Microbiology Study Guide: Prokaryotes, Eukaryotes, and Viruses (Chapters 11–13)

Chapter 11: The Prokaryotes – Domains Bacteria and Archaea

Basics of Prokaryotes

Prokaryotes are unicellular organisms lacking a membrane-bound nucleus and organelles. They include bacteria and archaea, which are distinguished by their cellular structures and habitats.

• Cell Type: Prokaryotic cells lack a true nucleus; their DNA is found in a nucleoid region.

• Cell Walls: Most bacteria have cell walls composed of peptidoglycan; archaea have cell walls without peptidoglycan.

• Special Structures:

  • Pili: Hair-like appendages for attachment and conjugation.

  • Flagella: Used for motility; structure differs from eukaryotic flagella.

  • Capsule: Gelatinous outer layer for protection and adherence.

  • Endospores: Highly resistant, dormant structures formed by some bacteria (e.g., Bacillus, Clostridium).

  • Reproduction: Mainly by binary fission (asexual).

Gram Staining

Gram staining differentiates bacteria based on cell wall structure:

• Gram-Positive: Thick peptidoglycan layer; stains purple (e.g., Staphylococcus, Bacillus).

• Gram-Negative: Thin peptidoglycan layer and outer membrane; stains pink (e.g., Escherichia, Pseudomonas).

Major Bacterial Groups and Features

Key genera and their characteristics:

• Streptococcus: Gram-positive cocci; causes strep throat, pneumonia.

• Pseudomonas: Gram-negative rods; opportunistic pathogens, resistant to antibiotics.

• Rickettsia: Obligate intracellular parasites; cause typhus, Rocky Mountain spotted fever.

• Bacteroides: Gram-negative anaerobes; abundant in human gut.

• Salmonella: Gram-negative rods; cause foodborne illness.

• Escherichia (E. coli): Gram-negative rods; normal flora and some pathogenic strains.

• Staphylococcus: Gram-positive cocci; cause skin infections, MRSA.

• Neisseria: Gram-negative diplococci; cause gonorrhea, meningitis.

• Streptomyces: Gram-positive; produce antibiotics.

• Spirochetes: Spiral-shaped; include Treponema (syphilis), Borrelia (Lyme disease).

• Wolbachia: Intracellular symbionts of insects.

• Yersinia: Gram-negative rods; cause plague.

• Bacillus: Gram-positive rods; form endospores, e.g., B. anthracis.

• Clostridium: Gram-positive rods; anaerobic, form endospores, e.g., C. botulinum.

• Cyanobacteria: Photosynthetic bacteria; produce oxygen.

Archaea: Types and Features

Archaea are prokaryotes distinct from bacteria, often found in extreme environments.

• Methanogens: Produce methane; live in anaerobic environments (e.g., swamps).

• Extreme Halophiles: Thrive in high-salt environments (e.g., salt lakes).

• Extreme Thermophiles: Live in very hot environments (e.g., hot springs).

• Unique Features: Cell membranes with ether-linked lipids; lack peptidoglycan.

Bacterial Shapes and Arrangements

• Coccus: Spherical

• Bacillus: Rod-shaped

• Spirillum/Spirilla: Spiral-shaped

• Arrangements: Chains (strepto-), clusters (staphylo-), pairs (diplo-)

Bacterial Motility

• Flagella: Most common; rotate for movement.

• Axial Filaments: Found in spirochetes; allow corkscrew motion.

• Gliding Motility: Some bacteria move without flagella (e.g., Myxococcus).

• Pili: Some use pili for twitching motility.

Chapter 12: The Eukaryotes – Fungi, Algae, Protozoa, and Helminths

Basics of Eukaryotic Cells

Eukaryotic cells have a true nucleus and membrane-bound organelles, distinguishing them from prokaryotes.

• Nucleus: Contains genetic material enclosed by a nuclear envelope.

• Organelles: Mitochondria, endoplasmic reticulum, Golgi apparatus, etc.

• Cell Wall: Present in fungi and algae (composition varies); absent in protozoa and animals.

Protists: Disease-Causing and Unique Types

• Giardia: Causes giardiasis (intestinal infection).

• Dinoflagellates: Some produce toxins causing red tides.

• Trypanosoma: Causes African sleeping sickness (transmitted by tsetse fly).

