Microbial Nutrition and Growth

Microbial Growth and Biofilms

Microbial growth refers to an increase in the number of cells, not the size of individual cells. Microbes often exist in complex communities called biofilms, which are groups of mixed microbes living together and interacting.

• Biofilm: A structured community of microorganisms encapsulated within a self-produced matrix, attached to a surface.

• Microbial growth: Increase in cell number, not cell size.

Normal Growth Curve

Bacterial populations typically follow a predictable pattern of growth in a closed system, known as the normal growth curve, which consists of four phases:

• Lag Phase: Cells adjust to their environment; little to no cell division occurs.

• Exponential (Log) Phase: Rapid cell division and population growth; cells are most metabolically active.

• Stationary Phase: Growth rate slows as nutrients deplete and waste accumulates; cell division equals cell death.

• Death Phase: Cells die at an exponential rate due to lack of nutrients and accumulation of toxic products.

Temperature Preferences of Microbes

Microorganisms are classified based on their optimal temperature for growth:

• Psychrophiles: Grow best at cold temperatures (typically 0–15°C).

• Psychrotrophs: Prefer cooler temperatures (15–30°C), but can grow at lower temperatures.

• Mesophiles: Grow best at moderate temperatures (20–45°C); most human pathogens are mesophiles.

• Thermophiles: Thrive at high temperatures (45–80°C).

Note: Placing a microbe above its maximal growth temperature causes enzyme and protein denaturation, leading to cell death.

Salt and Halophiles

• Halophiles: Microbes that thrive in high-salt environments (>3% NaCl).

Oxygen Requirements

Microbes vary in their oxygen requirements and tolerance:

• Obligate aerobes: Require oxygen for growth; possess enzymes (superoxide dismutase, catalase) to detoxify reactive oxygen species.

• Facultative anaerobes: Can grow with or without oxygen; use oxygen if present.

• Obligate anaerobes: Grow only in the absence of oxygen; lack enzymes to detoxify oxygen species.

Types of Media

• Differential media: Allows visual distinction between different microbes based on their biological characteristics.

• Selective media: Contains ingredients that promote the growth of certain microbes while inhibiting others.

Microbial Control Methods

• Decontamination: Removal or reduction of microbial populations to make an object safe for handling.

• Sterilization: Complete elimination of all microbes, including bacteria, viruses, and endospores.

• Pasteurization: High heat for a short time to reduce microbial load without sterilizing.

• Dry Heat: Incineration or hot-air ovens for sterilization or disinfection.

• Cide: Suffix indicating killing of microbes (e.g., bactericide).

• Static: Suffix indicating inhibition of microbial growth (e.g., bacteriostatic).

Common Chemical Agents

• Alcohols: Denature proteins and disrupt lipid membranes.

• Phenols: Destroy cell walls and interact with proteins.

• Halogens: Oxidize cell components.

• Ethylene oxide: Gaseous sterilant that damages proteins and nucleic acids.

Microbial Metabolism

Enzyme Inhibition

• Competitive inhibitors: Bind to the active site of an enzyme, blocking substrate binding.

• Non-competitive inhibitors: Bind to a regulatory (allosteric) site, altering the active site and preventing substrate binding.

• Substrate: The molecule upon which an enzyme acts, binding at the active site.

Enzyme Structure

• Holoenzyme: An active enzyme consisting of a protein component and one or more cofactors.

• Apoenzyme: The protein portion of an enzyme, inactive without its cofactor(s).

Catabolism and Anabolism

• Catabolism: Breakdown of large molecules into smaller ones, releasing energy (ATP).

• Anabolism: Synthesis of larger molecules from smaller ones, requiring energy (ATP).

Redox Reactions

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

Glycolysis and Krebs Cycle

• Glycolysis: Breakdown of glucose into 2 pyruvate molecules; net yield of 2 ATP and 2 NADH per glucose.

• Intermediate Step: Pyruvate is converted to Acetyl-CoA before entering the Krebs cycle.

• Krebs Cycle: Series of reactions that generate ATP, NADH, and FADH2 from Acetyl-CoA.

Amphibolic pathways have both catabolic and anabolic functions.

Electron Transport Chain (ETC)

• Series of redox reactions that transfer electrons from NADH and FADH2 to oxygen (in aerobes), generating ATP.

• Coenzymes like NAD+ and FAD act as electron shuttles.

Principles of Infectious Disease and Epidemiology

Pathogen Types and Disease Spread

• Opportunistic pathogens: Cause disease only in weakened hosts (e.g., immunocompromised individuals).

• True pathogens: Can cause disease in healthy hosts.

• Epidemic: Widespread outbreak in a specific region and time.

• Pandemic: Epidemic that spreads across countries or continents.

• Zoonotic diseases: Transmitted from animals to humans.

• Virulence: Severity of disease caused by a pathogen.

• Pathogenicity: Ability of an agent to cause disease.

Host-Microbe Interactions and Pathogenesis

Pathogens and Virulence Factors

• Pathogens: Microbes that cause disease.

• Infectious dose-50 (ID50): Number of organisms required to infect 50% of exposed hosts.

• Toxins: Two main classes:

  • Endotoxins: Released from the cell wall (Lipid A of LPS) upon cell lysis; found in Gram-negative bacteria.

  • Exotoxins: Secreted proteins that cause damage to the host.

• Adhesins: Molecules that allow pathogens to attach to host cells.

Biosafety Levels (BSL)

Antimicrobial Drugs

History and Definitions

• Alexander Fleming (1928): Discovered penicillin, the first true antibiotic.

• Bacteriostatic: Inhibit bacterial growth.

• Bactericidal: Kill bacteria.

Mechanisms of Action of Antibiotics

• Cell Wall Inhibitors:

  • Penicillin: Inhibits enzymes that form peptide bridges in peptidoglycan.

  • Isoniazid: Inhibits mycolic acid synthesis (important for Mycobacterium cell walls).

• Protein Synthesis Inhibitors:

  • Streptomycin: Binds 30S ribosomal subunit; causes misreading of mRNA.

  • Tetracycline: Binds 30S ribosomal subunit; blocks tRNA binding to A site.

  • Erythromycin: Binds 50S ribosomal subunit; inhibits ribosome translocation.

• Inhibitors of Essential Metabolites:

  • Sulfonamides (e.g., TMP/Sulfanilamide): Inhibit folic acid synthesis.

• Inhibitors of Nucleic Acid Synthesis:

  • Ciprofloxacin: Inhibits DNA gyrase.

  • Rifamycin: Inhibits RNA polymerase.

• Cell Membrane Damage:

  • Polymyxins: Disrupt membrane phospholipids, causing leakage of cell contents.

Antifungal Drugs

• Three main classes target fungal plasma membranes:

  • Azoles

  • Allylamines

  • Polyenes

Antimicrobial Resistance

• Intrinsic resistance: Natural resistance due to inherent structural or functional characteristics.

• Acquired resistance: Resistance gained through genetic mutation or acquisition of resistance genes.

Mechanisms of Drug Evasion

• Microbes evade antimicrobial drugs by:

  1. Altering the drug’s target

  2. Inactivating the drug

  3. Reducing drug concentration inside the cell (blocking entry or pumping out the drug)

Example Table: Mechanisms of Antibiotic Action

Key Equations

• ATP yield from glycolysis:

• Redox reaction:

Additional info: Where details were brief, standard microbiology context was added for clarity and completeness.