Microbial Culture: Essential Concepts for Class 12 Biotechnology

What is Microbial Culture and Its Importance in Biotechnology

Microbial culture refers to the process of growing microorganisms like bacteria, fungi, and algae in a controlled environment. In Class 12 Biotechnology, understanding microbial culture is crucial as it helps in:

• Isolating pure microbial strains for study
• Producing antibiotics, enzymes, and vaccines
• Studying microbial genetics and metabolism

Microbial cultures are grown on nutrient media, which provide essential nutrients for microbial growth. These cultures allow scientists to observe microbial behaviour, identify species, and apply microbes in industries and research.

Sterilisation Methods: Ensuring Contamination-Free Microbial Culture

Sterilisation is vital to prevent contamination during microbial culture. It eliminates all living microorganisms, including resistant spores, from media, equipment, and surfaces. Sterilisation methods are broadly classified into physical and chemical:

Physical Methods:

• Heat Sterilisation:
• Boiling: Heating at 100°C for 30 minutes, suitable for syringes and rubber goods.
• Autoclaving: Steam under 15 psi pressure reaches 121°C, sterilising media and instruments effectively.
• Pasteurisation: Mild heat treatment used in food industries to kill pathogens without full sterilisation.
• Dry Heat: Flaming inoculation loops and hot air ovens sterilise glassware.

• Radiation Sterilisation:
• UV Radiation: Germicidal UV rays (200–280 nm) disinfect surfaces but have limited penetration.
• Ionizing Radiation: Gamma rays and electron beams sterilise medical products and food.

• Filtration Sterilisation:
• Used for heat-sensitive liquids like serum and antibiotics using membrane filters (0.2–0.45 µm pores).

Chemical Methods:

• Alcohols (70% ethyl alcohol), aldehydes (formalin), heavy metals, and hydrogen peroxide disinfect surfaces and instruments.

Sterilisation ensures pure culture techniques succeed by preventing unwanted microbial growth.

Pure Culture Techniques: Isolating Single Microbial Species

Pure culture techniques isolate a single type of microorganism from a mixed population, essential for studying microbial properties accurately. Common methods include:

• Streak Plate Method: Microbial cells are spread on agar in decreasing density to obtain isolated colonies. The isolated colonies represent single species.

• Pour Plate Method: Diluted microbial samples are mixed with molten agar and poured into Petri dishes. Dilution reduces colony density to separate individual colonies.

• Spread Plate Method: A small volume of diluted sample is spread evenly on agar surface using a sterile L-shaped glass spreader.

These methods require sterile conditions to avoid contamination. Isolated colonies can be further sub-cultured for detailed studies.

Factors Affecting Microbial Growth in Culture

Microbial growth depends on environmental factors that influence metabolic activities:

• Temperature: Most microbes grow between 20°C and 45°C (mesophiles). Some prefer extreme temperatures (thermophiles or psychrophiles).

• pH: Fungi thrive in slightly acidic conditions, while bacteria prefer neutral to slightly alkaline pH.

• Oxygen Requirement: Microbes are classified based on oxygen needs:
• Aerobes: Require oxygen
• Obligate Anaerobes: Cannot tolerate oxygen
• Facultative Anaerobes: Can grow with or without oxygen
• Aerotolerant Anaerobes: Do not use oxygen but tolerate it

• Nutrient Availability: Carbon, nitrogen, minerals, and vitamins are essential for growth.

Understanding these factors helps optimize culture conditions for desired microbial growth.

Radiation and Filtration: Special Sterilisation Techniques for Sensitive Materials

Some materials used in microbial culture are sensitive to heat and require alternative sterilisation methods:

• Radiation Sterilisation:
• UV Radiation is used for surface sterilisation in laboratories and operation theatres. It damages microbial DNA but cannot penetrate solids or liquids.
• Ionizing Radiation like gamma rays and electron beams sterilise syringes, gloves, foods, and pharmaceuticals. They penetrate deeply and damage nucleic acids.

• Filtration Sterilisation:
• Used for heat-sensitive liquids such as serum, antibiotics, and sugars.
• Membrane filters with pore sizes between 0.2 and 0.45 micrometers physically remove microbes.
• High Efficiency Particulate Air (HEPA) filters remove 99.97% of particles larger than 0.3 micrometers and are used in biological safety cabinets.

These methods ensure sterilisation without damaging sensitive materials.

Chemical Sterilisation Agents and Their Applications

Chemical sterilisation uses disinfectants and germicides to kill or inhibit microbes:

• Alcohols (70% Ethyl Alcohol): Dehydrate cells, disrupt membranes, and coagulate proteins. Used for disinfecting surfaces and skin.

• Aldehydes (Formaldehyde/Formalin): Used for surface disinfection and fumigation due to strong germicidal properties.

• Heavy Metals (Mercury, Silver, Copper): Precipitate proteins and act as germicides. Used in antiseptics and preservatives.

• Hydrogen Peroxide: Disinfects wounds and deodorises dressings by producing reactive oxygen species.

Chemical methods complement physical sterilisation, especially for surfaces and equipment that cannot be autoclaved.