• Amoeba: Includes Entamoeba histolytica, causes amoebic dysentery.

• Euglena: Photosynthetic, flagellated protist.

• Phytophthora: Plant pathogen; causes potato blight.

• Plasmodium: Causes malaria (transmitted by Anopheles mosquito).

• Brown, Green, Red Algae: Photosynthetic; important in aquatic ecosystems.

Common Disease Vectors

• Mosquitoes: Transmit malaria, West Nile virus, yellow fever.

• Tsetse Flies: Transmit African trypanosomiasis.

• Ticks: Transmit Lyme disease, Rocky Mountain spotted fever.

• Fleas: Transmit plague.

Helminths: Tapeworms, Ringworms, Roundworms

• Tapeworms (Cestodes): Intestinal parasites; absorb nutrients through skin.

• Ringworm: Fungal infection of skin (not a worm).

• Roundworms (Nematodes): Intestinal and tissue parasites (e.g., Ascaris).

Fungi: Structure and Reproduction

• Cell Wall: Composed of chitin.

• Hyphae: Filamentous structures; form the mycelium.

• Mycelium: Mass of hyphae; main body of fungus.

• Reproduction: Sexual and asexual spores (e.g., conidia, sporangiospores).

Chapter 13: Viruses, Viroids, and Prions

Definitions

• Virus: Acellular infectious agent with DNA or RNA, protein coat (capsid), sometimes an envelope.

• Viroid: Infectious RNA molecule, no protein coat; infects plants.

• Prion: Infectious protein; causes neurodegenerative diseases (e.g., Creutzfeldt-Jakob disease).

Structure of Viruses

• Nucleic Acid: DNA or RNA (not both).

• Capsid: Protein coat surrounding nucleic acid.

• Envelope: Lipid membrane derived from host cell; present in some viruses.

• Spikes: Glycoproteins for attachment to host cells.

Viral Infection and Reproduction

• Attachment: Virus binds to host cell.

• Penetration: Entry of viral genome into host.

• Biosynthesis: Viral components are produced.

• Maturation: Assembly of viral particles.

• Release: New viruses exit host cell.

Lytic vs. Lysogenic Cycles

Feature	Lytic Cycle	Lysogenic Cycle
Viral DNA	Replicates independently	Integrates into host genome (prophage)
Host Cell Fate	Lysed (destroyed)	Survives, divides with viral DNA
End Result	Release of new viruses	Possible later entry into lytic cycle

Enveloped Viruses

• Unique Features: Envelope aids in entry/exit from host cells; sensitive to detergents.

• Examples: Influenza virus, HIV.

Latent Viruses

• Definition: Viruses that remain dormant in host cells and can reactivate.

• Examples: Herpes simplex virus, Varicella-zoster virus (chickenpox/shingles).

Types of Viruses

• RNA Viruses: Influenza, HIV, West Nile virus, COVID-19 (SARS-CoV-2).

• DNA Viruses: Herpesviruses, Hepatitis B, Adenoviruses.

• Retroviruses: HIV (reverse transcriptase enzyme; RNA to DNA).

Viruses and Cancer

• Oncogenes: Genes that can cause cancer when activated by viruses.

• Examples: Human papillomavirus (HPV) and cervical cancer; Hepatitis B and liver cancer.

Key Epidemiological Terms

• Emerging: Newly identified or increasing in incidence.

• Epidemic: Sudden increase in cases in a region.

• Pandemic: Worldwide epidemic.

• Pathogen: Disease-causing organism.

• Vector: Organism that transmits pathogens.

• Reservoir: Natural host or environment where pathogen lives.

Examples of Important Viruses

• Flu (Influenza): RNA virus; causes seasonal epidemics.

• Polio: RNA virus; causes poliomyelitis.

• Hepatitis: Several types (A, B, C); affect the liver.

• Ebola: RNA virus; causes hemorrhagic fever.

• Colds: Caused by rhinoviruses, coronaviruses.

• HIV and AIDS: Retrovirus; attacks immune system.

• West Nile Virus: RNA virus; transmitted by mosquitoes.

• COVID-19 (SARS-CoV-2): RNA virus; causes respiratory illness.

• Spanish Flu (H1N1): 1918 pandemic influenza virus.