Microbial Metabolism

Phototrophs, Chemotrophs, Autotrophs, and Heterotrophs

Microorganisms obtain energy and carbon in various ways, which are fundamental to their metabolism and ecological roles.

• Phototrophs: Organisms that use light as their energy source. Example: Cyanobacteria, algae.

• Chemotrophs: Organisms that obtain energy from chemical compounds. Example: Most bacteria, such as Escherichia coli.

• Autotrophs: Organisms that use carbon dioxide as their principal carbon source. Example: Plants, some bacteria.

• Heterotrophs: Organisms that require organic carbon sources. Example: Fungi, animals, many bacteria.

Additional info: Some organisms combine these strategies, such as photoautotrophs (light for energy, CO2 for carbon) and chemoheterotrophs (chemical energy, organic carbon).

Oxygenic vs. Anoxygenic Photosynthesis

Photosynthesis can be classified based on whether oxygen is produced.

• Oxygenic Photosynthesis: Produces oxygen; performed by cyanobacteria, algae, and plants.

• Anoxygenic Photosynthesis: Does not produce oxygen; performed by certain bacteria (e.g., purple sulfur bacteria).

• Key Difference: Oxygenic uses water as electron donor; anoxygenic uses other compounds (e.g., H2S).

Example: Cyanobacteria perform oxygenic photosynthesis, while green sulfur bacteria perform anoxygenic photosynthesis.

Dynamics of Microbial Growth

Definition and Requirements for Microbial Growth

Microbial growth refers to the increase in the number of cells, not cell size. Growth depends on physical and chemical factors.

• Physical Factors: Temperature, pH, osmotic pressure.

• Chemical Factors: Carbon, nitrogen, sulfur, phosphorus, oxygen, trace elements.

Effects of Temperature, pH, and Osmotic Pressure

• Temperature: Microbes are classified as psychrophiles (cold-loving), mesophiles (moderate temperature), thermophiles (heat-loving).

• pH: Most bacteria grow best near neutral pH (6.5–7.5); acidophiles thrive in acidic environments.

• Osmotic Pressure: High salt or sugar concentrations can inhibit growth; halophiles tolerate high salt.

Chemical Requirements for Growth

• Carbon: Needed for all organic molecules.

• Nitrogen: Required for proteins, nucleic acids.

• Sulfur: Needed for amino acids and vitamins.

• Phosphorus: Essential for nucleic acids, ATP.

• Oxygen: Used by aerobes; toxic to some anaerobes.

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

Oxygen Requirements of Microorganisms

• Obligate Aerobes: Require oxygen.

• Obligate Anaerobes: Cannot tolerate oxygen.

• Facultative Anaerobes: Can grow with or without oxygen.

• Microaerophiles: Require low oxygen concentrations.

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

Biofilms: Formation and Significance

Biofilms are communities of microorganisms attached to surfaces, embedded in a self-produced matrix.

• Formation: Cells adhere, multiply, and produce extracellular polymeric substances (EPS).

• Medical Significance: Biofilms cause persistent infections (e.g., on catheters).

• Industrial Significance: Biofilms can foul equipment and pipes.

Preservation of Microbial Cultures

• Methods: Deep freezing, lyophilization (freeze-drying), refrigeration.

• Purpose: Maintain cultures for long-term study or industrial use.

Bacterial Growth Curve

Bacterial populations grow in a predictable pattern in batch culture.

• Lag Phase: Adaptation, no increase in cell number.

• Log (Exponential) Phase: Rapid cell division.

• Stationary Phase: Growth rate slows, nutrients deplete.

• Death Phase: Cells die off.

Additional info: The growth curve is often plotted as log(cell number) vs. time.

Controlling Microbial Growth

Definitions: Sterilization, Disinfection, Antisepsis, Degerming, Sanitization, Biocide/Germicide

• Sterilization: Removal or destruction of all microbial life.

• Disinfection: Elimination of most pathogens (not necessarily spores).

• Antisepsis: Disinfection of living tissue.

• Degerming: Removal of microbes from a limited area (e.g., skin before injection).

• Sanitization: Lowering microbial counts to safe levels.

• Biocide/Germicide: Agents that kill microbes.

Patterns of Microbial Death and Factors Affecting Antimicrobial Treatments

• Microbial Death: Occurs at a constant rate; not all cells die at once.

• Factors: Number of microbes, environment, time of exposure, microbial characteristics.

Actions of Microbial Control Agents

• Cell Membranes: Disruption leads to leakage of cell contents.

• Proteins: Denaturation impairs function.

• Nucleic Acids: Damage prevents replication and metabolism.

