General Biology II Unit 15 Review: Viruses, Bacteria, and Archaea

What's This Unit All About?

• Explores the fascinating world of microorganisms, focusing on viruses, bacteria, and archaea
• Delves into the structure, function, and replication strategies of these microscopic entities
• Examines the ecological roles and diversity of microbes in various environments
• Investigates the impact of pathogenic microbes on human health and disease
• Discusses practical applications and current research in microbiology, including biotechnology and medical advancements

Key Concepts and Definitions

• Microorganisms: Tiny living entities that can only be seen with a microscope, including viruses, bacteria, archaea, and some eukaryotic organisms (protists and fungi)
• Viruses: Non-living entities consisting of genetic material (DNA or RNA) encased in a protein coat, requiring host cells for replication
• Bacteria: Prokaryotic microorganisms lacking membrane-bound organelles, with a cell wall composed of peptidoglycan
  • Gram-positive bacteria: Have a thick cell wall that retains the crystal violet stain in the Gram staining procedure
  • Gram-negative bacteria: Have a thin cell wall and an outer membrane, appearing pink after the Gram staining procedure
• Archaea: Prokaryotic microorganisms distinct from bacteria, often thriving in extreme environments (high temperature, salinity, or acidity)
• Pathogenic microbes: Microorganisms capable of causing disease in a host, often through the production of toxins or invasion of host tissues
• Microbial ecology: The study of microorganisms in their natural environments and their interactions with other organisms and the environment
• Biotechnology: The application of microorganisms or their components in industrial processes, such as the production of antibiotics, enzymes, or biofuels

Viral Structure and Replication

• Viruses are acellular entities composed of a protein capsid enclosing genetic material (DNA or RNA)
• Some viruses have an additional lipid envelope derived from the host cell membrane
• Viral genomes can be single-stranded or double-stranded, linear or circular, and vary in size and complexity
• Viruses replicate through a series of steps:
  1. Attachment: Viral surface proteins bind to specific receptors on the host cell
  2. Penetration: The virus or its genetic material enters the host cell
  3. Replication: Viral genes are expressed, and the host cell's machinery is hijacked to produce viral components
  4. Assembly: Viral components are assembled into new virions
  5. Release: New viruses are released from the host cell through lysis or budding
• Lytic cycle: Viral replication results in the lysis (rupture) of the host cell, releasing new virions
• Lysogenic cycle: Viral genome integrates into the host cell's genome (prophage) and replicates along with the host, potentially entering the lytic cycle later

Bacterial Cell Structure and Function

• Bacteria are prokaryotic cells with a simple cell structure lacking membrane-bound organelles
• Cell wall: Rigid structure composed of peptidoglycan, provides shape and protection
• Cell membrane: Selective barrier controlling the passage of substances in and out of the cell
• Cytoplasm: Gel-like substance containing enzymes, ribosomes, and other cellular components
• Nucleoid: Region containing the bacterial DNA, not enclosed by a nuclear membrane
• Plasmids: Small, circular DNA molecules that can replicate independently of the main chromosome and often carry genes for antibiotic resistance or other functions
• Ribosomes: Sites of protein synthesis, smaller (70S) than eukaryotic ribosomes
• Flagella: Long, whip-like structures used for motility in some bacteria
• Pili: Short, hair-like structures involved in attachment to surfaces or other cells, and in DNA transfer during conjugation

Archaea: The Third Domain of Life

• Archaea are prokaryotic microorganisms that form a separate domain of life, distinct from bacteria and eukaryotes
• Often thrive in extreme environments, such as hot springs, salt lakes, and deep-sea hydrothermal vents
• Cell wall: Varies in composition, lacking peptidoglycan found in bacterial cell walls
  • Some archaea have pseudopeptidoglycan, while others have S-layer proteins or lack a cell wall entirely
• Cell membrane: Contains unique lipids with ether linkages and branched hydrocarbon chains, contributing to their stability in extreme conditions
• Metabolism: Diverse metabolic capabilities, including methanogenesis (production of methane), sulfur reduction, and ammonia oxidation
• Evolutionary significance: Share some features with bacteria (prokaryotic cell structure) and eukaryotes (certain genes and metabolic pathways), providing insights into the early evolution of life

