microbial media and culture techniques

Chapter 5: Microbial Nutrition

Microbial Transport Mechanisms

• Facilitated Diffusion:

  • Transport proteins carry molecules across membranes along concentration gradients without energy.

• Active Transport:

  • Requires metabolic energy and moves substances against gradients using membrane carrier proteins.

  • Provides advantage over facilitated diffusion by enabling accumulation of molecules despite concentration gradients, essential for nutrient uptake in low availability.

• Group Translocation:

  • Involves the transport and modification of molecules (e.g., phosphorylation of sugars) during transport across the membrane.

Types of Transport Processes

• Symport and Antiport:

  • Symport: Moving two substances in the same direction across a membrane.

  • Antiport: One substance moves in while another moves out.

Iron Uptake in Microorganisms

• Siderophores:

  • Specialized molecules that bind ferric iron to facilitate its transport into bacterial cells.

  • Once inside, ferric iron is reduced to ferrous iron for use by the microorganism.

Culture Media in Microbiology

• Culture Medium: A nutritive solution for the growth of microorganisms.

• Types of Media:

  • Defined (Synthetic): All components known; used for specific microbial growth (e.g., BG-11 for cyanobacteria).

  • Complex: Ingredients not wholly known; supports broad microbial diversity (e.g., nutrient broth, tryptic soy broth).

• Functional Types of Media:

  • Supportive (General Purpose): Promote growth of many types.

  • Enriched: Contain special nutrients for fastidious organisms (e.g., blood agar).

  • Selective: Favor specific microorganisms while inhibiting others (e.g., MacConkey agar for gram-negative bacteria).

  • Differential: Distinguish between different species based on specific characteristics (e.g., blood agar for hemolytic activity).

Techniques for Isolation of Pure Cultures

• Spread Plate Technique:

  • A diluted microbial sample is spread across an agar plate to isolate colonies.

• Streak Plate Technique:

  • Dilution from a mixed culture onto an agar surface through a streaking process to separate colonies.

• Pour Plate Technique:

  • Diluting a microbial sample and mixing with liquid agar before plating to also achieve isolation of colonies.

Importance of Pure Cultures

• Pure cultures are critical for studying individual microorganisms, establishing relationships in disease, and metabolic studies.

Summary Points

• Microorganisms require a variety of nutrients categorized as macronutrients and micronutrients for growth.

• Different microorganisms have specific nutritional needs, influencing culture media design and selection methods.