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Microbial Growth and Culturing

Microbe Growth

Cell Division

Microbial growth is an increase in the number of cells in a population due to cell division, not the physical size of the microorganisms.

Unlike the growth of plants and animals, which is characterized by an increasing size, the growth of microorganisms (microbes) is characterized by an increase in population size. Microbes grow by increasing the number of cells within the population. The generation time is the amount of time it takes for a population to double in size. Generation times vary among microbes. For example, generation times of bacteria can range from minutes to almost a day.

Some microbes have short generation times, such as E. coli, a common cause of food poisoning, which has a generation time of 20 minutes. Some have long generation times, such as Mycobacterium tuberculosis, the causative agent of tuberculosis, which has a generation time of 15–20 hours. During this time, each bacterium within the population undergoes one round of cell division, in which one bacterium becomes two. Generation time can be measured by monitoring population number over time and determining when the population doubles. In many microbes, generation time can be affected by environmental conditions.

Microbial cell division takes place by one of two major mechanisms: binary fusion and budding. Binary fission is the process by which cells divide asexually. Under normal conditions bacteria and archaea divide by binary fission. Some algae, fungi, and protozoans also divide by binary fission. In binary fission the DNA replicates first. Then the copies of DNA move toward opposite sides of the cell. Finally, the cell membrane divides at the center of the cell, splitting it into two. These two cells are identical to the mother cell from which they were made and are referred to as daughter cells.

There are three general types of binary fission that differ based on the axis of the cell division takes place on. Most bacteria use transverse binary fission where the cell division occurs along the transverse, or short, axis of the cell. In longitudinal fission, the cell division occurs along the the long axis. In oblique fission, the division occurs along an oblique axis. Many eukaryotic organisms that reproduce by binary fission will produce multiple nuclei and then divide the single multinucleate cell into several daughter cells rather than two. There are protozoa that have even more complex binary fission mechanism that involve multinucleate parent and daughter cells.

Binary Fission

Binary fission is the process by which cells reproduce asexually. The cell's DNA is replicated, and each copy migrates to the opposite poles of the cell. The cell membrane then divides at the center of the cell, splitting it into two. The resulting daughter cells are genetically identical to the parent cell.
Another method by which some microbes divide is budding. Budding is a type of cell division in which the daughter cell develops from a bud growing off the mother cell. Instead of one cell dividing equally into two, a small bud-like projection grows progressively off of the mother cell while the cell's DNA simultaneously duplicates until the bud is large enough to split off of the mother cell and exist as its own cell. The mother cell is able to undergo the budding process multiple times and continue producing new daughter cells. The cell that has developed from the bud is identical to the mother cell. Budding is another form of asexual reproduction, as the offspring is genetically identical to the parent cell. Yeast, unicellular fungi, are the most common microbe to replicate by budding, but some bacteria are also able to replicate by this mechanism. Most bacteria that replicate by budding live in aquatic environments. Certain protozoans and apicomplexans, parasitic protists, also reproduce by budding.
Budding is a type of asexual reproduction in which the daughter cell develops from a bud growing off the mother cell. (scanning electron microscope)
Credit: CDC/Dr. Libero Ajello
The copying mechanism of asexual organisms ensures that the genome persists without being changed throughout generations. This is in contrast to sexual reproduction. Many fungi, algae, protozoans, and helminths reproduce sexually in a process that combines the genomes of two organisms to produce the daughter generation. Compared to sexual reproduction, asexual reproduction uses less cellular energy, does not require a compatible mate in close proximity, and reduces generation times. A major disadvantage of asexual reproduction is the lack of genetic recombination. Over evolutionary time, asexual organisms have less capability to adapt to changing conditions.

Phases of Bacterial Growth

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.

There are four phases of the bacterial growth cycle, each with specific characteristics regarding how the bacteria will behave. The four phases are lag, log, stationary, and death.

The lag phase is the initial stage of bacterial growth, where the number of bacterial cells does not change as the individual cells adjust to new environmental conditions. During this phase the bacteria increase in size, generating the cellular components required for binary fission, but they do not divide. This phase allows the bacteria to prepare for the cell division process. The bacteria replicate cell components such as organelles, proteins, and DNA that will eventually be needed by the daughter cells.

The log phase (exponential phase) is the second stage of bacterial growth, where the population is growing exponentially at its optimal growth rate. During this phase the bacteria are undergoing rapid cell division. Bacteria in the log phase will replicate exponentially, doubling in number according to their generation time. Eventually, the log phase stops, and the stationary phase begins.

The stationary phase is the third stage of bacterial growth, where the number of bacteria produced by cell division is equal to the number of bacteria dying. The stationary phase begins when environmental conditions such as pH, nutrient availability, or oxygen levels are unavailable for continued rapid cell division. In this phase the rate of cell division is equal to the death rate.

The death phase is the final stage of bacterial growth, where population size decreases. The death phase begins when the nutritional value of the environment has been depleted to no longer support bacteria life. These four phases are plotted on a growth curve. Growth curves are specific to each different type of bacteria and represent the amount of time that a population of bacteria spends in each phase.

Bacterial Growth Curve

Bacterial growth occurs in four distinct phases over time. In the lag phase, the number of bacterial cells does not change as the bacteria adjust to new environmental conditions. In the log phase, the bacterial population grows exponentially at its optimal growth rate. In the stationary phase, bacterial growth and bacterial death reach an equilibrium. In the death phase, the environment can no longer support bacterial life, and the population begins to decrease.
Bacteria grown in culture may progress through different stages of the cell cycle at the different times, or bacteria may progress through the cell cycle at the same time as one another. Asynchronous growth refers to bacteria that are at different points in the cell cycle while being in the same culture. For example, some cells may be in the process of replicating DNA while others are actively dividing in two. Most cultures in the laboratory—and microbial populations in the environment—experience asynchronous growth. If the bacteria are progressing through the cell cycle simultaneously, this is referred to as synchronous growth.

Synchronous culturing techniques are used for the study of the cell cycle and cell growth. Bacteria can be treated with chemicals that temporarily pause cell division at a specific step. Once this chemical has been removed, all bacteria will start at the same stage of cell division. This results in synchronous division. External conditions, such as light, temperature, and nutrients, can also be manipulated so that all bacteria synchronize to the same cell cycle stage.