Microbial Metabolism

Catabolism vs. Anabolism

• Catabolism: The set of metabolic pathways that break down molecules into smaller units to release energy. These reactions are generally exergonic (energy-releasing).

• Anabolism: The set of metabolic pathways that construct molecules from smaller units, requiring energy input. These reactions are endergonic (energy-consuming).

• Example: Glycolysis is a catabolic pathway, while protein synthesis is anabolic.

Types of Microbial Nutrition

• Chemoheterotroph: Organisms that obtain both energy and carbon from organic compounds. Most bacteria, fungi, and animals are chemoheterotrophs.

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

• Photoheterotroph: Organisms that use light for energy but require organic compounds as a carbon source. Example: Rhodobacter.

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

Enzymes: Properties and Function

• Structure: Enzymes are biological catalysts, typically proteins, with an active site where substrates bind.

• Factors Influencing Enzyme Activity:

  • Temperature

  • pH

  • Substrate concentration

  • Presence of inhibitors or activators

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

• Types of Enzyme Inhibition:

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

  • Noncompetitive inhibition: Inhibitor binds to an allosteric site, changing enzyme shape.

Cellular Respiration vs. Fermentation

• Cellular Respiration: Involves glycolysis, Krebs cycle, and electron transport chain. The final electron acceptor is usually oxygen (aerobic) or another inorganic molecule (anaerobic).

• Fermentation: Incomplete oxidation of glucose; the final electron acceptor is an organic molecule. Produces less ATP than respiration.

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

Oxidase Test

• Purpose: Detects the presence of cytochrome c oxidase enzyme in bacteria, which is part of the electron transport chain.

• Application: Differentiates between bacterial species, e.g., Pseudomonas (oxidase-positive) vs. Escherichia (oxidase-negative).

Lipid and Protein Catabolism

• Lipid Catabolism: Lipids are broken down by lipases into fatty acids and glycerol. Fatty acids enter β-oxidation to generate acetyl-CoA for the Krebs cycle.

• Protein Catabolism: Proteins are degraded by proteases into amino acids, which are deaminated and enter central metabolic pathways.

Importance of Microbial Metabolism

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

Microbial Growth

Growth Curve Phases

• Lag Phase: Cells adapt to new environment; no immediate increase in cell number.

• Log (Exponential) Phase: Rapid cell division; population doubles at a constant rate.

• Stationary Phase: Growth rate slows; nutrients deplete, waste accumulates; cell death equals cell division.

• Death Phase: Cells die at an exponential rate due to lack of nutrients and toxic waste buildup.

• Clinical Importance: Different phases affect susceptibility to antibiotics and infection progression.

Oxygen Requirements for Microbial Growth

• Obligate Aerobes: Require oxygen; use aerobic respiration.

• Facultative Anaerobes: Grow with or without oxygen; use aerobic respiration, anaerobic respiration, or fermentation.

• Obligate Anaerobes: Cannot tolerate oxygen; use anaerobic respiration or fermentation.

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

• Microaerophiles: Require low levels of oxygen.

• Protective Enzymes: Superoxide dismutase (SOD), catalase, and peroxidase protect cells from toxic oxygen radicals. 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 55–80°C.

• Hyperthermophiles: Grow above 80°C.

pH Requirements

• Acidophiles: Grow best at pH < 5.5.

• Neutrophiles: Grow best at pH 5.5–8.0.

• Alkaliphiles: Grow best at pH > 8.0.

Halophiles and Osmosis

• Obligate Halophiles: Require high salt concentrations for growth.

• Facultative Halophiles: Can 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 or removal of all forms of microbial life, including spores.

• Disinfection: Elimination of most pathogenic microorganisms (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, fungicidal).

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

Physical Control Methods

• Heat: Moist heat (autoclaving, boiling) and dry heat (oven) denature proteins and destroy microbes.

• Filtration: Physically removes microbes from liquids or air using filters.

Chemical Control Methods

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

• Halogens: Oxidize cellular 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 microbes (e.g., tetracycline).

• Narrow-Spectrum: Effective against specific groups (e.g., penicillin targets Gram-positive bacteria).

Superinfections

• Definition: Secondary infections caused by the overgrowth of non-susceptible microbes after antibiotic treatment.

• Significance: Disruption of normal flora can lead to infections like Clostridioides difficile colitis.

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 drugs enhance each other's effects.

• Negative Interactions (Antagonism): One drug reduces the effectiveness of another.

Mechanisms of Action of Antibacterial Drugs

• Inhibition of Cell Wall Biosynthesis: e.g., β-lactams (penicillins) inhibit peptidoglycan synthesis.

• Inhibition of Protein Synthesis: e.g., tetracyclines, macrolides target ribosomes.

• Disruption of Membrane Function: e.g., polymyxins disrupt bacterial membranes.

• Inhibition of Nucleic Acid Synthesis: e.g., quinolones inhibit DNA gyrase.

• Inhibition of Metabolic Pathways: e.g., sulfonamides inhibit folic acid synthesis.

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

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

• Antiprotozoal: Inhibit nucleic acid or protein synthesis (e.g., metronidazole).

• Antihelminthic: Disrupt metabolic processes or neuromuscular function (e.g., mebendazole).

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

Development and Acquisition of Drug Resistance

• Mechanisms:

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

  • Alteration of drug targets

  • Decreased permeability or increased efflux of drug

• Genetic Processes:

  • Transformation: Uptake of free DNA from the environment.

  • Transduction: Transfer of DNA by bacteriophages.

  • Conjugation: Direct transfer of DNA between bacteria via pili.

Mechanisms of Antimicrobial Drug Resistance

• Enzyme production that destroys or inactivates the drug

• Alteration of drug target sites

• Changes in membrane permeability or efflux pumps

• Bypass of metabolic pathways inhibited by the drug