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Microbial Growth and Culturing



Microorganisms, or microbes, require certain conditions for population growth, and these conditions determine where different microbes can survive. Microbial growth is an increase in the number of cells in a population due to cell division. These conditions must be replicated in a lab in order for the microbe population to grow in culture. Microbial growth can be measured by several direct and indirect methods. The conditions that support microbial life include the availability of nutrients, presence or absence of oxygen, temperature, salt level, and pH. Some microbes respond to their environment by protecting the interior of the cell from harmful materials in the environment, such as excess salt or highly acidic or basic conditions. Some microbes can produce structures that will protect the cell even under extreme environmental conditions that would not normally support growth. Microbial culturing techniques take into account the nutrient and environmental conditions the organism requires. Culture media can be utilized to select for specific species of microbe or to differentiate between species.

At A Glance

  • Microbial growth is an increase in the number of cells in a population due to cell division, not the physical size of the microorganisms.
  • The growth curve of bacteria is a predictable pattern consisting of the lag phase, in which bacteria grow slowly; the log phase, in which bacteria double consistently; the stationary phase, in which the doubling rate equals the death rate; and the death phase, in which the death rate is greater than the doubling rate.
  • Serial dilutions are used to count large population densities of microbes.
  • A quantity of microbes can be measured with a pour plate, spread plate, direct microscope count, and turbidity.
  • Obligate microbes have strict requirements for survival, while facultative microbes can adjust to tolerate other environmental conditions, such as pH, temperature, oxygen, salinity, and hydrostatic pressure.
  • Microbes require certain elements, such as carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur, to survive, which they can gain from their host or from inorganic sources.
  • Nutrient transport utilized by microbes can be passive, requiring no energy, or active, requiring energy.
  • Biofilms are complex communities of microbes that attach to a surface and share resources.
  • Endospores protect dormant cells from extreme environmental conditions.
  • The same physical and chemical requirements for microbial growth in the environment must be accurately mimicked in the lab for successful culturing.
  • Media used to grow microbes can be solid or liquid and are designed to provide the appropriate nutrients and moisture needed.
  • Selective media are used to enhance growth of certain microbes, and differential media provide a visual cue to separate microbial types.