Microbial Metabolism

Catabolism vs. Anabolism

Microbial metabolism encompasses all chemical reactions within a microbe, divided into two main processes: catabolism and anabolism.

• Catabolism: The breakdown of complex molecules into simpler ones, releasing energy. Example: Glycolysis breaks down glucose to pyruvate, generating ATP.

• Anabolism: The synthesis of complex molecules from simpler ones, requiring energy input. Example: Protein synthesis from amino acids.

Types of Microbial Nutrition

• Chemoheterotroph: Organisms that obtain energy and carbon from organic compounds. Most bacteria, fungi, and animals fall into this category.

• Chemoautotroph: Organisms that obtain energy from inorganic compounds and carbon from CO2. Example: Nitrifying bacteria.

• Photoheterotroph: Organisms that use light for energy but require organic compounds for carbon. Example: Some purple non-sulfur bacteria.

• Photoautotroph: Organisms that use light for energy and CO2 as a carbon source. Example: Cyanobacteria, plants.

Enzymes: Properties and Function

• Structure: Enzymes are biological catalysts, typically proteins, with a specific active site for substrate binding.

• Factors Influencing Rate: Temperature, pH, substrate concentration, enzyme concentration, and presence of inhibitors or activators.

• Function: Lower activation energy to speed up biochemical reactions without being consumed.

• Types of Inhibition:

  • Competitive inhibition: Inhibitor binds to the active site, blocking substrate.

  • Noncompetitive inhibition: Inhibitor binds elsewhere, altering enzyme shape and function.

Cellular Respiration vs. Fermentation

• Cellular Respiration: Complete oxidation of substrates with oxygen (aerobic) or other molecules (anaerobic) as final electron acceptors. Produces more ATP.

• Fermentation: Partial oxidation of substrates; organic molecules serve as final electron acceptors. Produces less ATP.

• Key Difference: Type of final electron acceptor (inorganic in respiration, organic in fermentation).

Oxidase Test

• Purpose: Detects the presence of cytochrome c oxidase enzyme in bacteria, helping differentiate between bacterial species (e.g., Pseudomonas vs. Enterobacteriaceae).

Lipid and Protein Catabolism

• Lipid Catabolism: Lipases break down lipids into fatty acids and glycerol. Fatty acids enter β-oxidation to form acetyl-CoA for the Krebs cycle.

• Protein Catabolism: Proteases degrade proteins into amino acids, which are deaminated and enter central metabolic pathways.

Importance of Microbial Metabolism

• Understanding metabolism aids in identifying microbes, developing antibiotics, and controlling microbial growth in clinical and industrial settings.

Microbial Growth

Growth Curve Phases

Bacterial populations grow in a predictable pattern known as the growth curve:

• Lag Phase: Adaptation, no cell division.

• Log (Exponential) Phase: Rapid cell division; most sensitive to antibiotics.

• Stationary Phase: Nutrient depletion slows growth; cell death equals cell division.

• Death Phase: Cells die faster than they divide.

Clinical Importance: The log phase is crucial for antibiotic effectiveness; stationary phase may increase resistance.

Oxygen Requirements for Growth

• Obligate Aerobes: Require oxygen; use aerobic respiration.

• Obligate Anaerobes: Oxygen is toxic; use anaerobic respiration or fermentation.

• Facultative Anaerobes: Can grow with or without oxygen; prefer aerobic respiration but can ferment.

• Microaerophiles: Require low oxygen levels.

• Aerotolerant Anaerobes: Do not use oxygen but tolerate its presence.

Protective Enzymes: Superoxide dismutase (SOD), catalase, and peroxidase detoxify reactive oxygen species. Aerobes and facultative anaerobes possess these enzymes.

Temperature Requirements

• Psychrophiles: Grow best at 0–15°C.

• Mesophiles: Grow best at 20–45°C (includes most human pathogens).

• Thermophiles: Grow best at 45–80°C.

• Hyperthermophiles: Grow above 80°C.

pH Requirements

• Acidophiles: Thrive at pH < 5.5.

