Microbiology Exam 2 Study Guide: Microbial Metabolism, Growth, Control, and Antimicrobial Drugs

Microbial Metabolism

Phototrophs, Chemotrophs, Autotrophs, and Heterotrophs

Microorganisms obtain energy and carbon in various ways, which are used to classify them based on their metabolic strategies.

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

• Chemotrophs: Organisms that obtain energy from chemical compounds. Example: Escherichia coli (uses organic compounds).

• Autotrophs: Organisms that use carbon dioxide as their principal carbon source. Example: Nitrifying bacteria.

• Heterotrophs: Organisms that require organic carbon sources. Example: Most fungi.

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 in microbes 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 purple and green sulfur bacteria.

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

Example: Cyanobacteria (oxygenic), Chlorobium (anoxygenic).

Dynamics of Microbial Growth

Definition and Requirements for Microbial Growth

Microbial growth refers to the increase in cell number, not cell size. Growth depends on both 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), thermophiles (heat-loving).

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

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

Chemical Requirements for Growth

• Carbon: Needed for all organic molecules.

• Nitrogen: For proteins, nucleic acids.

• Sulfur: For amino acids, vitamins.

• Phosphorus: For nucleic acids, ATP.

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

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

Oxygen Requirements of Microbes

• 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

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

• Formation: Initial attachment, maturation, dispersal.

• Medical Significance: Resistant to antibiotics; cause chronic infections.

• Industrial Significance: Can foul equipment, pipes.

Preservation of Microbial Cultures

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

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

Bacterial Growth Curve

Bacterial populations grow in a predictable pattern when cultured.

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

• Log (Exponential) Phase: Rapid cell division.

• Stationary Phase: Growth rate slows; nutrients depleted.

• Death Phase: Cells die faster than new ones are produced.

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 spores) on inanimate objects.

• Antisepsis: Removal of pathogens from living tissue.

• Degerming: Mechanical removal of microbes (e.g., hand washing).

• 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 spoilage organisms; does not sterilize.

• Limitations: Some spores and thermophiles survive.

Other Physical Methods

• Filtration: Removes microbes from liquids/air.

• Low Temperature: Inhibits growth; does not kill.

• High Pressure: Alters proteins; used for food preservation.

• Desiccation: Removes water; inhibits growth.

• Osmotic Pressure: High salt/sugar inhibits growth.

Radiation: Ionizing and Non-Ionizing

• Ionizing Radiation: X-rays, gamma rays; damages DNA.

• Non-Ionizing Radiation: UV light; causes thymine dimers in DNA.

• Uses: Sterilization, disinfection.

Principles of Effective Disinfection

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

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

Major Chemical Disinfectants and Antiseptics

• Phenolics: Disrupt membranes; used in soaps.

• Alcohols: Denature proteins; used for skin.

• Halogens: Iodine, chlorine; oxidize cell components.

• Heavy Metals: Silver, mercury; inactivate proteins.

• Surfactants: Lower surface tension; aid in degerming.

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

Chemical Sterilants and Gaseous Agents

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

• Peroxygens: Hydrogen peroxide; oxidizing agent.

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: Use of chemicals to treat disease.

• 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 drugs discovered from soil microbes.

Selective Toxicity

• Principle: Drug harms microbe, not host.

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

Modes of Action of Antimicrobial Drugs

• Inhibition of Cell Wall Synthesis: Penicillins, cephalosporins.

• Inhibition of Protein Synthesis: Tetracyclines, macrolides.

• Inhibition of Nucleic Acid Replication/Transcription: Rifamycins, quinolones.

• Disruption of Plasma Membrane: Polymyxins.

• Inhibition of Essential Metabolite Synthesis: Sulfonamides.

Representative Drugs

• Penicillins: Cell wall synthesis inhibitor.

• Tetracyclines: Protein synthesis inhibitor.

• Rifamycins: Inhibit RNA synthesis.

• Polymyxins: Disrupt membranes.

• Sulfonamides: Inhibit folic acid synthesis.

Broad-Spectrum vs. Narrow-Spectrum Antibiotics

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

• Narrow-Spectrum: Target specific microbes; preferred to minimize resistance.

Mechanisms of Bacterial Resistance

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

• Prevention of Drug Entry: Altered permeability.

• Target Alteration: Modified drug target.

• Efflux Pumps: Remove drug from cell.

Spread of Resistance Genes

• Horizontal Gene Transfer: Conjugation (plasmids), transformation (uptake of DNA), transduction (bacteriophage-mediated).

Antibiotic Misuse

• Causes: Overprescription, use in agriculture, improper dosing.

• 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: New resistant strains appear rapidly.

• Slow Drug Discovery: Few new drugs reach market.

Summary Table: Modes of Action and Representative Drugs

Mode of Action	Representative Drug	Example Pathogen Targeted
Cell Wall Synthesis Inhibition	Penicillin	Staphylococcus aureus
Protein Synthesis Inhibition	Tetracycline	Escherichia coli
Nucleic Acid Synthesis Inhibition	Rifampin	Mycobacterium tuberculosis
Plasma Membrane Disruption	Polymyxin B	Pseudomonas aeruginosa
Metabolite Synthesis Inhibition	Sulfonamide	Streptococcus pneumoniae

Additional info: This table summarizes the main modes of action and examples of drugs and pathogens.