship between algae and fungi that result in the formation of lichens is a classical example of mutualistic intermicrobial relationship. Lichens are composed of primary producer, the phycosymbiont (algae) and a consumer the mycosymbiont (fungus).
It is an association of mutual benefit between two populations, but not obligatory and only complementary. Both population are capable of surviving in their natural environment on their own, although when offered, the association offers some mutual advantages.
For an example, the mixed culture of Proteus vulgaris (pro-duce biotin/requiring nicotinic acid) and Bacillus polymyxa (produce nicotinic acid/ requiring biotin). Both grow as partner bacterium syn-thesizes the missing vitamin.
In a commercial relationship between two microbial populations, one population is benefited and other population remains unaffected. Commensalism is a unidirectional relationship between two popula-tions. The unaffected population does not benefit by the action of sec-ond population. For receiving population, the benefit provided may be essential.
In commensalism, the unaffected population modifies the habitat in such a way that another population benefits. For example, a popu-lation of facultative anaerobes utilizes oxygen and creates a habitat suit-able for the growth of anaerobes. In soil, vitamin and growth factors producing organisms benefit vitamin and growth factors requiring organisms.
The region which is adjacent to the root system is called rhizosphere. The microbial population on and around roots system considerably higher than that of root-free soil or non-rhizosphere soil. This may be due to the availability of nutrients from plant roots in the form of root nodules, secretions, lysates mucigel and sloughed off cells. Plant roots provide shelter to soil microbes in the rhizoplane (root surface) and endorhizosphere (inside root).
Bacteria predominate in rhizosphere soil and their growth is in-fluenced by nutritional substances released from the plant tissues eg. aminoacids, vitamins and other nutrients; the growth of the plant is in-fluenced by the products of microbial metabolism that are released into the soil. It has been reported that aminoacid - requiring bacteria exist in the rhizosphere in larger numbers than in the root-free soil. It has been demonstrated that the microflora of the rhizosphere is more active physiologically than that of non-rhizosphere soil. The rhizosphere effect improves the physiological conditions of the plant and ultimately result in higher yield. Greater rhizosphere effect is seen with bacteria (R:S ratio ranging from 10-20 times more) than with actinomycetes or fungi.
The Dutch Microbiologist Ruinen coined the term phyllosphere. The leaf surface has been termed as phylloplane and the zone on leaves inhabited by the microorganisms as phyllosphere. In forest vegetation, thick microbial epiphytic associations exist on leaves. The dominant and useful microorganisms on the leaf surfaces in the forest, vegetation happened to be nitrogen fixing bacteria such as Beijerinckia and Azotobacter. Apart from these nitrogen fixing bacteria, other generasuch as Pseudomonas, Pseudobacterium, Phytomonoas are also encountered on the leaf surface. The quantity and quality of phyllosphere organisms vary with the plant species and its morphological, physiological and environmental factors. The age of plant, its leaf spread, morphology and maturity level and the atmospheric factors greatly influence the phyllosphere microflora.
The region, which is adjacent to the seed surface is termed as spermosphere. Healthy seeds carry specific bacterial flora in respect of number and species. There are several reports in the litera-ture on the quantity and quality of microorganisms carried by the seeds of different plant species both externally and internally. Many of the organisms are harmless some may be positively beneficial and very few may be pathogenic under certain favourable conditions. It has been shown that some organisms have beneficial effects on the germinating seed through some biological products, such as growth hormones. It has been reported that the germinating seed excretes some chemicals, which influence the quality and quantity of the microorganisms on the seed. Picci defined the region of such influence around the seed as spermosphere and the phenomenon as spermosphere effect. When the seed is sown in soil, certain interactions between the seed-borne microflora and the soil microorganisms take place, under the influence of chemicals exuded by the germinating seed.
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