I) Neutralism (00): This is just the case of non interaction between component species of the environment. Lewis (1967) reported that mixed growth of the Lactobacillus species and a Streptococcus species in a chemostat culture formed individual population sizes that were same as those in divide monocultures under same growth conditions.
II) Mutualism (++): This kind of interaction takes place when both members of mixed culture derive some benefit from each other's presence in terms of increased growth rates or increased population sizes. Yeoh, Bungay and Krieg (1968) described the two membered mixed culture of Bacillus polymyxa and Proteus vulgaris grown in the carbon-limited chemostat in the simply growth medium that could not maintain growth of either population on its own. This specifies that each organism was in some way totally dependent on other population to complement its minimum growth requirements. Proteus species generates nicotinic acid that is necessary for growth of B. polymyxa, whereas Bacillus species reciprocated by excreting vitamin biotin that promotes Proteus vulgaris growth. At the point, concentration of the inhibiting protein was lowered adequately to cause the resurgence of B. polymyxa population and the whole cycle was repeated.
III) Commensalism (+0): This is the situation where one member of the community advantages from the presence of the second population that itself doesn't derive any benefit or disadvantage from activity of first organism. Commensalism is the very common interaction in nature and procedure of organism succession can be believed of as the chain of commensal relationships; growth of one population generating the particular set of conditions thus allowing the second population to develop. Commensalism can alternatively be known as metabiosis, which is the relationship when the organism causes the change in the conditions which then favors development of other species related with it. Metabiosis may be caused in any of the following ways:
(i) Supply of Nutrients: One organism may generate the substance that becomes the nutrient for another organism like the breakdown of large molecules in easily available smaller molecules like breakdown of starch by Micrococcus rouxianus to maltose for use by other micrococci or breakdown of starch by Aspergillus oryzae to glucose that are used by yeasts. The yeast Saccharomyces cerevisiae generates riboflavin (vitamin B2) that is required by Lactobacillus casei for growth. Stability of the community was verified to be because of the second interaction, namely competition for growth limiting quantities of carbon and other sources of energy, glucose.
(ii) Changes in pH: One organism may modify pH of the environment to favor growth of others e.g Leuconostoc mesenteriodes produces acid that lowers pH for Lactobacillus species to grow (in the production of yoghurt).
(iii) Changes in Electron Potential or Oxygen Tension (Eh): In cheese manufacture, Eh is decreased to approx-119mV to encourage growth of microaerophiles. Streptococcus species grow on whale meat and decreases Eh to the level which allows anaerobic clostridia to grow.
(iv) Elimination of antimicrobial factors: One organism may neutralize or devastate the antimicrobial factor that has been inhibiting growth of the second organism e.g. nisin that is synthesized by some strains of Streptococcus lactis is active against Gram positive bacteria comprising spores of Clostridium and Bacillus species. Though, some strains of Lactobacillus species are capable of inactivating nisin against the other organisms.
(v) Alternation of water activity (aw) by one organism may encourage growth of other organisms e.g. Osmophilic yeasts can grow in presence of high sugar concentrations, breakdown the sugar and lower the aw so that less tolerant organisms can grow in the solution.
IV) Amensalism (0-): This is the interaction in which growth of one population is limited by presence of the second population that is unaffected by metabolism of the inhibited population. Amensalism takes place when organisms generate antimicrobial compounds like antibiotics or colicins or through nonspecific effects like the elevation of dissolved oxygen tension or changes in PH.
Amensalism can be alternatively referred to as antagonism, in which case one organism makes the growth conditions less favorable for other organisms. Antagonism can be attained in many ways which comprise:
(i) Consumption of necessary nutrients: Here, one organism with the more rapid growth rate quickly consumes the necessary nutrients in the medium thus discouraging the others.
(ii) Accumulation of main metabolites to toxic levels: There are several metabolites which are produced up to toxic levels as end products of metabolism e.g. acids that are inhibitory both to other organism and producer. Growth slows down as acidity increases until it stops ultimately. This is clearly indicated in case of alcohol and cheese production.
(iii) Production of antibiotics: Nisin is generated by Streptococcus lactis which are found naturally in several products. Nisin is active against Gram positive bacteria, and inhibits unwanted organism such as Clostridium species.
V) Prey-predator relationship (+-): Here one organism, predator gains directly at the expense of the second organism, the prey as the prey forms absolute nutritional requirements of predator. In severe cases, the prey may go in extinction; but usually in open growth systems, continuous oscillations of two populations can be recognized with increasing stage of predator population lagging behind increase in prey population. This has been seen in the case of Dictyostelium discoidium (amoebae) feeding on Escherichia coli and the number of models have been suggested to explain population fluctuations.
VI) Competition (- -): In this case both populations are restricted either in terms of growth rate or ultimate population size by the common dependence on the external factor required for growth. Competition is extremely significant as it gives the selective mechanism for evolution. Competition as Gause seen is the basis of struggle for existence.
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