UNIT 3: Microbial Culture and Sterilization Techniques

Exam Focus: Understand the difference between the pure culture methods (Streak, Pour, Spread). For sterilization, the working principle (Temperature, Pressure, Time) of the **Autoclave** vs. **Hot Air Oven** is frequently examined.

Table of Contents

1. Microbial Culture Techniques
   • Preparation of Culture Media
   • Inoculation
   • Pure Culture Techniques
2. Sterilization Techniques
   • Physical Methods for Sterilization
   • Chemical Methods for Sterilization

1. Microbial Culture Techniques

These techniques encompass the methods used to grow microorganisms in a controlled environment to study their properties or produce products.

Preparation of Culture Media

Culture media (nutrient broth or agar) must provide all the essential nutrients and appropriate physical conditions (pH, osmolarity) for microbial growth.

• **Defined (Synthetic) Media:** Exact chemical composition is known.
• **Complex Media:** Chemical composition is unknown (e.g., uses yeast extract, peptone, or beef extract), used to grow most heterotrophic organisms.
• **Selective Media:** Inhibits the growth of unwanted organisms while allowing the target organism to grow (e.g., high salt media for staphylococci).
• **Differential Media:** Allows colonies of different organisms to be distinguished based on their metabolism (e.g., blood agar distinguishes hemolytic bacteria).

Inoculation

The deliberate introduction of microorganisms into a sterile culture medium. This process must be performed using **aseptic techniques** (working near a flame or in a laminar air flow hood) to prevent contamination from the air or surfaces.

Pure Culture Techniques

A pure culture contains only one species or strain of microorganism. These techniques separate individual cells so that they grow into isolated colonies (clones of a single cell).

• **Streak Plate Method:** Most common. A loop is used to sequentially dilute the inoculum over the surface of an agar plate, resulting in isolated colonies in the final quadrants.
• **Pour Plate Method:** Serial dilutions of the sample are mixed with molten agar and poured into plates. Colonies grow both on the surface and embedded within the agar.
• **Spread Plate Method:** A small volume of diluted sample is spread evenly over the surface of a pre-poured agar plate using a sterile spreader.

[Image of streak plate method steps]

2. Sterilization Techniques

Sterilization is the complete destruction or removal of all forms of microbial life, including spores.

Physical Methods for Sterilization

The primary choice for sterilizing equipment and media that can withstand heat.

• **Heat Sterilization:**
  • **Moist Heat (Autoclaving):** Uses **steam under pressure** (e.g., 121^° C at 15 psi for 15-20 minutes). It is highly effective, killing spores by denaturation of proteins. Used for culture media, water, and heat-resistant glassware.
  • **Dry Heat (Hot Air Oven):** Uses hot, dry air (e.g., 160^° C to 170^° C for 2 hours). Kills by oxidation. Used for glassware and metal instruments.
  • **Incineration/Flaming:** Used to sterilize inoculating loops and needles rapidly.
• **Filtration:** Physically removes microbes from liquids or gases by passing them through a filter with a pore size small enough to trap bacteria (0.22 μm filter for liquids). Used for heat-labile liquids (e.g., serum, antibiotics).
• **Radiation:** UV radiation (surface sterilization) and Ionizing radiation (penetrates materials).

Chemical Methods for Sterilization

Involve the use of liquid or gaseous chemical agents (sterilants).

• **Common Agents:** Ethylene oxide (gaseous sterilant for heat-sensitive equipment), Glutaraldehyde, Hydrogen Peroxide.
• **Mechanism:** These chemicals usually kill by alkylating proteins and nucleic acids.