Physical Parameters for Growth of Bacteria
Aside from the kind of media used to support bacterial growth, a number of environmental physical factors are crucial for the best possible growth. Every kind of microorganism needs to have its physical characteristics investigated. Temperature, pH, oxygen, light, hydrostatic pressure, and other factors are a few of these conditions.
One of the most significant physical parameters that greatly affects the development of microorganisms is temperature. The ideal growth temperature is one that permits quick development over a brief period of time. Maximum growth temperature is the highest temperature at which microorganisms grow, and minimum growth temperature is the lowest temperature at which microorganisms can grow.
Based on temperature tolerance and its influence on growth, bacteria may be classified as
Psychrophiles can grow at 0°C but prefer a temperature of 15°C or lower; these are known as obligate psychrophiles. Psychotropic or facultative psychrophiles can grow at 0°C but perform best at 20 to 30°C.
All the bacteria that are pathogenic to humans and warm-blooded animals are mesophiles. They can grow within the range of 20 to 40°C.
Thermophiles prefer to grow at high temperatures (45 to 70°C). Many thermophiles can grow at temperatures ranging from 37 to 55°C. These species are referred to as facultative thermophiles.
Some thermophiles can grow above 60°C; these are known as true, obligate, or steno thermophiles. Several mesophiles that can tolerate high temperatures (above 60°C but do not grow there) are known as thermoduric.
Table of temperature ranges for bacterial growth with examples
|Growth temperature °C
|0 to 5
|10 to 15
|0 to 5
|25 to 30
|15 to 20
|35 to 40
|35 to 40
|50 to 55
|50 to 55
|65 to 70
The pH of a bacteria’s growth medium has a significant impact on microorganism growth. Each microbial species has a specific pH range for growth and multiplication. Microorganisms are classified according to their optimum pH value
Acidophilic microorganisms have an optimum pH range between 1 to 6.5. Example- Thiobacillus thiooxidans (optimum pH 2 to 3.5) and Lactobacillus acidophilus (optimum pH 5.8 to 6.6)
The majority of bacteria, known as neutrophiles, grow in the narrow pH range of 6.5 to 7.5. Examples- Escherichia coli, salmonella typhi and all pathogenic bacteria.
Alkalophiles have an optimum pH range in between 7.5 to 14. Examples- Vibrio cholerae (optimum pH 9.0), Agarobacterium species (optimum pH 12.0)
The two main gases that influence the growth of bacteria are carbon dioxide and oxygen. A bacterium’s oxygen requirement is a reflection of its energy-requirement mechanism for oxygen. On the basis of oxygen requirement, bacteria can be classified as
As the name suggest these bacteria require oxygen for growth and can grow when incubated in an air atmosphere i.e. 21% of oxygen. Example- Escherichia coli.
The term “anaerobic bacteria” describes microorganisms that do not need oxygen to grow. Such cells are toxic to oxygen. Tolerant or non-stringent anaerobes are bacteria that can tolerate low oxygen concentrations. Strict or stringent anaerobes are certain other bacteria that are incapable of tolerating even very low oxygen concentrations and may die after a brief period of air exposure. Example-Clostridium species.
Facultatively anaerobic bacteria
The bacteria which can grow in both aerobic and anaerobic conditions are called as facultatively anaerobic bacteria. The above mention bacteria do not require oxygen for growth but if oxygen is available it is used for energy production. Example- Pseudomonas species.
Low level of oxygen requires for the growth of these bacteria but also, they cannot tolerate the levels of oxygen present in an air atmosphere. Example- Campylobacter jejuni, Lactobacillus plantarum.
Effect of oxygen
Oxygen serves as an electron acceptor in an aerobic system. For this reason, it is crucial for respiration and metabolic processes. Multiple essential reduced groups (thiol, -SH) or enzymes can be directly oxidized by molecular oxygen. When oxygen undergoes oxidation, it releases the harmful superoxide radical (O–2). Superoxide radicals have the ability to deactivate cell components and generate additional harmful compounds, like hydroxyl radicals (OH) and hydrogen peroxide(H2O2).
These radicals are potent oxidizers that disrupt metabolism and quickly degrade biological components. Two enzymes that eliminate harmful radicals are superoxide dismutase and catalase, which are found in both facultative anaerobic and aerobic bacteria.
Superoxide dismutase breaks down superoxide radicals into oxygen and hydrogen peroxide, while the catalase enzyme breaks down hydrogen peroxide into oxygen and water. Superoxide dismutase and catalase enzymes are not present in anaerobic bacteria hence, it does not show growth in the presence of oxygen.
