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The population of microorganisms in the biosphere remains roughly constant because the growth of microorganisms is balanced by the death of these organisms. The survival of any microbial group within an environmental niche is ultimately influenced by successful competition for nutrients and by maintenance of a pool of all living cells, often composed of human cells and a consortium of different microorganisms (referred to as the microbiome or microbiota). Understanding competition for nutritional resources within a given microenvironment is essential to understanding the growth, survival, and death of bacterial species (also known as physiology).

Much of our understanding of microbial physiology has come from the study of isolated cultures grown under optimal conditions in laboratories (nutrient excess). However, most microorganisms compete in the natural environment under nutritional stress. Furthermore, a vacant environmental microbial niche will soon be filled with a different microbiota composition. In the end, appreciating the complex interactions that ensure the survival of a specific microbiome is a balance between availability of nutrients and physiologic efficiency.


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 binary fission that leads to an increase in the number of single bacteria making up a population, referred to as a culture.

The Measurement of Microbial Concentrations

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 of a culture. Nor are they of equal significance: For example, 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.

A. Viable Cell Count

The viable cell count (Table 4-1) is typically considered the measure of cell concentration. For this, a 1-mL volume is removed from a bacterial suspension and serially diluted 10-fold followed by plating 0.1-mL aliquots on an agar medium. Each single invisible bacterium (or clump of bacteria) will grow into a visible colony that can be counted (see Chapter 5). For statistical purposes, plates containing between 30 and 300 colonies give the most accurate data. The plate count × the dilution × 10 will give the number of colony forming units (CFU)/mL in the undiluted bacterial suspension. Using this method, dead bacteria within the suspension do not contribute to the final bacterial count.

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