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

Chapter 11: The Prokaryotes—Domains Bacteria and Archaea

Overview 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 genetic material is found in a nucleoid region.

• Cell Wall: Most have a rigid cell wall composed of peptidoglycan (bacteria) or pseudopeptidoglycan (archaea).

• Special Structures: Many possess pili (for attachment and conjugation), flagella (for movement), capsules (for protection), and endospores (for survival in harsh conditions).

• Reproduction: Prokaryotes reproduce asexually by binary fission.

Gram Staining

Gram staining differentiates bacteria based on cell wall structure:

• Gram-Positive Bacteria: Thick peptidoglycan layer; stain purple.

• Gram-Negative Bacteria: Thin peptidoglycan layer and outer membrane; stain pink/red.

Key Bacterial Genera and Their Features

• Streptococcus: Gram-positive cocci; cause 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, can cause infections.

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

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

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

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

• Streptomyces: Gram-positive, filamentous; produce antibiotics.

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

• Wolbachia: Intracellular bacteria; affect reproduction in arthropods.

• Yersinia: Gram-negative rods; cause plague.

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

• Clostridium: Gram-positive rods; anaerobic, form endospores, cause tetanus, botulism.

• Cyanobacteria: Photosynthetic bacteria; produce oxygen, found in aquatic environments.

Archaea: Types and Characteristics

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

• 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 walls lack peptidoglycan; unique membrane lipids.

Bacterial Shapes and Arrangements

• Coccus: Spherical

• Bacillus: Rod-shaped

• Spirillum/Spirochete: Spiral-shaped

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

Bacterial Motility

• Flagella: Most common; rotate to propel cell.

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

• Gliding and Twitching: Some bacteria move without flagella using surface proteins or pili.

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

Defining Eukaryotic Cells

Eukaryotic cells have a true nucleus enclosed by a nuclear membrane and possess membrane-bound organelles. They are structurally more complex than prokaryotes.

• Key Differences: Presence of nucleus, organelles (mitochondria, ER, Golgi), larger size, complex cell division (mitosis/meiosis).

Protists: Disease-Causing and Unique Types

• Giardia: Causes giardiasis (intestinal infection).

• Dinoflagellates: Some produce toxins causing red tides; can cause shellfish poisoning.

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

• Amoeba: Some species cause amoebic dysentery.

• Euglena: Photosynthetic, flagellated; not typically pathogenic.

• Phytophthora: Plant pathogen; causes potato blight.

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

• Brown, Green, Red Algae: Important in aquatic ecosystems; some produce food products (e.g., agar from red algae).

Common Disease Vectors

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

• Tsetse Flies: Transmit African sleeping sickness.

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

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, Enterobius.

Fungi: Structure and Reproduction

• Cell Wall: Composed of chitin.

• Hyphae: Filamentous structures; form a network called mycelium.

• Reproduction: Both sexual and asexual spores.

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.

Structure of Viruses

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

• Capsid: Protein coat surrounding nucleic acid.

• Envelope: Lipid membrane derived from host cell (in enveloped viruses).

• Spikes: Glycoproteins for attachment to host cells.

Viral Infection and Reproduction

• Lytic Cycle: Virus replicates and lyses host cell.

• Lysogenic Cycle: Viral DNA integrates into host genome (prophage); can later enter lytic cycle.

• Latent Viruses: Remain dormant in host; can reactivate (e.g., herpes simplex).

Enveloped Viruses

• Unique Features: Envelope helps evade immune system; acquired from host cell membrane during budding.

Types of Viruses

• DNA Viruses: Herpesvirus, Adenovirus, Hepatitis B.

• RNA Viruses: Influenza, Polio, Hepatitis C, Ebola, West Nile, COVID-19 (SARS-CoV-2).

• Retroviruses: HIV; use reverse transcriptase to convert RNA to DNA.

Viruses and Cancer

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

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

Key Terms

• Emerging: Newly identified or increasing in incidence.

• Epidemic: Sudden increase in disease cases in a region.

• Pandemic: Global outbreak of disease.

• 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.

Additional info:

• For Gram staining, the difference in cell wall structure is critical for antibiotic susceptibility and identification.

• Some bacteria (e.g., Clostridium, Bacillus) form endospores, which are highly resistant to environmental stress.

• Fungi reproduce by forming spores, which can be sexual or asexual.

• Prion diseases include Creutzfeldt-Jakob disease and mad cow disease.