Major Historical Developments in Microbiology

Germ Theory and Koch’s Postulates

The Germ Theory established that microorganisms are the cause of many diseases. Koch’s Postulates are a set of criteria used to prove the causative relationship between a microbe and a disease.

• Koch’s Postulates:

  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 experimentally infected host and identified as identical to the original.

• Spontaneous Generation: The disproven idea that life arises spontaneously from non-living matter. Pasteur’s experiment used swan-neck flasks to show that microbes come from the environment, not spontaneously.

• Cultivation Techniques: Methods such as isolation and enrichment cultures allow scientists to grow and study specific microbes.

• Characteristics of Life: Includes cellular organization, metabolism, growth, reproduction, response to stimuli, and adaptation.

• Three Domains of Life: Bacteria, Archaea, and Eukarya, distinguished by genetic and structural differences.

• Beneficial Microbes: Microbes play roles in nutrient cycling, food production, and health.

• Key Figures: Louis Pasteur (disproved spontaneous generation, vaccines), Robert Koch (germ theory), Sergei Winogradsky (soil microbiology), Martinus Beijerinck (enrichment culture, viruses).

Major Developments in Microbiology

• Discovery of antibiotics, molecular genetics, genomics, and biotechnology.

Cell Structure, Function and Behavior

Cytoplasmic Membranes

The cytoplasmic membrane is a selectively permeable barrier, anchors proteins, and is involved in energy conservation.

• Selective Permeability: Controls entry and exit of substances.

• Protein Anchor: Holds proteins involved in transport and signaling.

• Energy Conservation: Site of generation and use of proton motive force.

Differences Among Domains

• Bacteria: Membranes contain ester-linked fatty acids.

• Archaea: Membranes contain ether-linked lipids, sometimes monolayers.

• Eukarya: Membranes similar to Bacteria, but with sterols.

Prokaryotic Cell Morphologies

• Common shapes: cocci (spherical), bacilli (rod-shaped), spirilla (spiral).

• Osmotrophy: Small size increases surface area-to-volume ratio, aiding nutrient uptake.

Cell Wall Structure and Function

• Prokaryotic Cell Wall: Provides shape and protection; composed of peptidoglycan in Bacteria.

• Gram Positive: Thick peptidoglycan layer, stains purple.

• Gram Negative: Thin peptidoglycan, outer membrane, stains pink.

• Outer Membrane: Contains lipopolysaccharides (LPS).

• Cell Wall vs Cytoplasmic Membrane: Wall is structural, membrane is functional barrier.

DNA Arrangement and Cell Structures

• Bacteria/Archaea: DNA in nucleoid, usually circular.

• Eukarya: DNA in nucleus, linear chromosomes.

• Flagella: Motility structures; Bacterial flagella rotate, Eukaryotic flagella whip, Archaeal flagella differ in structure.

• Chemotaxis: Movement toward/away from chemicals.

• Cell Surface Structures: Pili (attachment, conjugation), capsules (protection), etc.

• Cell Inclusions: Endospores (resistance), gas vesicles (buoyancy), storage polymers (energy).

Microbial Metabolism

Elemental Composition and Nutrient Acquisition

• Macronutrients: C, N, P, S, K, Mg, Ca, Na; required in large amounts.

• Micronutrients: Trace elements (Fe, Mn, Zn, etc.).

• Growth Factors: Organic compounds required for growth (vitamins, amino acids).

• Osmotrophs: Acquire nutrients via active transport.

Transport Systems

• Simple Transport: Driven by proton motive force.

• Group Translocation: Chemical modification during transport.

• ABC Systems: ATP-binding cassette, uses ATP.

Energy Classes and Bioenergetics

• Energy Classes: Phototrophs (light), chemotrophs (chemicals), organotrophs (organic), lithotrophs (inorganic).

• Free Energy (ΔG): Determines spontaneity of reactions.

  • Negative : Reaction is spontaneous.

  • Positive : Reaction is non-spontaneous.

• Catalysis: Enzymes lower activation energy, speeding reactions.

• Enzyme Structure: Active site binds substrate; specificity and efficiency depend on structure.

Redox Reactions and Electron Carriers

• Redox Reactions: Transfer of electrons; essential for energy generation.

• Redox Couples: Pair of oxidized/reduced forms.

• Electron Tower: Ranks redox couples by reduction potential.

• Common Electron Carriers: NAD+/NADH, FAD/FADH2, quinones.

Energy Conservation and ATP Synthesis

• High Energy Compounds: ATP, GTP, etc.

• Energy Storage: Glycogen, polyhydroxybutyrate.

• Principle: Energy from catabolism is conserved as ATP.

• ATP Synthesis:

  • Oxidative Phosphorylation: ATP generated via electron transport chain.

  • Substrate-Level Phosphorylation: ATP generated directly from metabolic intermediates.

Respiratory and Fermentative Pathways

• Aerobic Respiration: Uses O2 as terminal electron acceptor.

• Anaerobic Respiration: Uses other acceptors (NO3-, SO4-, etc.).

• Fermentation: No external electron acceptor; organic compounds serve as both electron donor and acceptor.

Electron Transport Chain and Proton Motive Force

• ETC: Series of membrane-bound carriers; electrons flow, protons pumped.

• Proton Motive Force (PMF): Drives ATP synthesis, solute transport, motility.

Citric Acid Cycle and Catabolic Diversity

• Citric Acid Cycle: Oxidizes acetyl-CoA; produces CO2, NADH, FADH2.

• Catabolic Diversity: Prokaryotes have greater metabolic diversity than Eukaryotes.

• Key Intermediates: 12 central metabolites produced by catabolic pathways.

Microbial Growth

Cellular vs Population Growth

• Cellular Growth: Increase in cell size and mass.

