Microbial Metabolism, Nutrition, and Growth: Study Notes

Microbial Metabolism

Introduction to Metabolism

Microbial metabolism encompasses the collection of controlled biochemical reactions that occur within a microbe. The ultimate function of metabolism is to enable the reproduction and survival of the organism.

• Metabolism: The sum of all chemical reactions within a cell.

• Purpose: To provide energy and building blocks for growth and reproduction.

Eight Elementary Statements Guiding Metabolic Processes

• Every cell acquires nutrients.

• Metabolism requires energy from light or from catabolism of nutrients.

• Energy is stored in adenosine triphosphate (ATP).

• Cells catabolize nutrients to form precursor metabolites.

• Precursor metabolites, energy from ATP, and enzymes are used in anabolic reactions.

• Enzymes plus ATP form macromolecules.

• Cells grow by assembling macromolecules.

• Cells reproduce once they have doubled in size.

Chemical Reactions Underlying Metabolism

Anabolism and Catabolism

Metabolism consists of two major classes of reactions: catabolic and anabolic pathways.

• Catabolic pathways: Break larger molecules into smaller products; these reactions are exergonic (release energy).

• Anabolic pathways: Synthesize large molecules from the smaller products of catabolism; these reactions are endergonic (require energy input).

ATP is the energy currency that links catabolism and anabolism.

Metabolic Cycle Diagram

Catabolism converts complex molecules (carbohydrates, lipids, proteins, DNA, RNA) into simple molecules (fatty acids, amino acids, sugars, nucleotides), releasing energy and producing ATP. Anabolism uses ATP to build complex molecules from simple ones.

Roles of Enzymes in Metabolism

Enzyme Function and Structure

Enzymes are organic catalysts that increase the likelihood of chemical reactions by lowering the activation energy required.

• Apoenzyme: The protein portion of an enzyme, inactive unless bound to a cofactor.

• Cofactor: Non-protein component (can be inorganic ions or organic molecules called coenzymes).

• Holoenzyme: The active enzyme formed by the combination of apoenzyme and its cofactor(s).

• Ribozymes: RNA molecules with catalytic activity.

Enzyme-Substrate Interaction

• Enzymes have an active site that binds specifically to the substrate, forming an enzyme-substrate complex.

• Enzymes lower the activation energy of reactions, increasing reaction rates.

Equation:

$E + S \rightarrow ES \rightarrow E + P$

Where E is enzyme, S is substrate, ES is enzyme-substrate complex, and P is product.

Factors Affecting Enzyme Activity

• Temperature: Affects enzyme structure and activity.

• pH: Alters enzyme shape and function.

• Enzyme and substrate concentrations: Influence reaction rates.

• Presence of inhibitors: Can block enzyme activity without denaturing the enzyme.

Types of Enzyme Inhibition

• Competitive inhibition: Inhibitor competes with substrate for the active site.

• Allosteric inhibition: Inhibitor binds to a site other than the active site, causing a conformational change that reduces activity.

• Allosteric activation: Activator binds to an allosteric site, increasing enzyme activity.

• Feedback inhibition: End product of a pathway inhibits an enzyme involved earlier in the pathway.

Carbohydrate Catabolism

Overview of Glucose Catabolism

Microorganisms oxidize carbohydrates, primarily glucose, to generate energy for anabolic reactions. Glucose catabolism occurs via two main processes: cellular respiration and fermentation.

Glycolysis

Glycolysis is the metabolic pathway that splits a six-carbon glucose molecule into two three-carbon pyruvic acid molecules. It occurs in the cytoplasm and involves substrate-level phosphorylation.

• Net gain: 2 ATP, 2 NADH, and 2 pyruvic acid molecules per glucose.

Equation:

$\text{Glucose} + 2 \text{NAD}^+ + 2 \text{ADP} + 2 \text{P}_i \rightarrow 2 \text{Pyruvate} + 2 \text{NADH} + 2 \text{ATP} + 2 \text{H}_2\text{O}$

Cellular Respiration

Cellular respiration is the complete oxidation of pyruvic acid to produce ATP through a series of redox reactions. It consists of three stages:

1. Synthesis of acetyl-CoA

2. Krebs cycle (Citric Acid Cycle)

3. Electron transport chain (ETC)

Synthesis of Acetyl-CoA

• Pyruvic acid is decarboxylated and combined with coenzyme A to form acetyl-CoA.

• Produces 2 acetyl-CoA, 2 CO2, and 2 NADH per glucose.

Krebs Cycle

• Occurs in the cytosol of prokaryotes and mitochondrial matrix of eukaryotes.

• Transfers energy to NAD+ and FAD.

