Lecture 14: Chemical Requirements for Microbial Growth

Introduction

• Focus: Chemical and molecular requirements for microbial growth
• Essential Chemical Elements: Carbon, hydrogen, nitrogen, sulfur, phosphorus, etc.

Major Chemical Requirements

• Macronutrients: Required in large amounts
  • CHNOPS: Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, Sulfur
  • Key Elements:
    • Carbon: Half of cell's dry weight, major element in macromolecules
    • Nitrogen: 13% of dry weight, key in proteins/nucleic acids
    • Oxygen & Hydrogen: Usually attached to carbon
    • Phosphorus: Nucleic acids, phospholipids, ATP
    • Sulfur: Amino acids (cysteine, methionine), vitamins

Sources of Macronutrients

• Heterotrophs: Obtain carbon from organic compounds (e.g., proteins, carbohydrates)
• Autotrophs: Use inorganic CO2; classified as:
  • Photoautotrophs: Use light
  • Chemoautotrophs: Oxidize inorganic substances

Nitrogen Sources

• Decomposing protein-containing material
• Ammonia and ammonium ions
• Nitrates or nitrogen gas (nitrogen fixation)

Sulfur and Phosphorus

• Sulfur: From sulfur-containing amino acids or inorganic sources
• Phosphorus: Supplied as inorganic phosphate

Micronutrients and Trace Elements

• Required in small amounts
• Examples: Iron, copper, molybdenum, zinc
• Role:
  • Iron: Cellular respiration, enzyme cofactors
• Growth Factors:
  • Amino acids, purines, pyrimidines, vitamins
  • Some microbes synthesize their own, others require environmental supply

Oxygen Requirements

• Oxygen's Role: Varies among microbes
  • Obligate Aerobes: Require oxygen, use it in respiration
  • Obligate Anaerobes: Oxygen is toxic
  • Facultative Anaerobes: Can grow with or without oxygen
  • Microaerophiles: Require low oxygen levels
  • Aerotolerant Anaerobes: Tolerate oxygen but don't use it

Reactive Oxygen Species

• Oxygen can form reactive, toxic byproducts
• Examples:
  • Singlet oxygen, superoxide anions, peroxide anions
• Defense Mechanisms:
  • Enzymes: Superoxide dismutase, catalase, peroxidase

Laboratory Assessment

• Thioglycolate Medium: Used to assess oxygen requirements
  • Contains reducing agents to remove traces of oxygen
  • Visual indicators like resazurin for oxygen presence

Additional Information

• Suggested Reading: Chapter 6 of the tutorial text
• Discussion of thio glycolate tube for practical lab application