Microbial Metabolism

Elemental and Macromolecular Composition of Bacterial Cells

Bacterial cells are composed of various elements and macromolecules essential for growth and metabolism. Understanding these components is fundamental to microbiology.

• Major Elements: Carbon (C), Nitrogen (N), Oxygen (O), Hydrogen (H), Phosphorus (P), Sulfur (S), Potassium (K), Magnesium (Mg), Calcium (Ca), Iron (Fe).

• Macromolecules: Proteins, nucleic acids, lipids, and polysaccharides make up the bulk of cellular material.

• Example: Escherichia coli cell dry weight composition: ~50% protein, ~20% RNA, ~10% lipids, ~10% polysaccharides, ~5% DNA.

Micronutrients Needed by Microorganisms

Micronutrients are required in trace amounts and are vital for enzymatic functions and cellular processes.

Culture Media for Microorganisms

Microorganisms require specific nutrients for growth, which are provided by culture media. Media can be simple or complex depending on the nutritional requirements of the organism.

• Simple Media: Contains basic nutrients for non-fastidious organisms.

• Complex Media: Contains additional growth factors for demanding organisms.

• Example: Minimal medium for E. coli contains glucose, ammonium, salts, and water.

Nitrogen Utilization and Fixation

Nitrogen is a critical element for microbial growth, used in amino acids, nucleic acids, and other cellular components.

• Sources: Ammonia (NH3), nitrate (NO3-), nitrogen gas (N2).

• Nitrogen Fixation: Conversion of atmospheric N2 to ammonia by nitrogenase enzyme complex.

• Equation:

• Protection from O2: Nitrogenase is oxygen-sensitive; protection mechanisms include high respiration rates and specialized cells (heterocysts).

Amino Acid Biosynthesis and Families

Amino acids are synthesized via multistep pathways and grouped into families based on their biosynthetic origins.

• Monomers of Proteins: Amino acids.

• Carbon Skeletons: Derived from glycolysis or citric acid cycle intermediates.

• Families: Glutamate family, aspartate family, serine family, etc.

• Example: Glutamate synthesized from α-ketoglutarate via reductive amination.

Sugar Metabolism

Sugars are metabolized through glycolysis and other pathways to provide energy and precursors for biosynthesis.

• Key Pathways: Glycolysis, pentose phosphate pathway, Entner-Doudoroff pathway.

• Example: Glucose is converted to pyruvate, generating ATP and NADH.

The Citric Acid Cycle

The citric acid cycle (Krebs cycle) is central to energy production and biosynthesis in aerobic organisms.

• Function: Oxidizes acetyl-CoA to CO2, generating NADH, FADH2, and ATP.

• Equation:

Microbial Growth and Its Control

Environmental Factors Influencing Growth

Microbial growth is influenced by several environmental factors, including temperature, pH, oxygen concentration, water activity, and salinity.

• Temperature: Microorganisms have minimum, optimum, and maximum growth temperatures.

• pH: Microbes are classified as acidophiles, neutrophiles, or alkaliphiles based on their pH preference.

• Oxygen: Aerobes, anaerobes, facultative anaerobes, microaerophiles, and aerotolerant organisms differ in oxygen requirements.

• Water Activity: Availability of water affects microbial growth; measured as aw.

• Salinity: Halophiles thrive in high salt concentrations; compatible solutes help maintain osmotic balance.

Relationships of Microorganisms to pH

Oxygen Relationships of Microorganisms

Enzymes That Destroy Toxic Oxygen Species

Microorganisms produce enzymes to neutralize reactive oxygen species (ROS).

• Catalase:

• Peroxidase:

• Superoxide Dismutase:

Temperature Classes of Microorganisms

Microbial Life in the Cold

Microorganisms adapt to cold environments through physiological and molecular mechanisms.

• Psychrophiles: Optimal growth at ≤15°C, minimum ≤0°C.

• Psychrotolerant: Can grow at 0°C, optimum 20-40°C.

• Adaptations: Enzymes with flexible structures, high unsaturated fatty acids in membranes, production of cryoprotectants (e.g., antifreeze proteins, exopolysaccharides).

• Example: Snow algae and Antarctic microbes thrive in seasonally or constantly cold habitats.

Microbial Life at High Temperatures

Thermophiles and hyperthermophiles are adapted to high-temperature environments, such as hot springs and hydrothermal vents.

• Thermophiles: Optimum growth between 45-80°C.

• Hyperthermophiles: Optimum growth above 80°C; some Archaea grow at up to 122°C.

• Adaptations: Heat-stable enzymes, saturated fatty acids in membranes, increased ionic bonds and hydrophobic interactions in proteins.

• Example: Thermus aquaticus produces Taq polymerase, used in PCR.

Water Activity and Compatible Solutes

Water activity (aw) measures the availability of water for microbial growth. Microorganisms use compatible solutes to survive in low water activity or high salt environments.

Effect of NaCl Concentration on Microbial Growth

Microorganisms vary in their tolerance to salt concentrations. Halophiles require high NaCl, while non-halophiles are inhibited by salt.

• Non-halophile: Optimal growth at low NaCl.

• Halotolerant: Can tolerate moderate NaCl.

• Halophile: Requires high NaCl for growth.

• Extreme halophile: Requires very high NaCl (e.g., Halobacterium).

*Additional info: Some details on enzyme adaptation, compatible solutes, and environmental classes were expanded for clarity and completeness.*