Bacterial nutrition requirements for growth and multiplication of bacteria are water, a source of carbon, a source of nitrogen and some inorganic salts. The water content of bacterial cells can vary from 75 to 90 per cent of the total weight and is the vehicle for the entry of all cells and for the elimination of all waste products. It participates in the metabolic reactions and also forms an integral part of the protoplasm.
Categories of Requirements for Microbial Growth
The requirements for microbial growth can be divided into two main categories:

(i) chemical and (ii) physical.

1. Chemical Requirements
Chemical requirements for microbial growth include sources of carbon, nitrogen, sulfur, phosphorus, trace elements, oxygen, and organic growth factors.
A. Major elements (Macroelements or macronutrients)
Elements that make up cell constituents are called major elements (macroelements or macronutrients) and over 95 per cent of cell dry weight is made up of a few major elements. These include carbon, oxygen, hydrogen, nitrogen, sulfur, phosphorus, potassium, magnesium, calcium, and iron.
B. Trace elements
Some elements termed trace elements or micronutrients are required in very minute amounts by all cells. They include cobalt, zinc, copper, molybdenum, and manganese. These elements form parts of enzymes or may be required for enzyme function which aids in the catalysis of reactions.
2. Growth Factors
Some bacteria require certain organic compounds in minute quantities known as growth factors or bacterial vitamins. A growth factor is an organic compound which a cell must contain in order to grow (also growth does not occur in their absence), but it is unable to synthesize. In many cases, bacterial vitamins are identical with the vitamins necessary for mammalian nutrition, particularly those belonging to the B group, thiamine, riboflavin, nicotinic acid, pyridoxine, folic acid and vitamin B12.
3. Energy Sources
Organisms derive energy either from sunlight or metabolizing chemical compounds.
Phototrophs: Organisms that gain energy from light are called phototrophs
Chemotrophs: Organisms that obtain energy by metabolizing chemical compounds are called chemotrophs (chemo means “chemical”).

2. Physical Factors Influencing Microbial Growth
1. Temperature
2. Oxygen
3. Carbon dioxide
4. Moisture and drying
5. pH
6. Light
7. Osmotic effect
8. Mechanical and sonic stresses

1. Temperature
Optimum Temperature
Each bacterial species has an optimal temperature for growth and a temperature range above and below which growth is blocked. The temperature at which growth occur best is known as the ‘optimum temperature’. Thus, bacteria pathogenic for humans usually grow at 37ºC. Bacteria are divided into
three groups on the basis of temperature ranges through which they grow:
i. Mesophilic: Bacteria which grow between 10ºC and 45ºC, with optimal growth between 20-40ºC.
Examples: All parasites of warm-blooded animals are mesophilic
ii. Psychrophilic: Psychrophilic bacteria (cold-loving) are organisms that grow between –5 to 30ºC, optimum at 10 to 20ºC.

Examples: They are soil and water saprophytes and though not of direct medical importance, may cause
spoilage of refrigerated food. These organisms may be capable of growth in food and pharmaceuticals
stored at normal refrigeration temperatures (0-8ºC).
iii. Thermophilic: Thermophiles (heat-loving) have growth range 25-80ºC, optimum at 50-60ºC. They may cause spoilage of underprocessed canned food and can be a source of proteins with remarkable thermotolerant properties such as taq polymerase, the key enzyme used in the polymerase chain reaction.
Examples: Some thermophiles (like Bacillus stearothermophilus) form spores that are exceptionally

Microbial Nutrition
2. Oxygen
Based on their O2 requirements, prokaryotes can be separated into aerobes and anaerobes.
A. Aerobic bacteria
Require oxygen for growth and may be:
i. Obligate aerobes: They have an absolute or obligate requirement for oxygen (O2), like the cholera
ii. Facultative anaerobes: They are ordinarily aerobic but can also grow in the absence of oxygen, though less abundantly, e.g. Staphylococcus spp.; Escherichia coli, etc.
iii. Microaerophilic organisms: They grow best at low oxygen tension (~5%) e.g. Campylobacter spp., Helicobacter app.

B. Anaerobic bacteria
Grow in the absence of oxygen.
Obligate anaerobes: They may even die on exposure to oxygen, e.g. Clostridium tetani, Bacteroides fragilis.

3. Carbon Dioxide
All bacteria require a small amount of carbon dioxide for growth. Thus, this requirement is usually met by the carbon dioxide present in the atmosphere, or produced endogenously by cellular metabolism. Some organisms such as Brucella abortus, require much higher levels of carbon dioxide (5-10%) for growth, especially on fresh isolation (capnophilic). Pneumococci and gonococci are other capnophilic which grow better in air supplemented with 5 to 10 per cent CO2.

4. Moisture and Drying
Moisture is very essential for the growth of the bacteria because water is an essential ingredient of bacterial protoplasm and hence drying is lethal to cells. However, the effect of drying varies in different species

i. Treponema pallidum is highly sensitive, while others like staphylococci withstand resistant to desiccation for months.
ii. Bacterial spores: Bacterial spores are particularly resistant to desiccation and survive in the dry state for several decades.
iii. Freeze-drying or lyophilization: Drying in a vacuum in the cold (freeze-drying or lyophilization) is
a method of preservation of bacteria, viruses and many labile biological materials. On a larger scale, it
is used for preserving therapeutic antisera, human plasma, antibiotics and vaccines.

5. pH
Most bacteria can live and multiply within the range of pH 5 (acidic) to pH 8 (basic) and have a pH optimum near neutral (pH 7). Most pathogenic bacteria grow best at a neutral or slightly alkaline pH (7.2 to 7.6). Some acidophilic bacteria such as lactobacilli grow under acidic conditions while cholera vibrio, grow at high degrees of alkalinity ( above pH 8), whereas most other bacteria grow in
a range of 6 to 7.5. Numerous fungi grow well at pH 4 or 5.

6. Light
Darkness provides a favourable condition for the growth and viability of bacteria. Bacteria are sensitive to ultraviolet light and other radiations as ultraviolet rays from direct sunlight or a mercury lamp are bactericidal. Bacteria are also killed by ionizing radiations. Exposure to light may influence pigment production. Photochromogenic mycobacteria form pigment only on exposure to light.

7. Osmotic Effect
Tolerance to osmotic variation: Bacteria are more tolerant to osmotic variation because of the mechanical strength of the cell wall. Except for the mycoplasma and other cell wall defective organisms, the majority of the bacteria are osmotically tolerant.
Plasmolysis: When microorganisms with rigid cell walls are placed in a hypertonic environment, water
leaves and the plasma membrane shrinks away from the wall, a process known as plasmolysis. This occurs more readily in gram-negative bacteria than in gram-positive bacteria.
Plasmoptysis: Sudden transfer of bacteria from a concentrated solution to distilled water may also cause plasmoptysis due to excessive osmotic imbibition of water leading to swelling and rupture of the cell.

8. Mechanical and Sonic Stresses
In spite of tough walls of bacteria, they may be ruptured by mechanical stress such as grinding or vigorous shaking with glass beads. Exposure to ultrasonic vibration may also disintegrate bacteria.