Some of the important factors affecting bacterial growth are:
1. Nutrition concentration
2. Temperature
3. Gaseous concentration
4. pH
5. Ions and salt concentration
6. Water
1. Nutrient concentration:
Bacterial growth is directly proportional to the available
nutrients. If culture media is rich in nutrients, the growth of bacteria occurs
faster and vice versa.
Nutritional requirement varies with the bacterial species.
The relationship between substrate concentration (nutrition) and growth rate is
shown in Fig.
Fig: Nutrient vs growth rate
With the increase in the concentration of nutrients, the growth
rate of bacteria increases up to a certain level, and then the growth rate
remains constant irrespective of nutrition addition.
2. Temperature:
The lowest temperature that allows growth is called minimum
temperature and the highest temperature that allows growth is called maximum
temperature.
There is no growth below minimum and above maximum
temperature.
Below minimum temperature cell membrane solidifies and
becomes stiff to transport nutrients into the cell, hence no growth occurs.
Cellular proteins and enzymes denature at maximum
temperature, so bacterial growth ceases.
The relationship between temperature and growth rate is
shown in Fig.
Fig: Temperature vs growth rate
The growth rate of bacteria increases because the rate of
metabolic reaction increases with an increase in temperature.
At a certain temperature the growth rate becomes maximum,
this temperature is known as the optimal temperature.
On further increasing the temperature above optimal, the growth
rate decreases drastically and completely ceases with reaching maximum
temperature.
Thermophile organisms are heat-loving bacteria. Their
optimum growth temperature is between 45°C and 70°C and are commonly found in
hot springs and compost heaps.
Hyperthermophile bacteria grow at very high temperatures.
Their optimum growth temperature is between 70°C and 110°C. They are usually
members of the Archaea. Psychrophiles are grown between −5°C to 20°C.
Mesophiles are grown between 20°C to 45°C.
3. pH:
pH affects the ionic properties of bacterial cells so it
affects the growth of bacteria.
Most of the bacteria grow at neutral pH (6.5-7.5). However, certain
bacteria grow best at acidic or basic pH.
The relationship between pH and bacterial growth is given in
Fig.
Fig: pH vs growth rate
Microorganisms can be placed in one of the following groups
based on their optimum pH requirements:
1. Neutrophiles grow best at a pH range of 5 to 8.
2. Acidophiles grow best at a pH below 5.5.
3. Alkaliphiles grow best at a pH above 8.5.
4. Ions and salt:
All bacteria require metal ions such as K+, Ca++,
Mg++, Fe++, Zn++, Cu++, Mn++ etc. to synthesize enzymes and proteins.
Most bacteria do not require NaCl in media and they can
tolerate very low concentrations of salt.
Some halophilic bacteria such as Archeobacteria require high
concentrations of salt in the media.
It is balanced with the Osmosis process.
Osmosis is the diffusion of water across a membrane from an
area of higher water concentration (lower solute concentration) to lower water
concentration (higher solute concentration) without the requirement of
metabolic energy. While water molecules are small enough to pass between the
phospholipids in the cytoplasmic membrane, their transport can be enhanced by
water-transporting transport proteins known as aquaporins. Based on the conditions,
a cell has three environments such as isotonic, hypertonic, and hypotonic. In
an isotonic environment both the water and solute concentration are the same
inside and outside the cell and water goes into and out of the cell at an equal
rate.
In a hypertonic environment, the water concentration is
greater inside the cell while the solute concentration is higher outside
whereas with hypotonic conditions, the water concentration is greater outside
the cell and the solute concentration is higher inside hence water goes into
the cell.
Most bacteria require an isotonic environment or a hypotonic
environment for optimum growth.
5. Gaseous requirement:
Oxygen and carbon dioxide are important gases that affect
the growth of bacteria.
Oxygen is required for aerobic respiration and obligate
aerobic bacteria must require O2 for growth. For example:
Mycobacterium, Bacillus.
For obligate anaerobes, oxygen is harmful or sometimes
lethal but facultative anaerobes can tolerate low concentrations of O2.
Carbon dioxide is needed for capnophilic bacteria. Such as
Campylobacter, and Helicobacter pylori.
6. Water:
Water is the most essential factor for bacterial growth.
Available water in the culture media determines the rate of
metabolic and physiological activities of bacteria.
Sugar, salts, and other substances are dissolved in water
and are made available for bacteria.
Preparation of Nutrient Agar:
1. 28 g of nutrient agar powder is suspended in 1 liter of
distilled water.
2. Heated the mixture while stirring to fully dissolve all
components.
3. Autoclave the dissolved mixture at 121°C for 15 minutes.
4. Allowed it to cool but not solidify.
5. Pour nutrient agar into each plate and leave the plates
on the sterile surface until the agar has solidified.
6. Stored the plates in a refrigerator.
Uses of Nutrients Agar
1. It is frequently used for the isolation and purification
of cultures.
2. It is also used for producing the bacterial lawns needed
for antibiotic sensitivity tests.