Microbiology Midterm Review Guide: Key Concepts and Definitions

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Microbiology Fundamentals

Definitions and Key Terms

This section covers essential definitions and concepts foundational to microbiology, including the nature of microorganisms, their classification, and their cellular structures.

Disease-causing microorganism: Also known as a pathogen, it is any microorganism (bacteria, virus, fungus, protozoan) capable of causing disease in its host.
Enzyme, coenzyme, and cofactor:
- Enzyme: Biological catalyst that speeds up chemical reactions in cells.
- Coenzyme: Organic molecule that assists enzymes, often derived from vitamins.
- Cofactor: Non-protein chemical compound (can be metal ions or organic molecules) required for enzyme activity.
Capsule: A polysaccharide or polypeptide layer outside the cell wall of some bacteria, providing protection and aiding in pathogenicity.
Peptidoglycan: A polymer forming the cell wall of most bacteria, providing structural strength. Found in both Gram-positive and Gram-negative bacteria, but thicker in Gram-positive.
Fungi: Eukaryotic organisms, some of which are pathogenic to humans (e.g., Candida species).
Difference between viruses and bacteria:
- Bacteria: Prokaryotic, living cells, reproduce independently.
- Viruses: Acellular, require host cells to replicate, contain either DNA or RNA.

Cell Structure and Function

Bacterial Cell Components

Bacterial cells possess unique structures that contribute to their survival, pathogenicity, and classification.

Flagella: Motility structures; types include monotrichous (single), lophotrichous (tuft), amphitrichous (both ends), peritrichous (all over).
DNA replication: The process by which bacteria duplicate their genetic material before cell division. Key steps: initiation, elongation, termination.
Binary fission: The primary method of bacterial reproduction, involving the division of a single cell into two identical daughter cells.
Magnetosome: Membrane-bound organelle in some bacteria containing magnetic iron minerals, allowing orientation along magnetic fields.
Peroxisome: Organelle involved in breakdown of fatty acids and detoxification of hydrogen peroxide (mainly in eukaryotes).
Golgi complex: Organelle in eukaryotic cells responsible for modifying, sorting, and packaging proteins and lipids.
Cell wall: Provides structural support; Gram-positive bacteria have thick peptidoglycan, Gram-negative have thin peptidoglycan and outer membrane.

Gram Staining and Cell Wall Differences

Gram-Positive vs. Gram-Negative Bacteria

Gram staining differentiates bacteria based on cell wall composition, which affects their response to antibiotics and immune defenses.

Gram stain: A differential staining technique; Gram-positive bacteria stain purple due to thick peptidoglycan, Gram-negative stain pink due to thin peptidoglycan and outer membrane.
Differences:
- Gram-positive: Thick peptidoglycan, teichoic acids, no outer membrane.
- Gram-negative: Thin peptidoglycan, outer membrane with lipopolysaccharide (LPS), periplasmic space.

Prokaryotic vs. Eukaryotic Cells

Comparative Features

Understanding the differences between prokaryotic and eukaryotic cells is fundamental in microbiology.

Prokaryotic cells: No nucleus, no membrane-bound organelles, single circular DNA.
Eukaryotic cells: Nucleus, membrane-bound organelles, multiple linear chromosomes.
Cell wall composition: Bacterial cell walls contain peptidoglycan; eukaryotic cell walls (in plants/fungi) contain cellulose or chitin.

Microbial Metabolism

Catabolism, Energy Production, and Fermentation

Microorganisms obtain energy through various metabolic pathways, including catabolism and fermentation.

Catabolic reactions: Break down molecules to release energy, often producing ATP.
Fermentation: Anaerobic process converting sugars to acids, gases, or alcohol.
Glycolysis: The metabolic pathway converting glucose to pyruvate, producing ATP and NADH. Equation:
Oxidative phosphorylation: ATP production using electron transport chain and oxygen as final electron acceptor.

Microbial Growth and Reproduction

Bacterial Growth Curve and Generation Time

Bacterial populations grow in distinct phases, and generation time refers to the time required for a cell to divide.

Bacterial growth curve: Four phases: lag, log (exponential), stationary, death.
Generation time: The time it takes for a bacterial population to double.

Microbial Genetics

DNA, RNA, and Genetic Variation

Genetic material in microorganisms determines their traits and ability to adapt.

