Microbial Metabolism

Catabolism vs. Anabolism

Microbial metabolism encompasses all chemical reactions that occur within a microbe. These reactions are divided into two main categories:

• Catabolism: The breakdown of complex molecules into simpler ones, releasing energy. Example: Glycolysis.

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

Types of Microbial Nutritional Patterns

• Chemoheterotroph: Organisms that obtain 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: Nitrifying bacteria.

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

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

• Factors Influencing Enzyme Activity:

  • Temperature

  • pH

  • Substrate concentration

  • Presence of inhibitors

• Function: Enzymes lower the activation energy of 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. Final electron acceptor is usually oxygen (aerobic) or another inorganic molecule (anaerobic).

• Fermentation: Involves glycolysis followed by conversion of pyruvate to various end products. Final electron acceptor is an organic molecule. Less ATP produced than respiration.

Oxidase Test

• Detects the presence of cytochrome c oxidase enzyme in bacteria.

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

Lipid and Protein Catabolism

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

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

Importance of Microbial Metabolism

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

• Metabolic pathways are targets for antimicrobial drugs.

Microbial Growth

Growth Curve Phases

Bacterial growth in a closed system follows a predictable pattern:

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

• Log (Exponential) Phase: Rapid cell division; cells are most susceptible to antibiotics.

• Stationary Phase: Nutrient depletion and waste accumulation slow growth; cell death equals cell division.

• Death Phase: Cells die at an exponential rate.

Clinical Importance: The log phase is critical for antibiotic effectiveness; stationary phase cells may be more resistant to treatment.

Oxygen Requirements for 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.

• Obligate aerobes and facultative anaerobes possess both enzymes; obligate anaerobes lack them.

Temperature Requirements

• Psychrophiles: Grow at 0–15°C.

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

• Thermophiles: Grow 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.

• Halophilism relates to osmosis, as high salt environments draw water out of cells, requiring adaptations to prevent dehydration.

Control of Microbial Growth

Terminology

• Sterilization: Destruction of all microbial life, including spores.

• Disinfection: Destruction of vegetative pathogens on inanimate objects.

• Antisepsis: Destruction of pathogens on living tissue.

• Sanitization: Lowering microbial counts to safe public health 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 (Examples)

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

• Filtration: Removes microbes from liquids or air using physical barriers.

Chemical Control Methods (Examples)

• 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 for Gram-positive bacteria).

Superinfections

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

Dosage and Route of Administration

• Proper dosage ensures drug effectiveness and minimizes resistance.

• Route (oral, intravenous, topical) affects drug absorption and distribution.

Drug Interactions

• Positive Interactions (Synergism): Combined drugs have enhanced effect (e.g., trimethoprim and sulfamethoxazole).

• Negative Interactions (Antagonism): One drug reduces the effect 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 helminth metabolism or neuromuscular function (e.g., mebendazole).

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

Development and Acquisition of Drug Resistance

• Microbes can acquire resistance through genetic changes:

  • Transformation: Uptake of free DNA from the environment.

  • Transduction: Transfer of DNA by bacteriophages.

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

Mechanisms of Antimicrobial Drug Resistance

• Enzymatic destruction or inactivation of the drug (e.g., β-lactamases).

• Alteration of drug target sites.

• Decreased permeability or increased efflux of the drug.

• Bypass of metabolic pathways inhibited by the drug.