• Population Growth: Increase in cell number.

Cell Division Processes

• Binary Fission: Cell divides into two equal parts.

• Budding: New cell forms as a protrusion.

• Stalked Organisms: Division involves specialized structures.

Mathematical Expressions of Growth

• Population growth can be modeled mathematically:

Growth Phases in Batch Cultures

• Lag Phase: Adaptation, no growth.

• Exponential Phase: Rapid growth.

• Stationary Phase: Growth ceases, nutrients depleted.

• Death Phase: Cell death exceeds growth.

Continuous Cultures (Chemostats)

• Chemostats maintain cells in a specific growth phase by controlling nutrient supply and waste removal.

Measuring Cell Concentrations

• Methods: Direct counts, viable plate counts, spectrophotometry.

Environmental Factors Affecting Growth

• Temperature: Cardinal temperatures (minimum, optimum, maximum).

• Oxygen: Aerobes, microaerophiles, aerotolerant, facultative, obligate anaerobes.

• pH, Water Activity: Affect enzyme activity and cell integrity.

Coping with Reactive Oxygen Species

• Enzymes: Superoxide dismutase, catalase, peroxidase.

Molecular Adaptations to Extremes

• Proteins, membranes, and genomes adapt to extreme temperatures, pH, salinity.

Sterilization and Disinfection

• Methods: Heat, filtration, chemicals.

• Bacteriostatic: Inhibits growth.

• Bacteriocidal: Kills cells.

• Bacteriolytic: Kills and lyses cells.

Viruses and Virology

Nature and Types of Viruses

• Viruses: Non-cellular entities; require host for replication.

• Types: DNA, RNA, double-stranded (ds), single-stranded (ss), retroviruses.

Virus Structure

• Simple: Only nucleic acid and capsid.

• Complex: Additional structures.

• Naked: No envelope.

• Enveloped: Lipid membrane from host.

Transmission and Infection

• Attachment to host via specific receptors.

• Host specificity: Receptive and permissive cells.

Replication Cycles

• Lytic Cycle: Virus replicates, lyses host.

• Lysogenic Cycle: Viral genome integrates, replicates with host.

• Other cycles: Chronic, latent infections.

Impact and Assays

• Viruses affect microbial food webs, nutrient transfer.

• Viral Titer: Concentration of virus particles; assayed by plaque method.

• Burst Size: Number of virions released per cell.

• Assembly: Formation of new virions.

• Latent Period: Time between infection and lysis.

• Capsomere: Protein subunit of capsid.

• Nucleocapsid: Capsid plus nucleic acid.

• Icosahedral Symmetry: Common viral shape.

Viral Enzymes and Immunity

• Viruses may carry enzymes for replication, integration, etc.

• Cell populations develop immunity via restriction enzymes, CRISPR.

Bacterial vs Animal Viruses

• Bacterial viruses (phages) often inject DNA; animal viruses enter via endocytosis.

Retroviruses, Viroids, Prions

• Retrovirus: RNA virus with reverse transcriptase.

• Viroid: Infectious RNA, no protein coat.

• Prion: Infectious protein, no nucleic acid.

Metabolic Regulation

DNA-Binding Proteins and Transcription Control

• DNA-binding proteins regulate gene expression by interacting with promoters and operators.

• Promoter: DNA sequence where RNA polymerase binds.

• Operator: DNA sequence where regulatory proteins bind.

• Transcription: Carried out by RNA polymerase.

Negative and Positive Control

• Negative Control: Repression (turns off gene), induction (turns on gene).

• Positive Control: Activator proteins enhance transcription.

Operons, Regulons, and Global Control

• Operon: Cluster of genes under single promoter.

• Regulon: Multiple operons regulated together.

• Global Control: Systems affecting many genes (e.g., catabolite repression).

Diauxic Growth and Two-Component Systems

• Diauxic Growth: Sequential use of two carbon sources.

• Two-Component Systems: Sensor kinase and response regulator; regulate processes like chemotaxis.

Quorum Sensing and Feedback Inhibition

• Quorum Sensing: Cell communication based on population density.

• Feedback Inhibition: End product inhibits pathway.

Post-Translational Modification

• Protein function regulated by chemical modifications after translation.

Genetics of Bacteria and Archaea

Mutations and Mutant Screening

• Mutation: Change in DNA sequence.

• Mutant: Organism with altered genotype.

• Reversion: Mutation returns to original state.

• Selectable Mutation: Confers advantage; easy to detect.

• Non-Selectable Mutation: No obvious advantage; harder to detect.

• Wild-Type Strain: Reference strain.

• Auxotroph: Mutant unable to synthesize a compound.

• Replica Plating: Technique to screen for mutants.

Types and Effects of Mutations

• Point Mutations: Single base change; can be silent, nonsense, or missense.

• Frameshift Mutations: Insertions/deletions shift reading frame.

• Mutation rates vary; mutagens increase rates.

• Mutagens: Chemicals, radiation causing mutations.

• DNA Repair: Mechanisms to fix mutations.

Recombination and Gene Transfer

• Recombination: Exchange of genetic material.

• Observed experimentally via genetic crosses.

• Transformation: Uptake of free DNA.

• Transduction: DNA transfer via virus.

• Conjugation: DNA transfer via cell contact.

• Conjugation involves pilus formation and plasmid transfer.

Transposable Elements and CRISPR

• Transposable Elements: DNA segments that move within genome.

• CRISPR: Adaptive immune system in prokaryotes; used for gene editing.

Example: The lac operon in Escherichia coli is regulated by both negative and positive transcriptional control, demonstrating operon and global control concepts.

Additional info: Academic context was added to clarify definitions, mechanisms, and examples for each topic, and to reconstruct tables for comparison and classification.