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The population of microorganisms in the biosphere remains roughly constant because the growth of microorganisms is in turn balanced by the death of these organisms. The survival of any microbial group, within a specific niche, is ultimately influenced by successful competition for nutrients and by maintenance of a pool of living cells, often composed of host cells and a consortia of different microorganisms. Consequently, understanding competition for nutritional resources within a given microenvironment is essential to understanding the growth, survival, and evolution of bacterial species (also known as physiology).

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Much of our understanding of microbial physiology has come from the study of isolated cultures grown under optimal conditions in laboratories (nutrient excess); these observations form the basis for this section. To the contrary, most microorganisms compete in the natural environment under nutritional stress. Furthermore, it should be recognized that a vacant microbial niche in the environment will soon be filled with another consortiae of bacteria. It is somewhat of a conundrum that public health procedures stress the elimination of microorganisms, yet by clearing their niche, these spaces can be filled with other bacterial species. In the end, understanding the complex interactions that ensure the survival of a specific bacterium in a microbially diverse biosphere is a matter of physiologic efficiency.

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Growth is the orderly increase in the sum of all the components of an organism. The increase in size that results when a cell takes up water or deposits lipid or polysaccharide is not true growth. Cell multiplication is a consequence of cell division of unicellular organisms, growth leads to an increase in the number of single bacteria making up a population, referred to as a culture.

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The Measurement of Microbial Concentrations

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Microbial concentrations can be measured in terms of cell concentration (the number of viable cells per unit volume of culture) or of biomass concentration (dry weight of cells per unit volume of culture). These two parameters are not always equivalent because the average dry weight of the cell varies at different stages in the history of a culture. Nor are they of equal significance: In studies of microbial genetics and the inactivation of microbes, cell concentration is the significant quantity; in studies on microbial biochemistry or nutrition, biomass concentration is the significant quantity.

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Cell Concentration

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The viable cell count (Table 4-1) is typically considered the measure of cell concentration. For most purposes, the turbidity of a culture, measured by photoelectric means, is related to the viable count in the form of a standard curve. As an alternative a rough visual estimate is sometimes possible: For example, a barely turbid suspension of Escherichia coli contains about 107 cells per milliliter, and a fairly turbid suspension contains about 108 cells per milliliter. In using turbidimetric measurements, the correlation between turbidity and viable count can vary during the growth and death of a ...

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