Microbial Growth, Metabolism, and Antimicrobial Control: Study Notes

Microbial Growth and Metabolism

Autotrophs vs. Heterotrophs

Microorganisms can be classified based on their carbon and energy sources. Understanding these classifications is essential for studying microbial ecology and metabolism.

• Autotrophs: Organisms that use inorganic carbon (usually CO2) as their carbon source. They can be photoautotrophs (using light energy) or chemoautotrophs (using chemical energy).

• Heterotrophs: Organisms that require organic compounds as their carbon source. They can be photoheterotrophs or chemoheterotrophs, depending on their energy source.

• Example: Cyanobacteria are photoautotrophs; Escherichia coli is a chemoheterotroph.

Types of Aerobes and Anaerobes

Microbes are also classified by their oxygen requirements, which determines where they grow in a broth culture tube.

• Obligate Aerobes: Require oxygen; grow at the top of the tube.

• Obligate Anaerobes: Killed by oxygen; grow at the bottom.

• Facultative Anaerobes: Grow with or without oxygen; more growth at the top.

• Aerotolerant Anaerobes: Do not use oxygen but tolerate it; even growth throughout.

• Microaerophiles: Require low oxygen; grow just below the surface.

Nitrogen and Biomolecules

Nitrogen is a critical element for the synthesis of several biomolecules:

• Amino acids (proteins)

• Nucleic acids (DNA, RNA)

• ATP and other cofactors

Nitrogen Cycling

Nitrogen cycles through the environment via microbial processes such as nitrogen fixation, nitrification, denitrification, and ammonification.

• Nitrogen fixation: Conversion of atmospheric N2 to ammonia (NH3).

• Nitrification: Ammonia is oxidized to nitrite (NO2-) and then nitrate (NO3-).

• Denitrification: Nitrate is reduced back to N2 gas.

Temperature Classification of Microbes

Microbes are classified by their optimal temperature ranges:

• Psychrophiles: 0–20°C

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

• Thermophiles: 45–80°C

• Hyperthermophiles: >80°C

Halophiles

Halophiles are organisms that thrive in high salt concentrations, such as those found in salt lakes.

Microbial Relationships

• Antagonistic: One organism harms another (e.g., antibiotics).

• Synergistic: Both benefit, but not required for survival.

• Symbiotic: Close, long-term interaction; can be mutualistic, commensal, or parasitic.

Biofilms

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

• Development: Attachment → Microcolony formation → Maturation → Dispersion

• Importance: Increased resistance to antibiotics and immune responses.

Media Types

• Differential Media: Distinguish between organisms based on metabolic traits (e.g., blood agar).

• Selective Media: Inhibit some microbes while allowing others to grow (e.g., MacConkey agar).

Aseptic Technique

Procedures used to prevent contamination of cultures and environments by unwanted microorganisms.

Binary Fission and Generation Time

• Binary Fission: Asexual reproduction in bacteria; one cell divides into two identical cells.

• Generation Time: Time required for a population to double.

• Calculation: $N_t = N_0 \times 2^n$ Where $N_t$ = final cell number, $N_0$ = initial cell number, $n$ = number of generations.

Phases of Bacterial Growth

• Lag Phase: Adaptation, no division.

• Log (Exponential) Phase: Rapid cell division.

• Stationary Phase: Nutrient depletion, growth rate = death rate.

• Death Phase: Cells die faster than they divide.

Measurement of Microbial Growth

• Direct Methods: Plate counts, microscopic counts.

• Indirect Methods: Turbidity (optical density), metabolic activity.

Microbial Metabolism

Major Metabolic Processes

Metabolism includes all chemical reactions in a cell, divided into catabolism (breakdown) and anabolism (synthesis).

• Aerobic Respiration: Requires oxygen; produces most ATP.

• Anaerobic Respiration: Uses other electron acceptors; less ATP.

• Fermentation: Anaerobic; organic molecules as electron acceptors; least ATP.

• Purpose: To generate energy (ATP) and building blocks for growth.

Glycolysis

• Breakdown of glucose to pyruvate.

