Microbiology Study Notes: Introduction, Prokaryotic Cells, and Genetics

Chapter 1: Introduction to Microbiology

Definition and Scope of Microbiology

• Microorganism: A microscopic organism, invisible to the naked eye. Examples include bacteria, archaea, fungi, protists, and helminths.

• Pathogen: A microbe that causes disease in a host.

• Opportunistic Pathogen: A microbe that causes disease only when the host's defenses are compromised or the normal microbiota is disrupted.

Historical Foundations

• Biogenesis vs. Spontaneous Generation:

  • Biogenesis: Life arises from pre-existing life.

  • Spontaneous Generation: Life can arise from non-living matter.

  • Louis Pasteur: Disproved spontaneous generation by demonstrating that microorganisms arise from other microorganisms, not spontaneously.

• Germ Theory of Disease (Robert Koch): Microorganisms are the cause of many diseases.

  • Koch's Postulates:

    1. Microbe must be present in all cases of the disease.

    2. Microbe must be isolated from the diseased host and grown in pure culture.

    3. Isolated microbe must cause the same disease when introduced into a healthy host.

    4. Microbe must be re-isolated from the experimentally infected host.

• Aseptic Technique: Procedures to prevent contamination and healthcare-associated infections.

• Contributors to Healthcare:

  • Semmelweis: Advocated handwashing to prevent disease transmission.

  • Joseph Lister: Introduced antiseptic techniques (e.g., mouthwash).

  • Florence Nightingale: Pioneered modern nursing and hygiene practices.

Classifying Microbes and Their Interactions

• Normal Microbiota (Normal Flora): The collection of microbes (bacteria, archaea, eukaryotic microbes) that live on and in the human body, playing roles in health and disease prevention.

• Symbiosis Types:

  • Parasitism: Microbe benefits, host is harmed.

  • Mutualism: Both microbe and host benefit.

  • Commensalism: Microbe benefits, host is neither harmed nor helped.

• Host–Microbe Interactions: Microbes have influenced human evolution, particularly immune system development.

• Microbes in Industry and Environment: Used in food production, waste decomposition, and biotechnology.

• Biofilms: Structured communities of microbes attached to surfaces, encased in a self-produced matrix. Biofilms are significant in healthcare due to their resistance to antibiotics and role in chronic infections.

Chapter 3: Introduction to Prokaryotic Cells

Prokaryotic Domains and Cell Structure

• Domains: Bacteria and Archaea

  • Both are unicellular and lack a membrane-bound nucleus and organelles.

  • Difference: Membrane structure; archaea have unique phospholipids and no known pathogens.

• Prokaryotic Cell Size: Small size increases surface area-to-volume ratio, facilitating efficient nutrient diffusion.

• Shapes: Bacilli (rod), Cocci (sphere), Vibrio (comma-shaped), Coccobacilli (ovoid), Spirochetes (corkscrew), Spirilla (spiral).

• Arrangements: Diplococci (pairs), Streptococci (chains), Staphylococci (clusters), Streptobacilli (chains of rods), Palisade (picket fence clusters).

• Pleomorphism: Ability to alter shape or size; enhances adaptability and pathogenicity.

• Binary Fission: Asexual reproduction where a cell divides into two genetically identical daughter cells.

Extracellular Structures

• Plasma Membrane: Semipermeable phospholipid bilayer; flexible and thin.

• Bacteria vs. Archaea Membranes and Cell Walls:

  • Bacteria: Linear fatty acids in phospholipids; cell wall contains peptidoglycan.

  • Archaea: Branched, long-chain fatty acids; can form monolayers; cell wall lacks peptidoglycan.

• Gram-Positive vs. Gram-Negative Cell Walls:

  • Gram-Positive: Thick peptidoglycan layer; stains purple; generally easier to kill.

  • Gram-Negative: Thin peptidoglycan layer; outer membrane; stains pink; more resistant to antibiotics.

• Acid-Fast Genera: Mycobacterium and Nocardia; contain mycolic acid; detected by acid-fast stain (red/pink); clinically important due to slow growth and treatment challenges.

• L-forms vs. Mycoplasma:

  • L-forms: Bacteria that have lost their cell wall.

  • Mycoplasma: Naturally lack a cell wall; live inside host cells (e.g., Mycoplasma pneumoniae).

• Transport Mechanisms:

  • Passive Transport: No ATP required (e.g., diffusion, osmosis, facilitated diffusion).

  • Active Transport: Requires ATP (e.g., pumps, group translocation).

• Osmosis: Movement of water across a semipermeable membrane.

  • Hypertonic Solution: Water leaves cell; plasmolysis occurs.

  • Hypotonic Solution: Water enters cell; cell may burst.

  • Isotonic Solution: No net water movement; cell remains stable.

• Surface Structures:

  • Flagella: Filamentous structures for motility; arrangements include monotrichous (single), lophotrichous (tuft), amphitrichous (both ends), peritrichous (all over).