Because of the mechanical strength of their cell walls, bacteria are more resistant to changes in osmotic pressure. They can thrive in media that contain wildly different amounts of sugar, salt, and other solutes. Plasmolysis can occur when bacteria are abruptly exposed to solutions with high salt concentrations. Therefore, to make the environment isotonic, 0.5% NaCl is added to nearly all culture media.
For most microorganisms, darkness is ideal for growth and viability. They are susceptible to various radiations, including direct light and ultraviolet radiation.
Growth can be defined as an orderly increase in cellular constituents. Growth is followed by cell division, resulting in an increase in the cell number.
Growth curve of bacteria
By introducing a small number of bacterial cells into an appropriate culture medium and counting the bacteria in aliquot samples on a regular basis, one can ascertain the normal growth curve of bacteria.
Plotting the logarithms of the viable cells against time on a graph paper produces a typical curve known as the bacterial growth curve, also known as the bacterial growth cycle. The growth has four phases namely, lag phase, log or log or exponential phase, stationary phase and death or decline phase.
When bacteria are introduced into a new medium, the microbial population remains stable for a while. The time between inoculation and the start of multiplication is known as the lag phase. During this phase, bacterial cells adjust to their new environment.
During this phase, the bacteria synthesize enzymes, coenzymes, and other essential molecules. The cells are metabolically and physiologically extremely active, but they do not divide. The length of the lag phase is determined by the medium’s composition, the species of microorganisms, and other physical and chemical growth factors.
Log phase or Exponential phase
The cells divide continuously and at a constant rate during this phase, producing a straight line when the log of the number of cells is plotted against time. The bacteria proliferate at their fastest rate, and as time passes, their population grows exponentially or geometrically.
Generation time is the amount of time needed for a single bacterial division during this phase. Every generational period has roughly twice as many bacteria as the one before it. The number of generations (n) that occur at a given time (t) can be used to calculate the generation time (g).
g = t / n = t / 3.3 (log N – log N0)
All bacteria do not have same generation time. Generation time is mainly dependent on type of species, nutrients in the medium and physical conditions.
Growth rate (R) i.e. number of generations per hour is equal to the reciprocal of generation time. It is also the slope of a straight line obtained by plotting the log number of cells versus time.
R = 1 / g = 3.3 (log N – log N0) / t
In stationary phase, a balance between cell division and cell death maintains a constant high cell count. The rate of multiplication is reduced due to nutrient depletion, toxic waste accumulation, very high cell concentration, and low partial pressure of oxygen. During the stationary phase, reserved food materials are consumed, a portion of ribosomes may be degraded, and enzymes may be synthesized. A viable population count at this point shows no change.
Death or decline phase
Death phase is also known as the logarithmic phase. During the death phase, the number of viable cells decreases exponentially, which is the inverse of growth during the log phase. A variety of conditions contribute to bacterial death, the most important of which are nutrient depletion and the accumulation of toxic waste products. Bacteria die and grow at different rates.
In between these phases, there is small curved portion called the transitional period.
Numerous environmental elements, such as temperature, pH, oxygen, light, and hydrostatic pressure, have an impact on the growth of bacteria. Temperature is a critical factor in the rapid development of bacteria, as thermophiles, mesophiles, psychrophiles, and thermoduric bacteria have different tolerance levels.
The ideal pH ranges for acidophiles are 1 to 6.5, neutrophiles are 6.5 to 7.5, and alkalophiles are 7.5 to 14. pH is an important consideration. Anaerobic bacteria do not require oxygen, whereas facultatively anaerobic bacteria can grow in both carbon dioxide and oxygen environments. Aerobic bacteria have an oxygen requirement of 21%.
The four phases of the bacterial growth curve, also referred to as the bacterial growth cycle, are the lag phase, log phase, stationary phase, and death or decline phase. During the lag phase, bacteria produce necessary molecules and adapt to their new surroundings.
While the stationary phase keeps the balance between cell division and death, the log phase involves continuous cell division. Cell death occurs exponentially during the death phase, also referred to as the logarithmic phase.
Frequently asked questions
Classify bacteria depending on the physical requirements for growth?
Bacteria Can be classified on some of these physical parameters like temperature, pH, gaseous requirement etc
Psychrophiles are defined as the bacteria that can grow at 0°C but prefer a temperature of 15°C or lower; these are known as obligate psychrophiles. Psychotropic or facultative psychrophiles can grow at 0°C but perform best at 20 to 30°C.
What are Neutrophiles?
The bacteria which can grow best in narrow range of pH between 6.5 and 7.5 are called as neutrophiles.
Define anaerobic bacteria?
The bacteria that do not require oxygen for energy and growth are called as anaerobic bacteria.