Introduction
Bacteria are capable of growing in different types of laboratory media when provided with essential nutrients and suitable environmental conditions. Their growth depends on sources of energy, carbon, nitrogen, minerals, pH, temperature and oxygen availability. Bacterial multiplication occurs through binary fission, where one cell divides into two identical daughter cells.
Nutritional Requirements of Bacteria
Bacteria require several chemical substances for growth. These nutrients help in energy production, synthesis of cellular components and metabolic activities.
1. Source of Energy
- Phototrophs: Use sunlight for energy (e.g., Rhodospirillum rubrum).
- Chemotrophs: Obtain energy from chemical reactions (e.g., Escherichia coli).
2. Source of Electrons
- Lithotrophs: Use inorganic compounds as electron donors (e.g., Pseudomonas pseudoflava).
- Organotrophs: Use organic compounds as electron donors (e.g., E. coli).
Sub-divisions
Based on their energy and electron source:
- Photolithotrophs: Use sunlight + inorganic electron donors (e.g., Chromatium okenii).
- Photoorganotrophs: Sunlight + organic electron donors.
- Chemolithotrophs: Chemical energy + inorganic donors (e.g., Nitrosomonas).
- Chemoorganotrophs: Chemical energy + organic donors.
3. Source of Carbon
- Autotrophs: Use CO₂ as main carbon source (e.g., Chromatium okenii).
- Heterotrophs: Require organic compounds as carbon source (e.g., E. coli).
4. Nitrogen Requirements
Nitrogen is essential for proteins and nucleic acids. Bacteria utilize nitrogen from atmospheric nitrogen, nitrates, nitrites, ammonium salts or amino acids.
5. Sulphur Requirements
Sulphur is required for amino acids like cystine and methionine. Bacteria use sulphur from organic and inorganic sources.
6. Phosphorus
Usually supplied as phosphate; required for nucleic acids, nucleotides and phospholipids.
7. Mineral Salts
Necessary salts include sodium, potassium, magnesium, iron, calcium, phosphates and sulphates. They support enzyme activity and osmotic balance.
8. Growth Factors (Bacterial Vitamins)
Some bacteria require small quantities of vitamins or organic compounds for growth. Others can synthesize all vitamins internally. When required vitamins are absent, growth fails.
9. Water
Water forms 80–90% of bacterial cell weight. It dissolves nutrients and supplies micronutrients and trace elements needed for bacterial life.
Physical Conditions Required for Growth
Apart from nutrients, bacteria require favourable environmental conditions for optimum multiplication.
1. Temperature
Bacteria have minimum, optimum and maximum temperature ranges.
- Psychrophiles: Grow at 0°C, optimum 15°C.
- Mesophiles: Grow between 20–40°C (includes human pathogens).
- Thermophiles: Grow at 45–70°C.
- Thermoduric species: Do not grow at high temp but survive above 60°C.
2. pH
- Acidophiles: pH 1–6.5.
- Neutrophiles: pH 6.5–7.5 (most pathogens).
- Alkalophiles: pH 7.5–14.
3. Gaseous Requirements
Dependency on oxygen defines several bacterial groups:
- Aerobes: Require oxygen (e.g., Staphylococcus aureus).
- Anaerobes: Oxygen is toxic; cannot grow in air (e.g., Clostridium).
- Facultative anaerobes: Grow with or without oxygen.
- Microaerophiles: Require low oxygen levels (e.g., Campylobacter).
Oxygen Toxicity
Oxygen generates toxic radicals like superoxide and hydrogen peroxide. Aerobic and facultative anaerobic bacteria produce superoxide dismutase and catalase to neutralize these. Anaerobes lack these enzymes, so oxygen is harmful to them.
4. Osmotic Pressure
Bacteria tolerate a wide range of osmotic conditions due to their cell wall strength. High salt concentrations may cause plasmolysis. Most media contain 0.5% NaCl to maintain isotonic conditions.
5. Light
Most microorganisms prefer darkness because they are sensitive to ultraviolet and strong visible light.
Growth Pattern of Bacteria
Bacteria in liquid media follow a characteristic growth curve showing lag phase, log phase, stationary phase and death phase. Continuous growth can be maintained by adding fresh medium to the culture system.
Bacterial numbers can be estimated by:
- Direct microscopic count
- Plate count (dilution and inoculation)
- Turbidity measurement
- Dry/wet weight
- Biochemical assays
Cultivation of Bacteria
Bacteria are grown on artificial media containing carbon, nitrogen, minerals and water. Both aerobic and anaerobic cultivation techniques are used.
Cultivation of Aerobic Bacteria
- Grow in flasks, tubes or bottles under normal atmospheric conditions.
- Shaking liquid cultures improves aeration.
Cultivation of Anaerobic Bacteria
Anaerobes require oxygen-free environments.
- Candle jar: Burns oxygen but leaves small amounts behind.
- Reducing agents: Thioglycollate, glucose or ascorbic acid reduce oxygen in media.
- Vacuum desiccator: Partial removal of air using vacuum.
- Pyrogallic acid method: Alkaline pyrogallol absorbs oxygen.
- McIntosh and Fildes jar: Removes air, replaces with hydrogen and uses catalyst to remove oxygen.
- Gaspak system: Convenient chemical sachets that generate hydrogen and CO₂, creating anaerobic conditions.
Key Points
- Bacteria need carbon, nitrogen, minerals, water and energy sources for growth.
- Physical factors like pH, temperature and oxygen strongly influence bacterial multiplication.
- Aerobic and anaerobic bacteria require different cultivation techniques.
- Understanding nutritional needs improves laboratory cultivation and identification of microbes.
Detailed Notes:
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