• Neutrophiles: Thrive at pH 5.5–8.0 (most pathogens).

• Alkaliphiles: Thrive at pH > 8.0.

Halophiles and Osmosis

• Obligate Halophiles: Require high salt concentrations for growth.

• Facultative Halophiles: Tolerate high salt but do not require it.

• Relation to Osmosis: High salt environments cause water to leave cells (osmosis); halophiles have adaptations to prevent dehydration.

Control of Microbial Growth

Terminology

• Sterilization: Complete destruction of all microbial life, including spores.

• Disinfection: Elimination of most pathogens (not spores) on inanimate objects.

• Antisepsis: Destruction of pathogens on living tissue.

• Sanitization: Reduction of microbial population to safe levels.

-cidal vs. -static Treatments

• -cidal: Agents that kill microbes (e.g., bactericidal).

• -static: Agents that inhibit growth without killing (e.g., bacteriostatic).

Physical Control Methods

• Heat: Moist heat (autoclaving) and dry heat (oven) denature proteins and kill microbes.

• Filtration: Physically removes microbes from liquids or air.

Chemical Control Methods

• Alcohols: Denature proteins and disrupt membranes (e.g., ethanol, isopropanol).

• Halogens: Oxidize cell components (e.g., chlorine, iodine).

Antimicrobial Drugs

Bacteriostatic vs. Bactericidal

• Bacteriostatic: Inhibit bacterial growth; immune system clears infection.

• Bactericidal: Kill bacteria directly.

Broad-Spectrum vs. Narrow-Spectrum Drugs

• Broad-Spectrum: Effective against a wide range of bacteria (both Gram-positive and Gram-negative).

• Narrow-Spectrum: Target specific types of bacteria.

Superinfections

• Occur when normal microbiota are disrupted by antibiotics, allowing opportunistic pathogens to proliferate (e.g., Clostridioides difficile infection after broad-spectrum antibiotic use).

Dosage and Route of Administration

• Dosage: Must achieve therapeutic levels without toxicity.

• Route: Oral, intravenous, or topical; affects drug absorption and effectiveness.

Drug Interactions

• Positive Interactions: Synergism—combined effect greater than sum (e.g., penicillin and aminoglycoside).

• Negative Interactions: Antagonism—one drug reduces the effect of another.

Mechanisms of Action of Antibacterial Drugs

• Inhibition of Cell Wall Biosynthesis: e.g., Penicillins, cephalosporins.

• Inhibition of Protein Synthesis: e.g., Tetracyclines, macrolides.

• Disruption of Membrane Function: e.g., Polymyxins.

• Inhibition of Nucleic Acid Synthesis: e.g., Quinolones, rifamycins.

• Inhibition of Metabolic Pathways: e.g., Sulfonamides (folic acid synthesis inhibitors).

Modes of Action: Antifungal, Antiprotozoan, Antihelminthic, and Antiviral Drugs

• Antifungal: Target ergosterol in fungal membranes (e.g., amphotericin B).

• Antiprotozoan: Inhibit DNA synthesis or metabolic pathways (e.g., metronidazole).

• Antihelminthic: Disrupt microtubule function or energy metabolism (e.g., mebendazole).

• Antiviral: Inhibit viral entry, replication, or release (e.g., acyclovir, oseltamivir).

Development and Acquisition of Drug Resistance

• Transformation: Uptake of free DNA from the environment.

• Transduction: Transfer of DNA by bacteriophages.

• Conjugation: Direct transfer of DNA via cell-to-cell contact (plasmids).

Mechanisms of Antimicrobial Drug Resistance

• Enzymatic drug inactivation (e.g., β-lactamases).

• Alteration of drug targets (e.g., mutated ribosomes).

• Decreased permeability or increased efflux of drugs.

• Bypass of metabolic pathways targeted by drugs.

Additional info: Understanding these mechanisms is crucial for developing new drugs and managing antibiotic resistance in clinical settings.