Unit 3: Microbial Culture and Sterilization Techniques

Microbial Culture Techniques

To study microbes, we must be able to grow them in the lab in a pure culture (a culture containing only one species or strain). This is done using culture media and aseptic techniques.

Culture Media

A culture medium (plural: media) is a nutrient solution (liquid or solid) prepared in the lab to support the growth of microorganisms.

Classification by Physical Form:

• Liquid Media (Broth): Used to grow large numbers of bacteria.
• Solid Media (Agar): Contains a solidifying agent called agar (a polysaccharide from seaweed). Agar is ideal because it melts at ~95°C, solidifies at ~42°C, and is not digested by most microbes. Used to isolate colonies.

Classification by Functional Type:

• Complex Media: The exact chemical composition is not known. Contains "rich" ingredients like yeast extract, beef extract, or peptones. (e.g., Nutrient Broth/Agar).
• Defined (Synthetic) Media: The exact chemical composition is known. Made from pure chemical compounds.
• Selective Media: Suppresses the growth of unwanted microbes and encourages the growth of desired ones. (e.g., MacConkey agar selects for Gram-negative bacteria).
• Differential Media: Makes it easy to distinguish colonies of different microbes. (e.g., Blood agar, where some bacteria lyse red blood cells and others don't).

Inoculation and Aseptic Techniques

• Inoculation: The process of introducing microbes into a sterile culture medium.
• Aseptic Technique: A set of practices used to prevent contamination of a culture and the lab environment.
  • Key Principle: Always assume microbes are everywhere.
  • Examples:
    • Sterilizing media and equipment (e.g., in an autoclave).
    • Flaming the inoculating loop before and after use.
    • Working near a Bunsen burner (creates an updraft of sterile air).
    • Minimizing the time culture plates or tubes are open.

Sterilization Techniques

Key Definitions

• Sterilization: The complete removal or destruction of all forms of microbial life, including endospores.
• Disinfection: The destruction of vegetative (non-spore) pathogens. Does not kill spores. Used on inanimate objects (e.g., bleach).
• Antisepsis: Disinfection of living tissue (e.g., iodine on a wound).

Physical Methods for Sterilization

1. Heat: The most common method.
   • Moist Heat (Autoclave): This is the gold standard. Uses steam under pressure (121°C, 15 psi, 15-20 minutes). It is effective because the high pressure allows steam to reach temperatures above boiling, and the moisture rapidly denatures proteins. Kills endospores.
   • Dry Heat: Used for glassware, powders, and oils that would be damaged by steam. Requires higher temperatures and longer times (e.g., 170°C for 2 hours in a hot-air oven).
   • Pasteurization: This is not sterilization! It is a mild heating process (e.g., 72°C for 15 seconds) that kills pathogens and reduces spoilage microbes in food, but does not kill spores.
2. Filtration:
   • Mechanism: Physically removes microbes by passing the liquid or gas through a filter with microscopic pores (e.g., 0.22 µm).
   • Use: For heat-sensitive liquids, such as antibiotics, vitamins, and some vaccines.
3. Radiation:
   • Ionizing Radiation (X-rays, Gamma rays): Has high energy and deep penetration. Used to sterilize disposable medical supplies (syringes, gloves) and some food.
   • Non-ionizing Radiation (UV light): Has low penetration. Used to sterilize surfaces, air, and water. It works by damaging DNA.

Chemical Methods for Sterilization

These are often disinfectants, but some can achieve sterilization with long exposure.

• Alcohols (e.g., Ethanol, Isopropanol): Good disinfectants, but not sporicidal (don't kill spores). Used at 70-90% concentration.
• Halogens (e.g., Chlorine/Bleach, Iodine): Broad-spectrum disinfectants.
• Phenols (e.g., Lysol): Effective surface disinfectants, stable in the presence of organic matter.
• Gases (e.g., Ethylene Oxide): A true sterilizing gas. Used for heat-sensitive hospital equipment (e.g., plastic tubing, electronics). It is highly penetrating but also toxic and explosive.