• Produces 2 ATP, 6 NADH, 2 FADH2, and 4 CO2 per glucose.

Electron Transport Chain (ETC)

• Located in the inner mitochondrial membrane (eukaryotes) or cytoplasmic membrane (prokaryotes).

• Electrons are passed through a series of carriers to a final electron acceptor (O2 in aerobic respiration).

• Energy from electrons pumps protons, creating a proton gradient used to generate ATP via oxidative phosphorylation.

• Total ATP yield in eukaryotes: ~38 ATP per glucose.

Fermentation

Fermentation is an alternative pathway for energy production when cells cannot fully oxidize glucose via respiration. It regenerates NAD+ by transferring electrons to organic molecules, allowing glycolysis to continue.

• Produces less ATP than respiration.

• Common products: lactic acid, ethanol, CO2, and others.

Microbial Nutrition and Growth

Microbial Growth

Microbial growth refers to an increase in the population of microbes, typically resulting in the formation of colonies derived from a single parent cell.

• Growth is dependent on nutrient availability and environmental conditions.

Chemical Requirements for Growth

• Carbon: Autotrophs use CO2; heterotrophs use organic carbon.

• Energy: Phototrophs use light; chemotrophs use chemical compounds.

• Electrons: Organisms may use organic or inorganic sources.

Energy Source	Carbon Source	Example Organisms
Light	CO2	Plants, algae, cyanobacteria (photoautotrophs)
Chemicals	CO2	Hydrogen, sulfur, nitrifying bacteria (chemoautotrophs)
Light	Organic compounds	Green and purple non-sulfur bacteria (photoheterotrophs)
Chemicals	Organic compounds	Most bacteria, fungi, protozoa, animals (chemoheterotrophs)

Oxygen Requirements

• Obligate aerobes: Require oxygen for growth.

• Obligate anaerobes: Oxygen is toxic; cannot grow in its presence.

• Facultative anaerobes: Can grow with or without oxygen.

• Aerotolerant anaerobes: Tolerate oxygen but do not use it.

• Microaerophiles: Require low levels of oxygen.

Toxic forms of oxygen include singlet oxygen, superoxide radicals, peroxide anion, and hydroxyl radical. These can cause cellular damage.

Nitrogen, Phosphorus, Sulfur, and Trace Elements

• Nitrogen: Essential for amino acids and nucleotides; some bacteria fix atmospheric nitrogen.

• Phosphorus: Needed for nucleic acids, ATP, and membranes.

• Sulfur: Required for some amino acids and vitamins.

• Trace elements: Required in small amounts (e.g., iron, copper, zinc).

• Growth factors: Organic compounds that some organisms cannot synthesize (e.g., vitamins).

Physical Requirements for Growth

• Temperature: Affects protein structure and membrane fluidity. Microbes are classified by optimal growth temperature:

  • Psychrophiles: Cold-loving (0–20°C)

  • Mesophiles: Moderate temperature (20–40°C)

  • Thermophiles: Heat-loving (40–80°C)

  • Hyperthermophiles: Extreme heat (>80°C)

• pH: Most microbes are neutrophiles (pH ~7); acidophiles prefer acidic environments; alkalinophiles thrive in basic conditions.

• Water: Essential for dissolving nutrients and metabolic reactions. Some microbes withstand dry conditions by forming endospores or cysts.

• Osmotic pressure: Influences water movement; halophiles thrive in high-salt environments.

• Hydrostatic pressure: Barophiles live under high pressure, such as deep-sea environments.

Microbial Associations and Biofilms

Types of Relationships

• Antagonistic: One organism harms another.

• Synergistic: Both organisms benefit, but not required for survival.

• Symbiotic: Close, long-term interaction; often required for survival.

Biofilms

Biofilms are complex communities of microorganisms attached to surfaces and embedded in an extracellular matrix. They facilitate nutrient acquisition, protection, and communication (quorum sensing).

• Biofilms can increase microbial resistance to antibiotics and host defenses.

Growth of Microbial Populations

Generation Time and Growth Curve

Generation time is the time required for a bacterial cell to grow and divide. It depends on chemical and physical conditions.

Microbial growth follows a characteristic curve:

• Lag phase: Cells adapt to environment; little growth.

• Log (exponential) phase: Rapid cell division and population increase.

• Stationary phase: Nutrient depletion slows growth; population stabilizes.

• Death (decline) phase: Cells die due to lack of nutrients and accumulation of waste.

Equation for exponential growth:

$N_t = N_0 \times 2^n$

Where $N_t$ is the population at time t, $N_0$ is the initial population, and $n$ is the number of generations.