PHYSICAL OR ENVIRONMENTAL FACTORS AFFECTING BACTERIAL GROWTH
The various factors affecting bacterial growth are;
- OXYGEN
- TEMPERATURE
- HYDROGEN ION CONCENTRATION (PH)
- OSMOTIC PRESSURE
- WATER
- CARBON DIOXIDE
- LIGHT
1. Oxygen
Oxygen is a universal component of cells and is always provided in large amounts by H₂O. Bacteria differ in their requirement for molecular oxygen and are mainly divided into two groups;
a. Aerobes
They require oxygen for growth.
b. Anaerobes
They do not require oxygen for growth.
They are further divided into following groups;
i. Obligate /stringent or strict aerobes
They only grow in presence of oxygen.
ii. Obligate /stringent or strict anaerobes
They only grow in absence of oxygen.
iii. Facultative/non-stringent aerobes
They are anaerobes that can grow even in the presence of oxygen.
iv. Facultative /non-stringent anaerobes
They are aerobes that can grow even in absence of oxygen.
There are some organisms which can grow best in an atmosphere of 5-10% CO2, such organisms are called ‘capnophilic’. Still others require low levels of oxygen but cannot tolerate the level of oxygen present in air atmosphere; such organisms are called ‘microaerophilic’.
2.Temperature
It is the most important factor that determines the rate of growth, multiplication and death of all living organisms. A particular microorganism will exhibit a range of temperature over which it can grow, defined by three cardinal points in the same manner as pH;
a. Minimum temperature
It is the lowest temperature required for growth.
b. Maximum temperature
It is the highest temperature required for growth.
c. Optimum temperature
It is the temperature at which rapid growth in short period of time occurs.
On the basis of their temperature relationships, bacteria may be divided into three main groups;
1. PSYCHROPHILES or CRYOPHILES (Greek “psychros” meaning cold)
These are the bacteria which are able to grow at 0 °C or lower though they grow best at higher temperatures. The term psychrophile is now restricted by many microbiologists to organisms that can grow at 0 °C but have an optimum temperature of 15 °C or lower and maximum temperature of 20 °C -30 °C.
They are further subdivided into two groups;
i. Obligate or strict psychrophiles
They cannot grow at temperature above 20 °C – 22 °C and die when exposed to room temperature.
ii. Facultative psychrophiles
They are able to grow at 0 °C but can grow best at temperature in the range of 20 °C -30 °C.
2. MESOPHILES:
These are the bacteria which are able to grow within temperature range of 25 °C to 40°C.
They are further subdivided into two groups;
i. Those mesophiles with optimum temperature of 20 °C-35 °C Bacteria which live as saprophytes and plant pathogens are included in this group.
ii. Those mesophiles with optimum temperature of 35 °C – 45 °C. Bacteria which are pathogenic to animals are included in this group.
Thermotolerants or Thermoduric: Mesophiles those are capable of withstanding high temperature, though they do not multiply at these elevated temperatures. Psychro or Cryotolerants: They are also called psychroduric or cryoduric. These are mesophiles that can survive at very low temperatures but do not grow and multiply at such temperatures
3. THERMOPHILES (Greek “thermos” meaning heat)
These are the bacteria which grow best at optimum temperature of 45 °C or higher but can also grow over a range of 40°C -75°C.
They are further subdivided into two groups;
i. Obligate thermophiles
They grow only at temperature usually above 50°C and cannot grow in mesophilic range.
ii. Facultative thermophiles
They grow at high temperature and can also grow in mesophilic range i.e., grow at temperature range of 37°C -50°C.
3. HYDROGEN ION CONCENTRATION (PH)
The pH or hydrogen ion concentration [H+] of the growth medium of bacteria has a profound effect upon the multiplication of microorganisms. Each microbial species has a definite pH range and depending upon this optimum pH value.
They can be classified as:
i. Acidophiles
These microorganisms have optimum pH range in between 1-6.5 and grow at an optimum pH well below neutrality.
ii. Neutrophiles
Most bacteria fall in this category and grow best in pH range between 6.5-7.5.
iii. Alkalophiles
These bacteria have an optimum pH range between 7.5-14.
4. OSMOTIC PRESSURE
Because of presence of cell wall, bacteria are able to withstand a wide range of external osmotic pressure. The only common solute in nature that occurs over a wide concentration range is salt (NaCI) 0.5% strength is added in most culture media for creating a suitable environment for bacterial growth. On the basis of osmotic concentrations of substrates upon which the micro-organisms grow, microorganisms are classified as; 4.
i. Osmophobic
Those micro-organisms that die of dehydration if subjected to substrates of high osmotic concentrations.
ii. Osmophilic
Those microorganisms that best grow on substrates of high osmotic concentrations.
iii. Halophilic
Halophiles represent those microbes that preferably grow in high osmotic concentrations produced by dissolved salts.
iv. Osmoduric
These are those microbes that grow normally on substrates of moderate osmotic concentrations but prove to be resistant to wide osmotic changes in their substratum.
5. WATER
Water is the solvent in which the molecules of life are dissolved, and the availability of water is therefore a critical factor that affects the bacteria growth as 80% of bacterial cell consists of water. The availability of water for a cell depends upon its presence in the atmosphere (relative humidity) or its presence in solution or a substance (water activity). The water activity (Aw) of pure H₂O is 1.0 (100% water). Water activity is affected by the presence of solutes such as salts or sugars that are dissolved in the water. The higher the solute concentration of a substance, the lower is the water activity and vice-versa. Bacteria live over a range of Aw from 1.0 to 0.7.
6. CARBON DIOXIDE:
Small amount of CO₂ is required by all bacteria which is usually made available endogenously (inside culture medium in which bacteria is growing) as a product of cellular metabolism or by CO2 present in atmosphere.
7. LIGHT
Bacteria (except phototrophic species) usually grow in darkness.
BACTERIAL GROWTH CURVE
A characteristic growth curve obtained in batch culture (culture in which no fresh nutrients are added) consists of four phases.
1. Lag phase
During this phase bacteria adapt themselves to growth conditions. It is a phase of intense metabolic activity in which bacteria prepare for reproduction; synthesizing DNA, enzymes and other macromolecules needed for cell division. Therefore, during this phase there may be increase in size (volume)but no increase in cell number. The length of this phase depends on type of bacteria, quality of culture medium in which bacteria is growing.
2. Log phase or Exponential phase
This phase is characterized by cell doubling. During this phase the bacteria multiply at maximum rate and their number increases exponentially or by geometric progression: 1, 2, 4, 8, etc. or 20, 21,22, 23, 24…2n (where n=number of generations) with time. For this type of exponential growth, plotting the natural logarithm of cell num against time produces a straight line. Since the bacteria is growing in a constant medium of batch culture(no fresh nutrients added), therefore the duration of this phase is limited because of exhaustion of nutrients ,accumulation
the duration of this phase is of toxic metabolic end products, decrease in oxygen tension and increase in cell density.
3. Stationary phase
During this phase the growth rate slows down as a result of nutrient depletion and accumulation of inhibitory end (toxic) products. Sooner or later, the bacterial cells start dying and number of such cells balances the number of new born cells. In other words growth rate becomes equal to death rate in this phase.
4. Death or decline phase
During this phase death rate exceeds reproduction rate and thus number of viable bacterial cells start declining. Finally after a variable period, the entire bacterial population dies and culture becomes sterile.