The Contribution of Culture Based Studies of Microorganism to General Ecology

In addition to understanding the diversity, function, and interactions of microorganisms as they occur in nature, microbiology has a history of contributing to general ecology by providing simple, experimentally tractable systems to test ecological theory. The short generation times, ease of experimental manipulation, and analysis of many cultured microorganisms make them ideal for conducting experiments in evolution and ecology that would be impractical with communities of macroorganisms. The archetype for this approach is the study of competition and predator-prey interactions by Gause with cultures of Paramecium, yeast, and bacteria that ultimately led to the development of the competitive exclusion principle and the niche concept (Figure 6).

Since these seminal experiments, microbial cultures have been used to test diversity-stability relationships

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Gause Curve Bacteria

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Figure 6 The growth measured by volume of Paramecium caudatum and P. aurelia cultivated separately and in the mixed population on buffered medium supplied with a defined 'half-loop' concentration of bacteria. When grown separately both organisms show similar growth curves, but when grown together P. aurelia displaces P. caudatum as a result of competition for food. From Gause GF (1934) The Struggle for Existence. Baltimore, MD: Williams & Wilkins.

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Figure 6 The growth measured by volume of Paramecium caudatum and P. aurelia cultivated separately and in the mixed population on buffered medium supplied with a defined 'half-loop' concentration of bacteria. When grown separately both organisms show similar growth curves, but when grown together P. aurelia displaces P. caudatum as a result of competition for food. From Gause GF (1934) The Struggle for Existence. Baltimore, MD: Williams & Wilkins.

and competition between different taxa and between different genetic variants of the same taxon in the context of demographic tradeoffs. Simple microbial culture systems also provide insights on such fundamental theories as the tendency for mutualistic versus competitive and parastitic interactions in the context of kin selection; resource competition; coexistence of generalist and specialist bacteria; the role of spatial structure in dictating microbial community interactions and diversity.

Microbial cultures have also been employed in studies of evolution and adaptation showing that adaptive and genetic changes can occur remarkably quickly in a microbial population. Pseudomonas fluorescens in unshaken cultures show adaptive radiation where genetically distinct subpopulations which have distinct colony morphology exploit different niches that result from spatial structure in the simple culture system. The ecologically distinct nature of the different subpopulations has subsequently been shown in laboratory competition experiments under different growth conditions.

Longer-term evolutionary experiments have demonstrated that replicated populations of E. coli, grown for 30 000 generations in identical cultures, diverge from the parent strain and the extant cultures show increased fitness (faster growth rates) and increased cell size which can be associated with specific mutations in the derived strains. Such fundamental insights on evolution and ecology can only be achieved using simple systems based on pure cultures of microorganisms.

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