DNA location in bacteria: Main chromosome in nucleoid region; plasmids in cytoplasm.
Types of RNA: mRNA (messenger), tRNA (transfer), rRNA (ribosomal).
Gene transfer: Movement of genetic material between cells; includes transformation, transduction, conjugation.
Base substitution: Mutation where one nucleotide is replaced by another.
Frameshift mutation: Insertion or deletion of nucleotides altering the reading frame.
MicroRNA: Small non-coding RNA molecules regulating gene expression (mainly in eukaryotes).

Microbial Classification and Identification

Taxonomy and Diagnostic Techniques

Microorganisms are classified based on morphology, genetics, and biochemical properties.

Identification: Determining the species or strain of a microorganism using tests and observations.
Culture: Growing microorganisms in controlled conditions.
Broth and agar: Liquid and solid media for culturing microbes.
Serial dilution: Stepwise dilution of a substance to estimate concentration or isolate colonies.

Microbial Pathogenesis and Disease

Pathogens, Transmission, and Disease Vectors

Microorganisms cause disease through various mechanisms and can be transmitted by vectors.

Pathogenic bacteria: Bacteria capable of causing disease (e.g., Streptococcus).
Vector of Lyme disease: The black-legged tick (Ixodes scapularis).
Chagas disease: Caused by Trypanosoma cruzi, transmitted by triatomine bugs.
Superficial mycoses: Fungal infections affecting skin, hair, or nails.
Systemic mycoses: Fungal infections affecting internal organs.
Helminths: Parasitic worms; some are human parasites.

Microbial Physiology and Adaptation

Environmental Responses and Metabolic Types

Microorganisms adapt to diverse environments and exhibit various metabolic strategies.

Halophile: Organism that thrives in high salt concentrations.
Psychrophile: Organism that grows best at low temperatures (0–15°C).
Chemotroph: Organism that obtains energy from chemical compounds.
Phototroph: Organism that obtains energy from light.
Aerotolerant: Can survive in presence of oxygen but does not use it.
Anaerobe: Grows without oxygen.
Microaerophile: Requires low levels of oxygen.
Aerobe: Requires oxygen for growth.

Microbial Structures and Motility

Flagella, Pseudopods, and Cell Morphology

Microorganisms use various structures for movement and interaction with their environment.

Flagella: Composed of filament, hook, and basal body; used for motility.
Pseudopods: Temporary projections of cytoplasm for movement (in protozoa).
Coccus, spiral forms: Bacterial shapes; cocci are spherical, spirals are twisted.

Microbial Processes and Biochemical Reactions

Enzyme Activity, Photosynthesis, and Respiration

Microorganisms carry out diverse biochemical reactions essential for survival.

Catalase: Enzyme that breaks down hydrogen peroxide into water and oxygen.
Photosynthesis: Conversion of light energy to chemical energy; equation:
Enzyme inhibition: Competitive inhibitors bind active site; noncompetitive inhibitors bind elsewhere, altering enzyme function.

Genetic Mechanisms and Gene Transfer

Transformation, Transduction, and Conjugation

Bacteria exchange genetic material through several mechanisms, contributing to genetic diversity.

Transformation: Uptake of free DNA from environment.
Transduction: Transfer of DNA via bacteriophages.
Conjugation: Direct transfer of DNA between bacteria via pilus.

Additional Key Terms and Concepts

Other Important Definitions

Transcription: Synthesis of RNA from DNA template.
Codon: Sequence of three nucleotides in mRNA specifying an amino acid.
Substitution: Mutation replacing one base with another.
Stages of bacterial growth curve: Lag, log, stationary, death.

Table: Comparison of Gram-Positive and Gram-Negative Bacteria

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Layer	Thick	Thin
Teichoic Acids	Present	Absent
Outer Membrane	Absent	Present
Stain Color	Purple	Pink
Lipopolysaccharide (LPS)	Absent	Present

Table: Types of Gene Transfer in Bacteria

Type	Description
Transformation	Uptake of naked DNA from environment
Transduction	DNA transfer via bacteriophage
Conjugation	Direct transfer via pilus between cells

Example Applications

Gram stain: Used in clinical labs to identify bacterial infections and guide antibiotic therapy.
Gene transfer: Basis for genetic engineering and antibiotic resistance spread.
Fermentation: Used in food production (e.g., yogurt, cheese, alcohol).

Additional info: Some definitions and explanations have been expanded for clarity and completeness based on standard microbiology curriculum.