Microbial Metabolism

Types of Nutritional Groups

Microorganisms are classified based on their energy and carbon sources. Understanding these groups is essential for studying microbial metabolism.

• Phototrophs: Use light as their energy source. Example: Cyanobacteria.

• Chemotrophs: Obtain energy from chemical compounds. Example: Nitrosomonas (uses ammonia).

• Autotrophs: Use carbon dioxide as their carbon source. Example: Algae.

• Heterotrophs: Require organic carbon sources. Example: Escherichia coli.

Additional info: Some organisms are photoautotrophs (light + CO2), while others are chemoheterotrophs (chemical + organic carbon).

Oxygenic vs. Anoxygenic Photosynthesis

Photosynthesis in microbes can be classified based on oxygen production.

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

• Anoxygenic Photosynthesis: Does not produce oxygen; performed by purple sulfur bacteria and green sulfur bacteria.

Example: Oxygenic: Cyanobacteria; Anoxygenic: Chlorobium.

Microbial Growth

Definition and Requirements for Growth

Microbial growth refers to the increase in cell number, 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 Environmental Factors

Environmental conditions affect microbial growth rates and survival.

• 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 inhibit growth; halophiles tolerate high salt.

Chemical Requirements for Growth

• Carbon: Structural backbone; autotrophs use CO2, heterotrophs use organic carbon.

• Nitrogen: Needed for proteins, nucleic acids; obtained from NH4+, NO3-, or N2 (nitrogen fixation).

• Sulfur: Used in amino acids and vitamins.

• Phosphorus: Component of nucleic acids, ATP, phospholipids.

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

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

Oxygen Requirements

Microbes differ in their oxygen needs and tolerances.

• Obligate Aerobes: Require oxygen.

• Obligate Anaerobes: Cannot tolerate oxygen.

• Facultative Anaerobes: Grow with or without oxygen, but better with oxygen.

• Microaerophiles: Require low oxygen concentrations.

• Aerotolerant Anaerobes: Do not use oxygen but tolerate it.

Biofilms

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

• Formation: Cells adhere, secrete extracellular polymeric substances (EPS), and develop complex structures.

• Significance: Biofilms are resistant to antibiotics and disinfectants; cause medical device infections and industrial fouling.

Preservation of Microbial Cultures

Long-term storage methods are used to maintain microbial viability.

• Deep Freezing: -80°C storage.

• Lyophilization (Freeze-drying): Removes water under vacuum.

• Refrigeration: Short-term storage.

Bacterial Growth Curve

Bacterial populations follow a characteristic growth curve in batch culture.

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

• Log (Exponential) Phase: Rapid cell division.

• Stationary Phase: Growth rate slows; nutrients depleted.

• Death Phase: Cell death exceeds cell division.

Control of Microbial Growth

Definitions of Control Methods

Different terms describe methods for reducing or eliminating microbes.

• Sterilization: Removal of all microbial life.

• Disinfection: Removal of pathogens from surfaces.

• Antisepsis: Removal of pathogens from living tissue.

• Degerming: Mechanical removal of microbes.

• Sanitization: Lowering microbial counts to safe levels.

• Biocide/Germicide: Agents that kill microbes.

Patterns of Microbial Death and Factors Affecting Treatment

Microbial death occurs at a constant rate; effectiveness depends on several factors.

• Number of microbes

• Environment (organic matter, temperature, biofilms)

• Time of exposure

• Microbial characteristics

Actions of Control Agents

Agents act on cellular structures:

• Membranes: Disrupt permeability.

• Proteins: Denature enzymes.

• Nucleic Acids: Damage genetic material.

Physical and Chemical Methods of Control

Methods are classified as physical or chemical.

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

• Chemical: Disinfectants, antiseptics, sterilants.

Heat Methods

• Moist Heat: Boiling, autoclaving; denatures proteins.

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

• Pasteurization: Reduces spoilage organisms; does not sterilize.

Other Physical Methods

• Filtration: Removes microbes from liquids/air.

• Low Temperature: Slows growth.

• High Pressure: Alters proteins.

• Desiccation: Removes water; inhibits growth.

• Osmotic Pressure: High salt/sugar inhibits growth.

Radiation

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

• Non-ionizing Radiation: UV light; causes thymine dimers.

Principles of Effective Disinfection

• Concentration of disinfectant

• Contact time

• Nature of material

• Presence of organic matter

Major Chemical Disinfectants and Antiseptics

• Phenolics: Disrupt membranes.

• Alcohols: Denature proteins, dissolve lipids.

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

• Heavy Metals: Inactivate proteins (e.g., silver, mercury).

• Surfactants: Lower surface tension.

• Aldehydes: Cross-link proteins.

Chemical Sterilants and Gaseous Agents

• Ethylene Oxide: Used for medical devices; alkylates DNA.

Peroxygens and Oxidizing Agents

• Hydrogen Peroxide: Used for disinfection and sterilization.

• Peracetic Acid: Effective against spores.

Bactericidal vs. Bacteriostatic Actions

• Bactericidal: Kills bacteria.

• Bacteriostatic: Inhibits growth; used when host defenses can eliminate microbes.

Antimicrobial Drugs

Definitions and Historical Development

Antimicrobial drugs are used to treat infections. Chemotherapy refers to the use of chemicals to treat disease; antibiotics are natural substances produced by microbes.

• Paul Ehrlich: Developed the concept of selective toxicity.

• Alexander Fleming: Discovered penicillin.

Selective Toxicity

Drugs should harm pathogens but not the host. Achieving selective toxicity is harder with eukaryotic pathogens due to similarities with host 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

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 disruption of normal flora.

Mechanisms of Bacterial Resistance

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

• Prevention of Drug Entry: Altered permeability.

• Target Alteration: Modified drug targets.

• Efflux Pumps: Remove drugs from cell.

Spread of Resistance Genes

• Conjugation: Transfer via plasmids.

• Transformation: Uptake of naked DNA.

• Transduction: Transfer by bacteriophages.

Antibiotic Misuse

• Medicine: Overprescribing, incomplete courses.

• Agriculture: Use in animal feed.

• Consumer Products: Antibacterial soaps.

Consequences: Increased resistance, fewer effective drugs.

Bactericidal vs. Bacteriostatic Drugs

• Bactericidal: Used for severe infections.

• Bacteriostatic: 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: Rapid evolution of resistant strains.

• Slow Drug Discovery: Few new drugs reach the market.

Additional info: Combination therapy and stewardship programs are used to combat resistance.