What is Growth?
In microbiology, an increase in the number of cells is nothing but growth.
The growth of micro-organisms may Halophiles are a group of extremophiles that require high salt be defined as development in cellular components or constituents. It leads to a rise m cell number when microorganisms reproduce by processes like budding or binary fission.
Finally, individual cells multiply and divide to produce two identical progeny. Growth also results when cells simply become longer or larger.
If the microorganism is coenocytic—that is, a multinucleate organism in which nuclear divisions are not accompanied by cell divisions growth results in an increase in cell size but not cell number.
Measurement of Bacterial Growth
Growth may be defined as an orderly increase in cellular constituents.
If the microorganism is coenocytic (multinucleated in which nuclear divisions are not accompanied by cell divisions), growth results in an increase in cell size but not cell number.
Growth leads to an increase in cell number when microorganisms reproduce by processes as budding or binary fission.
It is not usually convenient to investigate the growth and reproduction of individual microorganisms because of their small size.
Therefore, when studying growth, microbiologists often observe total changes in the total population number of cells.
What is Growth Curve?
Population growth is studied by analyzing the growth curve of microbial culture.
When microorganisms are cultivated in a liquid medium, they usually are grown in batch culture or closed system.
Because no fresh medium is provided during incubation, nutrient concentrations decline, and the concentration of waste increases.
The growth of microorganisms reproducing by binary fission can be plotted as the logarithm of cell number versus the incubation time; the resulting curve has four distinct phases.
After the introduction of cells into the fresh culture medium, no intermediate increase in cell or mass occurs, and therefore this phase is called as ‘Lag phase’.
In this phase, cells are synthesizing new components.
This lag phase may be old depleted of ATP, essential cofactors, and ribosomes, which must be synthesized at the initial growth.
If the medium is different from the one in which the cells were originally growing, new enzymes would be needed to use the different nutrients.
This phase may be quite long if the inoculum is from an old culture or one that has been refrigerated/ Inoculation ofa culture into a chemically different medium.
If the cells in the inoculums have been injured due to some stress, they require time to recover and start growing.
The lag phase varies considerably in length depending on the microorganisms and the nature of the medium.
To get minimum lag, a young vigorously growing exponential phase culture grown in the same medium should be used.
A lag phase before the start of cell division may be required for a variety of reasons:
- The cells may be old and depleted of ATP, essential cofactors, and ribosomes; these must be synthesized before growth can begin.
- The medium may be different from the one the microorganism was growing in previously. Here new enzymes will be needed to use some various nutrients.
- Perhaps the microorganisms have been damaged and require time to recover or heal.
This phase is also referred to as the “Log phase”.
Exponential growth is balanced growth.
During the exponential or log phase, microorganisms are growing and dividing at the maximal rate possible given their genetic potential, the nature of the medium, and the conditions under which they are growing.
Their rate of growth is constant during the exponential phase – the microorganisms are dividing and doubling in number at regular intervals.
Because each individual divides at a slightly different moment, the growth curve rises smoothly rather than in discrete jumps.
The population is most uniform in terms of chemical and physiological properties during this phase hence are usually used in biochemical and physiological studies.
In this phase, the microorganisms are growing at the maximal rate possible under the set of conditions and their inherent growth potential.
The rate of growth in this phase is constant; that is, the microorganisms are dividing and doubling in number at regular intervals since individuals in the culture rise smoothly instead of discrete jumps.
“In this phase, the population is most uniform in terms of chemical and physiological properties, therefore exponential phase cultures are usually used in biochemical and physiological studies.”
“Eventually, population growth ceases and the growth curve becomes horizontal.”
This stationary phase usually is attained by bacteria at a population level of around 109 cells per ml.
Other microorganisms generally do not reach such high population densities, protozoan, and algal cultures usually having maximum concentrations of about 106 cells per ml.
The final population size depends on the availability of nutrients and other substances, as well as the type of microorganism cultivated. Some bacteria can survive starvation for years.
These considerations are critical to the effectiveness of medical and industrial microbiology.
Salmonella typhimurium and some other bacterial pathogens become more virulent and infectious when they are starved.
Microbial populations enter stationary phases for several reasons:
- Nutrient depletion – if an essential nutrient is severely depleted, population growth will slow down.
- Aerobic organisms are often limited by oxygen availability.
- Oxygen is not very soluble and may be depleted quickly such that only the surface of culture will have oxygen concentration adequate for growth.
- The cells beneath the surface will not be able to grow unless the culture is shaken or aerated in another way.
- Population growth may also cease due to the accumulation of toxic waste products.
- This factor seems to limit the growth of many anaerobic microorganisms. For example, acid production from sugar fermentation can inhibit growth.
Death is defined to be the irreversible loss of the ability to reproduce.
The death rate may decrease after the population has been drastically reduced- due to the extended survival of, particularly resistant cells.
Ultimately nutrient depletion can buildup of toxic waste products leads to a decline in the number of viable cells, thus the death phase.
This phase, like growth in the exponential phase, is usually logarithmic (a constant proportion of cells dying every hour).
In this phase, it is possible that the cell number may remain the same because the dead cells fail to lyse after dying.
The death rate may decrease after the population has been significantly reduced.
This is due to the extended survival of particular resistant cells.
The knowledge of microbial growth rates in the exponential phase is important for microbiologists.
Such studies contribute to basic physiological and ecological research and the solution of applied problems in the industry.
Effect of pH on Bacterial Growth
pH is another key factor in bacterial growth. As we know pH range less than 7 is acidic and the pH range greater than 7 is Basic. Bacteria that grow best in high pH are known as Alkaliphiles. They can survive and thrive in highly acidic conditions (usually at pH 2.0).
Bacteria that grow better in low pH values are known as Alkaliphiles. They can live and thrive in environments with extremely high pH values (9-13) with the optimal pH being 10.
Neutrophils are bacteria that grow at normal pH i.e. pH 7.
Effect of Temperature on Bacterial Growth
Temperature plays the most important role in the growth of bacteria. Bacteria that grow best in low temperatures are known as Psychrophiles. They having an optimum growth temperature of greater than 15°C and a maximum growth temperature of greater than 20°C and a minimum of 0°C.
Those bacteria that grow best in high temperatures are generally known as Thermophiles. They can survive higher than those (25-40°C) that support many species.
Bacteria having higher temperatures are Hyperthermophilles. They can survive and thrive in temperatures above 80°C.
Effect of Salt on Bacterial Growth
Salty condition is also one of the important factor in Bacterial growth. Halophiles are a group of extremophiles that require high salt concentration in order to survive and grow. Halophiles are of two types; obligate halophiles that require a NaCl concentration of 3% or more and halotolerant that survive at both average salt concentrations and higher.
Effect of Radiation on Bacterial Growth
Bacteria that can survive in extreme forms of radiation like ionizing radiant (gamma rays) and UV radiation are known as Radiophiles. Studies on radiophiles are quite limited because these bacteria are to be isolated from outer space and other planets.