• Net gain: 2 ATP, 2 NADH per glucose.

• Occurs in cytoplasm; does not require oxygen.

Fermentation

• Regenerates NAD+ from NADH.

• End products: lactic acid, ethanol, etc.

• Occurs when oxygen is absent.

TCA Cycle (Krebs Cycle)

• Oxidizes acetyl-CoA to CO2.

• Produces NADH, FADH2, and GTP/ATP.

Electron Transport Chain (ETC)

• Located in the cell membrane (prokaryotes) or mitochondria (eukaryotes).

• Uses NADH and FADH2 to generate ATP via oxidative phosphorylation.

• Oxygen is the final electron acceptor in aerobic respiration.

Enzyme Structure

• Apoenzyme: Protein portion.

• Cofactor: Non-protein helper (metal ion or coenzyme).

• Holoenzyme: Apoenzyme + cofactor (active form).

Microbial Control

Action of Antimicrobial Agents

• Disrupt cell walls, membranes, proteins, or nucleic acids.

• Can be microbicidal (kill) or microbiostatic (inhibit growth).

Microbial Growth Terminology

• Sterilization: Complete removal of all microbes.

• Aseptic: Free of contamination.

• Disinfectant: Chemical used on inanimate objects.

• Antiseptic: Chemical used on living tissue.

• Biocide: Agent that kills microbes.

Microbial Control Methods

• Physical Methods: Heat (autoclaving, pasteurization), filtration, radiation.

• Chemical Methods: Alcohols, phenolics, halogens, heavy metals, aldehydes.

Biosafety Levels (BSL)

• BSL-1: Non-pathogenic microbes.

• BSL-2: Moderate risk; lab coats, gloves.

• BSL-3: Aerosol transmission; special ventilation.

• BSL-4: Dangerous/exotic; full-body suits, isolated facilities.

Evaluating Disinfectants and Antiseptics

• Use-dilution test: Determines effectiveness against microbes.

• Disk-diffusion method: Measures zone of inhibition on agar plates.

Antimicrobial Drugs

Discovery and Key Scientists

• Alexander Fleming: Discovered penicillin.

• Howard Florey: Developed clinical use and culturing methods for penicillin.

• Gerhard Domagk: Discovered sulfa drugs.

Mechanisms of Action (6 Main Types)

Mechanism	Example Drug
Inhibit cell wall synthesis	Penicillins, Vancomycin
Inhibit protein synthesis	Tetracycline
Disrupt cell membrane	Polymyxins
Inhibit nucleic acid synthesis	AZT
Inhibit metabolic pathways	Sulfonamides
Inhibit attachment/entry	Echinocandins

Clinical Considerations

• Selective Toxicity: Drug harms microbe, not host.

• Therapeutic Index (TI): Ratio of toxic dose to therapeutic dose. $TI = \frac{\text{Toxic Dose}}{\text{Therapeutic Dose}}$

• Routes of Administration: Oral, intravenous, topical, etc.

• Spectrum of Action: Narrow (few species) vs. Broad (many species).

• Effectiveness: Measured by diffusion susceptibility (Kirby-Bauer), Minimum Inhibitory Concentration (MIC), and Minimum Bactericidal Concentration (MBC).

• Safety and Side Effects: Consider TI and therapeutic range.

Mechanisms of Resistance and Evasion

• Enzyme production (e.g., beta-lactamases)

• Altered target sites

• Efflux pumps

• Reduced permeability

Development of Resistance

• Mutation

• Horizontal gene transfer (conjugation, transformation, transduction)

Special Drugs

• Vancomycin: Inhibits cell wall synthesis (binds D-Ala-D-Ala).

• Beta-lactams: Inhibit cell wall synthesis (block transpeptidase enzymes).

• AZT: Inhibits reverse transcriptase (antiviral).

• Echinocandins: Inhibit fungal cell wall synthesis.

• Tetracycline: Inhibits protein synthesis (binds 30S ribosomal subunit).

Additional info: Where content was brief, academic context and definitions were added for clarity and completeness.