  • Periplasmic Flagella: Found in spirochetes; located in periplasmic space; enable corkscrew motion.

  • Fimbriae: Short, bristle-like; aid in attachment.

  • Pili: Longer, rigid; involved in conjugation and attachment.

  • Glycocalyx: Carbohydrate-rich layer (slime layer or capsule); protects against desiccation and immune attack.

Intracellular Structures

• Nucleoid: Region containing the prokaryotic chromosome (usually single, circular DNA).

• Cytoskeleton: Provides structural support; less complex than in eukaryotes.

• Ribosomes: Sites of protein synthesis; prokaryotic ribosomes (70S) support the endosymbiotic theory.

• Inclusion Bodies: Storage sites for nutrients or waste (e.g., glycogen granules).

• Endospores: Dormant, tough structures formed by some bacteria (e.g., Bacillus, Clostridium); highly resistant to heat, chemicals, and desiccation; significant in healthcare due to persistence and difficulty in eradication.

Chapter 5: Genetics

Heredity Basics

• Genotype: The genetic makeup of an organism.

• Phenotype: Observable traits determined by genotype.

• Genetic Material Organization:

  • Eukaryotes: Multiple, linear chromosomes in a nucleus; organized with histone proteins.

  • Prokaryotes: Single, circular chromosome in nucleoid; histone-like proteins.

• DNA and RNA Nucleotides: DNA contains deoxyribose, bases A, T, C, G; RNA contains ribose, bases A, U, C, G (U replaces T).

• Phosphodiester Bonds: Link nucleotides in a 5' to 3' direction, forming the backbone of DNA and RNA.

• Central Dogma: Flow of genetic information: DNA → RNA → Protein (transcription then translation).

DNA Replication

• Purpose: Accurate copying of genetic material before cell division.

• Key Enzymes:

  • Helicase: Unwinds DNA helix.

  • Primase: Synthesizes RNA primers.

  • DNA Polymerase III: Synthesizes new DNA strands.

  • DNA Polymerase I: Replaces RNA primers with DNA.

  • Ligase: Seals gaps in the sugar-phosphate backbone.

• Origin of Replication: Specific sequence where replication begins; forms a replication fork.

• Directionality: DNA is synthesized in the 5' to 3' direction; leading strand is continuous, lagging strand is discontinuous (Okazaki fragments).

• Prokaryotic vs. Eukaryotic Replication: Eukaryotes have multiple origins and more proteins; process is slower.

Protein Synthesis (Gene Expression)

• Definition: The process by which genetic information is used to synthesize proteins, impacting phenotype.

• Steps:

  1. Transcription: DNA is transcribed to mRNA.

  2. Translation: mRNA is translated to protein.

• Differences in Prokaryotes vs. Eukaryotes: Prokaryotes couple transcription and translation; eukaryotes separate these processes (nucleus vs. cytoplasm).

• RNA Types:

  • mRNA: Messenger RNA; carries genetic code.

  • tRNA: Transfer RNA; brings amino acids to ribosome.

  • rRNA: Ribosomal RNA; structural and catalytic component of ribosomes.

• Genetic Code: Universal, redundant, and unambiguous; allows for efficient and accurate protein synthesis.

• Reverse Transcription: Synthesis of DNA from an RNA template (e.g., retroviruses).

• mRNA Splicing (Eukaryotes): Removal of introns and joining of exons to produce mature mRNA.

• Nonstandard Amino Acids: Rare amino acids incorporated by special mechanisms (e.g., selenocysteine).

• Post-Translational Modifications: Chemical changes to proteins after synthesis (e.g., phosphorylation, methylation).

Regulating Protein Synthesis

• Constitutive Genes: Expressed continuously (housekeeping genes).

• Facultative Genes: Expressed only when needed.

• Pretranscriptional Regulation: Involves transcription factors, DNA methylation, quorum sensing, and operons.

• Operons: Clusters of genes regulated together.

  • Inducible Operons: Usually off; turned on in response to a substrate (e.g., lac operon).

  • Repressible Operons: Usually on; turned off when end product is abundant (e.g., trp operon).

• Epigenetic Regulation: Heritable changes in gene expression not involving changes to DNA sequence (e.g., DNA methylation).

• Quorum Sensing: Cell-to-cell communication in bacteria to coordinate gene expression based on population density.

• Post-Transcriptional Regulation: Controls mRNA stability, alternative splicing (eukaryotes), small noncoding RNAs, and riboswitches.

Essay Topic: Disruptions in Normal Microbiota and Infection

• Loss of Colonization: Reduced competition allows opportunistic pathogens to thrive.

• Overgrowth: Antibiotics may kill beneficial bacteria, enabling harmful microbes (e.g., yeast) to proliferate.

• Altered Microbiota: Environmental changes (e.g., pH shifts) can favor pathogenic bacteria.

• Compromised Immune System: Weak host defenses or disrupted microbiome increase infection risk.