• Growth of bacteria is affected by many factors such as nutrition concentration and other environmental factors.

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. Available water

1. Nutrient concentration:

• If culture media is rich in growth promoting substance, growth of bacteria occurs faster. Decrease in nutrient concentration decreases the growth rate.
• Different bacteria have different nutritional requirement.

The relationship between substrate concentration (nutrition) and growth rate is shown in figure.

                                                figure: nutrient vs growth rate

• With increase in concentration nutrition, growth rate of bacteria increases up to certain level and then growth rate remains constant irrespective of nutrition addition.

2. Temperature:

• Temperature affects the growth of bacteria by various ways.
• The lowest temperature that allows the 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 become stiff to transport nutrients in to the cell, hence no growth occurs.
• Above maximum temperature, cellular proteins and enzymes denatures, so the bacterial growth ceases.

The relationship between temperature and growth rate is shown in figure below.

figure: temperature vs growth rate

• When temperature is increases continuously from its minimum, growth rate of bacteria increases because the rate of metabolic reaction increases with increase in temperature.
• At certain temperature the growth rate become maximum, this temperature is known as optimal temperature.
• On further increasing the temperature above optimal, growth rate decreases abruptly and completely ceases with reaching maximum temperature.

3. pH:

• pH affects the ionic properties of bacterial cell so it affects the growth of bacteria.
• Most of the bacteria grow at neutral pH (60.5-7.5). However there are certain bacteria that grow best at acidic or basic pH.
• relationship between pH and bacterial growth is given in figure below.

figure: pH vs growth rate

4. Ions and salt:

• All bacteria requires 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 however they can tolerate very low concentration of salt.
• There is some halophilic bacteria such as Archeobacteria that require high concentration of salt in media.

5. Gaseous requirement:

• Oxygen and carbon-dioxide are important gases that affects the growth of bacteria.
• Oxygen is required for aerobic respiration and obligate aerobic bacteria must require O2 for growth. Eg. Mycobacterium, Bacillus
• For obligate anaerobes Oxygen is harmful or sometime lethal. However facultative anaerobes can tolerate low concentration of O2.
• Carbon-dioxide is needed for capnophilic bacteria. Such as Campylobacter, Helicobacter pylori

6.  Available 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.

Factor affecting bacterial growth

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