Physical vs. Chemical Methods of Microbial Control

• Physical Methods: Heat, filtration, radiation, low temperature, high pressure, desiccation, osmotic pressure.

• Chemical Methods: Disinfectants, antiseptics, sterilants.

Heat Methods: Moist Heat, Dry Heat, Pasteurization

• Moist Heat: Boiling, autoclaving; denatures proteins.

• Dry Heat: Flaming, incineration; oxidizes cell components.

• Pasteurization: Reduces pathogens; does not sterilize.

• Limitations: Some spores and thermophiles survive.

Other Physical Methods: Filtration, Low Temperature, High Pressure, Desiccation, Osmotic Pressure

• Filtration: Removes microbes from liquids or air.

• Low Temperature: Slows microbial growth.

• High Pressure: Alters protein structure.

• Desiccation: Removes water; inhibits growth.

• Osmotic Pressure: High salt/sugar inhibits growth.

Radiation: Ionizing and Non-Ionizing

• Ionizing Radiation: (e.g., X-rays, gamma rays) damages DNA.

• Non-Ionizing Radiation: (e.g., UV light) causes thymine dimers in DNA.

• Uses: Sterilization of medical equipment, disinfection of surfaces.

Principles of Effective Disinfection

• Factors: Concentration, contact time, presence of organic matter, pH.

• Selection: Based on target microbes, application, safety.

Major Chemical Disinfectants and Antiseptics

• Phenolics: Disrupt membranes; used in soaps.

• Alcohols: Denature proteins; used for skin.

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

• Heavy Metals: (e.g., silver, mercury) inhibit enzymes.

• Surfactants: Lower surface tension; aid removal.

• Aldehydes: Cross-link proteins; used for sterilization.

Chemical Sterilants and Gaseous Agents

• Ethylene Oxide: Gaseous sterilant; used for medical devices.

• Peroxygens: (e.g., hydrogen peroxide) oxidize cell components.

Bactericidal vs. Bacteriostatic Actions

• Bactericidal: Kills bacteria.

• Bacteriostatic: Inhibits growth; allows immune system to clear infection.

• Appropriate Use: Bactericidal for severe infections; bacteriostatic for less critical cases.

Antimicrobial Drugs

Chemotherapy, Antimicrobial Drugs, and Antibiotics

• Chemotherapy: Treatment of disease with chemicals.

• Antimicrobial Drugs: Agents that kill or inhibit microbes.

• Antibiotics: Naturally produced antimicrobial drugs.

Historical Development of Antimicrobial Drugs

• Paul Ehrlich: Developed first synthetic drug (Salvarsan).

• Alexander Fleming: Discovered penicillin.

• Additional info: Many modern drugs are derivatives or modifications of natural compounds.

Selective Toxicity

• Principle: Drug should harm pathogen, not host.

• Challenge: Harder with eukaryotic pathogens due to similarity to human cells.

Modes of Action of Antimicrobial Drugs

• Inhibition of Cell Wall Synthesis: e.g., penicillins.

• Inhibition of Protein Synthesis: e.g., tetracyclines.

• Inhibition of Nucleic Acid Replication and Transcription: e.g., rifamycins.

• Disruption of Plasma Membrane: e.g., polymyxins.

• Inhibition of Essential Metabolite Synthesis: e.g., sulfonamides.

Representative Drugs for Each Mode of Action

Broad-Spectrum vs. Narrow-Spectrum Antibiotics

• Broad-Spectrum: Effective against many types of microbes; used when pathogen is unknown.

• Narrow-Spectrum: Targets specific microbes; reduces risk of resistance and side effects.

Mechanisms of Bacterial Resistance

• Enzymatic Destruction: e.g., beta-lactamases.

• Prevention of Drug Entry: Altered permeability.

• Target Alteration: Mutation changes drug target.

• Efflux Pumps: Expel drug from cell.

Spread of Resistance Genes

• Horizontal Gene Transfer: Conjugation, transformation, transduction.

Antibiotic Misuse: Causes and Consequences

• Causes: Overprescription, use in agriculture, consumer products.

• Consequences: Increased resistance, loss of drug efficacy.

Bactericidal vs. Bacteriostatic Drugs

• Bactericidal: Kills bacteria; used for life-threatening infections.

• Bacteriostatic: Inhibits growth; used when host immunity is sufficient.

Mechanisms of Antiviral, Antifungal, Antiprotozoan, and Antihelminthic Drugs

• Antiviral: Inhibit viral replication (e.g., acyclovir).

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

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

• Antihelminthic: Disrupt worm metabolism (e.g., mebendazole).

Challenges in Developing New Antimicrobial Agents

• Emerging Resistance: Pathogens evolve rapidly.

• Slow Drug Discovery: Few new drugs reach market.