Unit 4: Comprehensive Study Notes

Cell Structure and Classification

Gram Positive vs. Gram Negative Cell Walls

The structure of bacterial cell walls is fundamental to microbiology, influencing staining, pathogenicity, and antibiotic susceptibility.

• Gram Positive Cell Walls: Thick peptidoglycan layer, teichoic acids, retain crystal violet stain (purple).

• Gram Negative Cell Walls: Thin peptidoglycan layer, outer membrane with lipopolysaccharides, do not retain crystal violet (appear pink after counterstain).

• Gram Stain: Differential staining technique used to classify bacteria based on cell wall structure.

• Examples: Staphylococcus (Gram positive), Escherichia coli (Gram negative).

Unique Bacterial Genera: Mycoplasma and Mycobacterium

• Mycoplasma: Lacks a cell wall, making it resistant to antibiotics targeting peptidoglycan.

• Mycobacterium: Has a waxy, lipid-rich cell wall (mycolic acids), acid-fast staining required.

Endospore Structure and Function

• Endospore: Dormant, highly resistant structure formed by some Gram positive bacteria (e.g., Bacillus, Clostridium).

• Sporulation: Process of endospore formation triggered by harsh conditions.

• Purpose: Survival during environmental stress.

Plasmids

• Plasmid: Small, circular DNA molecule independent of chromosomal DNA; often carries antibiotic resistance genes.

Microbial Metabolism (Chapter 5)

Aerobic vs. Anaerobic Respiration

Microorganisms obtain energy through various metabolic pathways, which can be classified based on oxygen requirements.

• Aerobic Respiration: Uses oxygen as the final electron acceptor; produces more ATP.

• Anaerobic Respiration: Uses inorganic molecules other than oxygen as final electron acceptors (e.g., nitrate, sulfate).

• Fermentation: Anaerobic process; organic molecules serve as electron acceptors; less ATP produced.

Electron Transport Chain

• Final Electron Acceptors:

  • Aerobic: Oxygen

  • Anaerobic: Nitrate, sulfate, etc.

  • Fermentation: Organic molecules (e.g., pyruvate)

Microbial Growth (Chapter 6)

Oxygen Requirements of Microorganisms

Microorganisms are classified based on their oxygen requirements, which affect their growth and metabolism.

• Obligate Aerobe: Requires oxygen for growth.

• Obligate Anaerobe: Cannot tolerate oxygen; grows only in its absence.

• Facultative Anaerobe: Can grow with or without oxygen.

• Aerotolerant Anaerobe: Does not use oxygen but tolerates its presence.

• Microaerophile: Requires low levels of oxygen.

Metabolic Pathways and Applicability

• Aerobic Respiration: Used by obligate aerobes and facultative anaerobes in oxygen-rich environments.

• Anaerobic Respiration: Used by obligate anaerobes and some facultative anaerobes.

• Fermentation: Used by aerotolerant anaerobes and facultative anaerobes in absence of oxygen.

Microbial Genetics

Genetic Transfer and Recombination

Genetic variation in bacteria is achieved through several mechanisms of gene transfer and recombination.

• Horizontal Gene Transfer: Movement of genetic material between organisms other than by descent.

• Types of Genetic Transfer:

  • Transformation: Uptake of naked DNA from environment.

  • Conjugation: Transfer of DNA via direct cell-to-cell contact, often involving plasmids.

  • Transduction: Transfer of DNA by bacteriophages.

• Genetic Recombination: Integration of foreign DNA into host genome, increasing genetic diversity.

• R Factor: Plasmid conferring antibiotic resistance.

Plasmids and Genetic Transfer

• Plasmid: Can transfer genes via conjugation; F plasmid enables transfer of chromosomal genes.

Classification – Eukaryotes (Chapter 12)

Plasmodium and Malaria

Plasmodium is a protozoan parasite responsible for malaria, transmitted by the Anopheles mosquito.

• Vector: Anopheles mosquito.

• Definitive Host: Host in which sexual reproduction occurs (mosquito for Plasmodium).

• Causative Agent: Plasmodium species cause malaria.

Classification – Viruses (Chapter 13)

DNA Virus Families

DNA viruses are classified based on envelope presence, replication site, and genome structure.

Family	Envelope	Replication Site	Transforming Members
Papillomaviridae	No	Nucleus	Human Papillomavirus
Herpesviridae	Yes	Nucleus	Varicella Zoster Virus (VZV)
Poxviridae	Yes	Cytoplasm	Variola Virus

RNA Virus Families

Family	Envelope	Genome	Strand Type	Example
Picornaviridae	No	Segmented	Positive	Poliovirus
Rhabdoviridae	Yes	Non-segmented	Negative	Rabies virus

Retroviruses

• Effect on Infection: Integration into host genome, persistent infection (e.g., HIV).

Microbial Mechanisms of Pathogenicity

Exotoxins vs. Endotoxins

Bacterial toxins are classified based on their source, structure, timing of release, and immunogenicity.

Feature	Exotoxin	Endotoxin
Source	Secreted by living bacteria	Component of Gram negative outer membrane (LPS)
Structure	Protein	Lipid A of LPS
Release	Actively secreted	Released upon cell lysis
Immunogenicity	Highly immunogenic	Weakly immunogenic

Types of Exotoxins

• Cytotoxins: Damage host cells.

• Neurotoxins: Affect nervous system.

• Enterotoxins: Target intestinal tract.

Exotoxin Production in Pathogenic Bacteria

• Clostridium tetani: Tetanus toxin (neurotoxin).

• Clostridium botulinum: Botulinum toxin (neurotoxin).

• Staphylococcus aureus: Enterotoxins, toxic shock syndrome toxin.

• Vibrio cholerae: Cholera toxin (enterotoxin).

Cytopathic Effect (CPE)

• CPE: Observable changes in host cells due to viral infection.

• Types:

  • Syncytia formation

  • Inclusion bodies

  • Cell lysis and rounding up

Host Defenses (Chapters 15 and 16)

Phagocytosis and Pattern Recognition

Phagocytosis is a key innate immune response, enhanced by pattern recognition receptors (PRRs) and opsonization.

• Steps of Phagocytosis:

  1. Chemotaxis

  2. Adherence

  3. Ingestion

  4. Digestion

  5. Exocytosis

• PRRs: Detect pathogen-associated molecular patterns (PAMPs).

• Opsonization: Coating of pathogens to enhance phagocytosis.

Complement Pathways

• Classical Pathway: Activated by antigen-antibody complexes.

• Alternative/Lectin Pathways: Activated by microbial surfaces or lectins.

Functions of Complement

• Opsonization: Enhances phagocytosis.

• Inflammation: Attracts immune cells.

• Lysis: Destroys pathogens by forming membrane attack complex.

Primary Cell-Mediated Response (Th1)

• Th1 Cells: Activate macrophages and cytotoxic T cells.

• Function: Defense against intracellular pathogens.

Additional info: These notes expand on the original questions by providing definitions, examples, and structured tables for comparison and classification, ensuring a comprehensive review for exam preparation.