Microbial Ecology and Diversity

• Microorganisms are ubiquitous and play crucial roles in various ecosystems
• Nutrient cycling: Microbes are key drivers of biogeochemical cycles, such as carbon, nitrogen, and sulfur cycles
  • Decomposers: Break down dead organic matter, releasing nutrients back into the environment
  • Nitrogen fixation: Convert atmospheric nitrogen ($N_2$) into biologically available forms (ammonia, $NH_3$)
• Symbiotic relationships: Microbes form diverse relationships with other organisms
  • Mutualism: Both partners benefit (e.g., rhizobia bacteria and legume plants)
  • Commensalism: One partner benefits while the other is unaffected (e.g., gut microbiome and human host)
  • Parasitism: One partner (the parasite) benefits at the expense of the other (the host)
• Microbial diversity: Microorganisms exhibit an astonishing range of metabolic capabilities and adaptations to various environments
  • Extremophiles: Thrive in extreme conditions, such as high temperature (thermophiles), acidity (acidophiles), or salinity (halophiles)
  • Anaerobic microbes: Survive and grow in the absence of oxygen, using alternative electron acceptors (e.g., sulfate reducers, methanogens)

Pathogenic Microbes and Human Health

• Pathogenic microbes can cause infectious diseases in humans, leading to significant morbidity and mortality
• Transmission: Pathogens can spread through various routes, such as air, water, food, or direct contact
• Virulence factors: Microbial components or products that contribute to their ability to cause disease
  • Adhesins: Allow pathogens to attach to host cells or tissues
  • Toxins: Substances that damage host cells or disrupt normal cellular functions (e.g., botulinum toxin, tetanus toxin)
  • Enzymes: Aid in the invasion of host tissues or evasion of the immune system (e.g., hyaluronidase, coagulase)
• Immune response: The human body has evolved complex mechanisms to defend against microbial infections
  • Innate immunity: Non-specific, rapid response to pathogens (e.g., skin barrier, inflammatory response)
  • Adaptive immunity: Specific, targeted response to pathogens, involving antibodies and T cells
• Antibiotic resistance: The ability of bacteria to survive and grow in the presence of antibiotics, a growing public health concern
  • Caused by the overuse and misuse of antibiotics, leading to the selection and spread of resistant strains
  • Addressed through the development of new antibiotics, improved antibiotic stewardship, and alternative therapies (e.g., phage therapy, antimicrobial peptides)

Practical Applications and Current Research

• Microbiology has numerous practical applications and is a rapidly advancing field of research
• Biotechnology: Harnessing microorganisms or their components for various industrial and medical purposes
  • Biopharmaceuticals: Production of drugs, vaccines, and other therapeutics using genetically engineered microbes (e.g., insulin, monoclonal antibodies)
  • Bioremediation: Using microbes to clean up environmental pollutants (e.g., oil spills, heavy metals)
  • Food and beverage production: Fermentation processes involving microbes to produce foods and drinks (e.g., yogurt, beer, cheese)
• Microbiome research: Investigating the diverse microbial communities associated with the human body and their roles in health and disease
  • Gut microbiome: The collection of microbes in the human digestive tract, influencing nutrition, immune function, and mental health
  • Probiotics: Beneficial microbes consumed to promote a healthy gut microbiome and overall well-being
• Synthetic biology: Designing and constructing novel biological systems or organisms with specific functions
  • Metabolic engineering: Modifying microbial metabolic pathways to produce desired compounds (e.g., biofuels, pharmaceuticals)
  • Biosensors: Genetically engineered microbes that detect and respond to specific environmental stimuli (e.g., pollutants, disease biomarkers)
• Antimicrobial discovery: Searching for new antibiotics and alternative therapies to combat the rise of antibiotic-resistant pathogens
  • Natural product discovery: Screening microbes from diverse environments for novel antimicrobial compounds
  • Phage therapy: Using viruses that specifically infect and kill bacteria as a targeted treatment for bacterial infections