Introduction to Microbiology

What is a Microorganism?

Microorganisms are living organisms that are too small to be seen with the naked eye. They are found everywhere in nature, making them ubiquitous.

• Types of Microorganisms: Includes bacteria, protozoa, fungi, algae, viruses, and archaea.

• Habitats: Microorganisms are found in soil, water, air, and within other organisms.

Pathogens are microorganisms that cause disease, but most microbes are beneficial (e.g., fermenting food, decomposing waste).

• Example: Lactobacillus species ferment milk to produce yogurt.

Prokaryotes vs. Eukaryotes

Cells are classified as either prokaryotic or eukaryotic based on their structural features.

• Prokaryotes: Lack membrane-bound organelles; DNA is not enclosed in a nucleus (e.g., bacteria, archaea).

• Eukaryotes: Have membrane-bound organelles, including a nucleus (e.g., protozoa, fungi, algae, plants, animals).

Historical Experiments and Theories

• Pathogen: A microorganism that causes disease.

• Florence Nightingale: Observed that pathogens have optimal growing conditions, influencing infection rates during the Crimean War.

• Koch's Postulates: Four criteria to establish a causative relationship between a microbe and a disease:

  1. The microorganism must be found in all organisms suffering from the disease, but not in healthy organisms.

  2. The microorganism must be isolated from a diseased organism and grown in pure culture.

  3. The cultured microorganism should cause disease when introduced into a healthy organism.

  4. The microorganism must be re-isolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.

• Louis Pasteur's Swan Neck Flask Experiment: Disproved spontaneous generation by showing that microorganisms in the air, not the broth itself, caused contamination.

• Endosymbiotic Theory: Eukaryotic cells originated from a symbiotic relationship between pre-eukaryotic cells and certain prokaryotes (e.g., mitochondria evolved from engulfed bacteria).

Basic Chemistry and Biomolecules

Chemical Bonds

• Ionic Bonds: Formed when electrons are transferred from one atom to another, creating charged ions.

• Covalent Bonds: Formed when atoms share electrons.

  • Non-polar Covalent: Electrons shared equally.

  • Polar Covalent: Electrons shared unequally, creating partial charges.

Hydrophilic vs. Hydrophobic Compounds

• Hydrophilic: Water-loving; often charged or polar (e.g., ions, carbohydrates).

• Hydrophobic: Water-fearing; nonpolar (e.g., lipids).

• Example: Lipids are hydrophobic and form the core of biological membranes.

Major Biomolecules

• Lipids: Include phospholipids, triglycerides, and sterols (e.g., cholesterol). Phospholipids form cell membranes.

• Proteins: Polymers of amino acids; function as enzymes, antibodies, and structural components.

• Carbohydrates: Polymers of monosaccharides (e.g., glucose, fructose, ribose). Disaccharides (e.g., sucrose), polysaccharides (e.g., starch, cellulose, glycogen).

• Polymers: Large molecules made of repeating monomers (e.g., proteins from amino acids, polysaccharides from monosaccharides).

Cell Structure and Function

• Nucleus: Membrane-bound organelle housing DNA (as chromatin with histone proteins).

• Plasma Membrane: Composed of a double lipid layer with embedded proteins; regulates entry and exit of substances.

• Mitochondria: Site of ATP (energy) production.

• Organelles: Specialized structures within eukaryotic cells (e.g., rER, sER, nucleolus).

• Movement of Water:

  • Isotonic Solution: No net movement of water.

  • Hypotonic Solution: Water enters the cell; cell may swell.

  • Hypertonic Solution: Water leaves the cell; cell may shrink.

Cells: Structure and Function

Bacterial Structures

• Flagella: Tail-like structures for movement; enable chemotaxis (movement toward or away from chemicals).

• Fimbriae and Pili: Surface structures for attachment and conjugation.

• Shapes of Bacteria: Cocci (spherical), rods (bacilli), chains (streptococci), clusters (staphylococci).

• Cell Envelope: Includes inner cell membrane, peptidoglycan (PG) layer, and (in Gram-negative bacteria) an outer membrane (OM).

Gram-Positive vs. Gram-Negative Bacteria

• Gram-Positive: Thick peptidoglycan layer, no outer membrane.

• Gram-Negative: Thin peptidoglycan layer, outer membrane present, contains lipopolysaccharide (LPS).

• LPS (Endotoxin): Found in the outer membrane of Gram-negative bacteria; can trigger strong immune responses.

• Antibiotics: Used to treat bacterial infections.

Eukaryotic Cell Structures

• Glycocalyx: Outer coating for protection and cell recognition.

• Nucleus: Contains DNA; nucleolus is the site of rRNA synthesis.

• Cell Wall: Provides structure (in plants, fungi, some protists).

• rER (Rough Endoplasmic Reticulum): Protein synthesis.

• sER (Smooth Endoplasmic Reticulum): Lipid synthesis.

• Mitochondria: ATP production.

• Nuclear Pores: Allow transport of molecules in and out of the nucleus.

• Cytoskeleton: Provides cell shape, support, and movement.

Viruses

Structure of Viruses

• Capsid: Protein shell; can be helical or icosahedral.

• Naked vs. Enveloped Viruses: Naked viruses lack an envelope; enveloped viruses have a lipid membrane derived from the host.

• Spikes: Surface proteins for attachment to host cells.

• Nucleic Acid: Can be DNA or RNA.

• Virion: Complete, infectious virus particle.

Viral Entry and Replication

• Attachment: Virus binds to host cell receptors.

• Entry: By direct endocytosis or membrane fusion.

• Lytic Cycle: Virus replicates and lyses (kills) the host cell.

• Lysogenic Cycle: Viral DNA integrates into host genome and replicates with it (may later enter lytic cycle).

• Budding: Enveloped viruses exit the cell by budding from the membrane.

Microbial Nutrition and Growth

Organic vs. Inorganic Compounds

• Organic Compounds: Contain carbon and hydrogen (e.g., glucose).

• Inorganic Compounds: Do not contain both carbon and hydrogen (e.g., water, salts).

Types of Microbial Nutrition

• Autotrophs: Use CO2 as a carbon source.

• Heterotrophs: Require organic carbon.

• Photoautotrophs: Use light for energy and CO2 for carbon.

• Chemoautotrophs: Use inorganic chemicals for energy and CO2 for carbon (e.g., aerobic respiration).

• Parasites: Obtain nutrients from living hosts.

• Facultative Anaerobes: Can grow with or without oxygen.

Essential Nutrients

• Essential Nutrients: Required for growth; cannot be synthesized by the organism.

• Macronutrients: Needed in large amounts (e.g., C, H, O, N, P, S).

• Micronutrients/Trace Elements: Needed in small amounts (e.g., Fe, Zn, Cu).

Biofilms and Quorum Sensing

• Biofilm: Community of microorganisms attached to a surface and embedded in a self-produced matrix.

• Formation: Bacteria adhere to surfaces, multiply, and produce extracellular polymeric substances.

• Quorum Sensing: Cell-to-cell communication to coordinate activity based on population density.

Summary Table: Key Differences Between Prokaryotes and Eukaryotes

Key Equations

• General Equation for Aerobic Respiration:

• Osmosis (Water Movement):

Additional info: Academic context and definitions have been expanded for clarity and completeness.