Microbial Metabolism, Growth, Control, and Antimicrobial Drugs – Study Guide

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 species.

• Photoheterotroph: Organisms that use light for energy but require organic compounds as a carbon source. Example: 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 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 Enzymatic 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: Involves glycolysis followed by conversion of pyruvate to various end products (e.g., lactic acid, ethanol). The final electron acceptor is an organic molecule.

• Main Difference: Cellular respiration yields more ATP and uses an electron transport chain; fermentation yields less ATP and does not use an electron transport chain.

Oxidase Test

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

• Application: Used to differentiate between bacterial species, especially Gram-negative rods.

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 converted into intermediates for the Krebs cycle.

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; little to no cell division.

• 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 accumulation of toxic products.

• 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): Converts superoxide radicals to hydrogen peroxide.

  • Catalase: Converts hydrogen peroxide to water and oxygen.

  • Peroxidase: Also breaks down hydrogen peroxide.

  • Distribution: Obligate aerobes and facultative anaerobes possess these enzymes; obligate anaerobes typically lack them.

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: Destruction of all microbial life, including spores.

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

• Antisepsis: Destruction of pathogens on living tissue.

• Sanitization: Reduction of microbial population to safe levels.

-cidal vs. -static Treatments

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

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

• Bactericidal: Kill bacteria directly (e.g., penicillin).

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 G targets Gram-positive bacteria).

Superinfections

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

• Significance: Can complicate therapy and patient recovery (e.g., Clostridioides difficile after broad-spectrum antibiotics).

Dosage and Route of Administration

• Dosage: Must achieve effective concentration without toxicity.

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

Drug Interactions

• Positive Interactions (Synergism): Combined drugs enhance each other's effects (e.g., trimethoprim and sulfamethoxazole).

• 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, 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 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

  • Bypass of metabolic pathways

• Genetic Processes:

  • Transformation: Uptake of naked DNA from the environment.

  • Transduction: Transfer of DNA by bacteriophages.

  • Conjugation: Transfer of plasmids via direct cell-to-cell contact.

Summary Table: Mechanisms of Antimicrobial Drug Resistance

Mechanism	Description	Example
Enzymatic Inactivation	Microbe produces enzymes that destroy or inactivate the drug	β-lactamase inactivates penicillins
Altered Target Site	Mutation or modification of drug target	Altered ribosomal proteins prevent macrolide binding
Decreased Permeability	Changes in membrane proteins reduce drug uptake	Porin mutations in Gram-negative bacteria
Efflux Pumps	Active transport of drug out of the cell	Tetracycline efflux pumps
Metabolic Bypass	Alternative pathways circumvent drug action	Acquisition of new enzymes for folic acid synthesis