Honors Biology Unit 13 ReviewMicrobiology – Prokaryotes and Viruses

Key Concepts and Definitions

• Prokaryotes single-celled microorganisms that lack a nucleus and other membrane-bound organelles (bacteria and archaea)
• Viruses non-living infectious agents that consist of genetic material (DNA or RNA) encased in a protein coat
• Capsid protein shell that encases and protects the viral genetic material
• Bacteriophages viruses that specifically infect bacterial cells
• Lytic cycle viral replication process that results in the lysis (rupture) of the host cell and release of new viruses
• Lysogenic cycle viral replication process where the viral genome integrates into the host cell's genome and replicates along with it
• Pathogenicity ability of a microorganism to cause disease in a host
• Antibiotics substances that inhibit the growth or survival of bacteria

Prokaryotic Cell Structure

• Cell wall rigid structure surrounding the cell membrane provides protection and maintains cell shape
  • Peptidoglycan polymer consisting of sugars and amino acids forms the cell wall in most bacteria
• Cell membrane phospholipid bilayer that encloses the cytoplasm and controls the passage of substances in and out of the cell
• Nucleoid region where the circular DNA chromosome is located not enclosed by a nuclear membrane
• Ribosomes sites of protein synthesis smaller and simpler than eukaryotic ribosomes (70S vs. 80S)
• Plasmids small, circular DNA molecules that replicate independently of the main chromosome can carry genes for antibiotic resistance or virulence factors
• Flagella long, whip-like structures used for motility and chemotaxis
• Pili short, hair-like structures involved in attachment to surfaces and DNA transfer between cells

Bacterial Classification and Diversity

• Morphology bacteria classified based on their shape (cocci, bacilli, spirilla)
• Gram staining technique divides bacteria into two groups based on cell wall composition (Gram-positive and Gram-negative)
  • Gram-positive bacteria have a thick peptidoglycan layer and appear purple when stained
  • Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane appear pink when stained
• Metabolism bacteria classified based on their energy and carbon sources (phototrophs, chemotrophs, autotrophs, heterotrophs)
• Oxygen requirements bacteria classified as obligate aerobes, facultative anaerobes, or obligate anaerobes
• Extremophiles bacteria adapted to survive in extreme environments (high temperatures, low pH, high salinity)
• Phylogenetic classification based on genetic relatedness using 16S rRNA gene sequencing

Bacterial Growth and Reproduction

• Binary fission asexual reproduction process where a bacterial cell divides into two identical daughter cells
• Growth curve represents the phases of bacterial growth in a closed system (lag, exponential, stationary, death)
  • Lag phase period of adaptation to new environment minimal growth
  • Exponential (log) phase period of rapid growth and cell division
  • Stationary phase growth rate equals death rate due to nutrient depletion or waste accumulation
  • Death phase number of viable cells decreases as the death rate exceeds the growth rate
• Doubling time time required for a bacterial population to double in size varies among species
• Nutrient requirements bacteria need water, carbon, nitrogen, phosphorus, and other trace elements for growth
• Environmental factors temperature, pH, and oxygen availability influence bacterial growth

Viral Structure and Classification

• Nucleic acid type viruses classified based on their genetic material (DNA or RNA)
  • DNA viruses (adenoviruses, herpesviruses)
  • RNA viruses (influenza viruses, retroviruses)
• Capsid symmetry viruses classified based on the arrangement of their capsid proteins (icosahedral, helical, complex)
• Presence of an envelope some viruses have an additional lipid bilayer derived from the host cell membrane (enveloped vs. non-enveloped)
• Size viruses range in size from 20 to 400 nm much smaller than bacteria
• Host range viruses classified based on the type of cells they infect (animal, plant, bacterial)
• Baltimore classification system categorizes viruses into seven groups based on their genome and replication strategy

Viral Replication Cycles

• Attachment virus binds to specific receptors on the host cell surface
• Penetration viral genetic material enters the host cell through fusion or endocytosis
• Replication viral genome is replicated using host cell machinery
  • DNA viruses replicate in the nucleus using host DNA polymerase
  • RNA viruses replicate in the cytoplasm using viral RNA-dependent RNA polymerase
• Assembly viral proteins and genetic material are assembled into new virus particles
• Release new viruses are released from the host cell through lysis or budding
• Lytic cycle results in the destruction of the host cell and the release of new viruses
• Lysogenic cycle viral genome integrates into the host genome (prophage) and replicates along with it until induced to enter the lytic cycle

Prokaryotes and Viruses in Disease

• Pathogenic bacteria cause diseases by invading tissues, producing toxins, or triggering immune responses (Streptococcus, Salmonella, Mycobacterium)
• Viral pathogens cause diseases by infecting and killing cells, altering cell function, or inducing immune-mediated damage (influenza, HIV, measles)
• Zoonotic diseases infectious diseases that can be transmitted from animals to humans (rabies, Ebola, COVID-19)
• Antibiotic resistance ability of bacteria to survive and grow in the presence of antibiotics major public health concern
  • Resistance genes can be acquired through mutations or horizontal gene transfer (plasmids, transposons)
• Vaccines stimulate the immune system to produce antibodies against specific pathogens prevent infections
• Antiviral drugs target specific stages of the viral replication cycle to inhibit viral growth (reverse transcriptase inhibitors, protease inhibitors)

Ecological Roles and Applications

• Nutrient cycling prokaryotes play crucial roles in the carbon, nitrogen, and sulfur cycles
  • Nitrogen fixation conversion of atmospheric nitrogen (N2) into ammonia (NH3) by bacteria (Rhizobium)
  • Decomposition breakdown of organic matter by bacteria and fungi releases nutrients back into the environment
• Bioremediation use of microorganisms to clean up environmental pollutants (oil spills, heavy metals)
• Food production bacteria used in the production of fermented foods (yogurt, cheese, sauerkraut)
• Biotechnology genetically engineered bacteria used to produce insulin, enzymes, and other valuable compounds
• Probiotics beneficial bacteria that improve gut health when consumed (Lactobacillus, Bifidobacterium)
• Phage therapy use of bacteriophages to treat bacterial infections alternative to antibiotics
• Biocontrol use of viruses to control insect pests in agriculture (baculoviruses)