Introductory Microbiology

Beneficial/ positive interactions

a. Neutralism
b. Symbiosis / mutualism
c. Protoco-operation
d. Commensalism

a. Neutralism
It is a type of neutral association, in which two microorganisms behave entirely independently. e.g., Each could utilize different nutrients without producing metabolic end products that are inhibitory. This might be transitory as the condition change in the environment, particularly the availability of nutrients, the relationship might change.

b. Symbiosis / Mutualism
It is the type of association in which two symbionts relying on one another and both are benefited by the relationship. The manner in which benefit derived varies. The living together of two or more organisms; Symbiotic association is evident in soil among several groups of organisms. Algae and fungi in lichens, bacteria residing within protozoan cells, bacteria and roots in the Rhizobium-legume symbiosis, fungi and roots in mycoorhizae. In lichens, the algae and fungi are in such an intimate physical and physiological relationship that the lichens they make are classified as distinct organism.
The alga benefits in part because of the protection afforded to it by the hyphae that envelop and protect it from environmental stresses. While the fungi gains by making use of the CO2 fixed by its photosynthetic partner. Symbiotic relationship exist between micro and macro organisms. In Rhizobium-legume symbiosis, Rhizobium fixes N2 and transferred to host, inturn Rhizobium nutrients from the host.

c. Proto co-operation
One type of mutualistic association is that involving the exchange of nutrients between two species, a phenomenon called synergism. Many microorganisms synthesize the vitamins and organic acids in excess of their nutritional requirements. Others have a requirement of one or more of these nutrient. Hence certain combinations of species will grow together but not apart when nutrient levels are very low.
As association of mutual benefit to the two species, but without the co-operation being obligatory for their existence or for their performance of some reaction.
Nutritional proto co-operation has been demonstrated in cultures. Eg: In a medium deficient in nicotinic acid and biotin, neither Proteus vulgaris nor Bacillus polymyxa will multiply as the former (B) requires nicotinic acid and the latter requires biotin. In mixed culture, in the same medium however both have grown since the partner bacterium synthesizes the missing vitamins.

d. Commensalisms
It is the type of beneficial association, in which only one species derives benefit while the other is unaffected. This occurs commonly in soil with respect to degradation of complex molecules like cellulose and lignin. One patter can attack a substrate not available to the second organism, but the decomposition results in the formation of products utilized by the second.
• The one which offer benefit is called commensalism eg: (1) Many fungi able to degrade cellulose and yield glucose and organic acids. This can serve as a which source for many bacteria and fungi, which are non cellulolytic (2) The second type of commensal association arises from the need of many microorganism for growth factors. These compounds are synthesized by many micro organism and their excertion permits the proliferation of nutritionally fastidious soil inhabitants.

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