Microbiology Study Guide: Chapters 7–13 (Genetics, Microbial Control, Antimicrobials, Prokaryotes, Eukaryotes, Viruses, and Prions)

Chapter 7: Microbial Genetics

Structure of DNA

DNA (deoxyribonucleic acid) is the hereditary material in all living organisms and many viruses. Its structure is essential for its function in storing genetic information.

• Nitrogenous Bases: The five nitrogenous bases are adenine (A), guanine (G), cytosine (C), thymine (T), and uracil (U; found in RNA). In DNA, A, G, C, and T are present.

• Attachment: Each base is attached to a deoxyribose sugar and a phosphate group, forming a nucleotide.

• Base Pairing: Bases pair via hydrogen bonds: A with T (2 bonds), G with C (3 bonds). This is called complementary base pairing.

Example: The double helix structure of DNA is stabilized by these base pairs, forming the genetic code.

Plasmids: Structure and Function

• Plasmids are small, circular, double-stranded DNA molecules found in prokaryotes and some eukaryotes.

• They replicate independently of the chromosomal DNA and often carry genes for antibiotic resistance, virulence factors, or metabolic pathways.

Eukaryotic vs. Prokaryotic Chromosomes

• Prokaryotic chromosomes: Usually single, circular, and located in the nucleoid region.

• Eukaryotic chromosomes: Multiple, linear, and contained within a membrane-bound nucleus.

• Eukaryotes also have histone proteins for DNA packaging; prokaryotes generally do not.

DNA Replication

• Begins at the origin of replication.

• Enzymes involved: Helicase (unwinds DNA), DNA polymerase (synthesizes new DNA), primase (lays RNA primers), ligase (joins Okazaki fragments).

• Leading strand: Synthesized continuously.

• Lagging strand: Synthesized discontinuously as Okazaki fragments.

Equation: $\text{DNA}_{\text{template}} + \text{dNTPs} \xrightarrow{\text{DNA polymerase}} \text{DNA}_{\text{new}}$

Genotype and Phenotype

• Genotype: The genetic makeup of an organism.

• Phenotype: The observable traits, determined by gene expression.

Central Dogma of Genetics

• Describes the flow of genetic information: DNA → RNA → Protein.

Equation: $\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

Mutations

• Mutation: A change in the nucleotide sequence of DNA.

• Types: Point mutations (substitution, insertion, deletion), frameshift mutations, silent, missense, nonsense mutations.

• UV Light: Causes thymine dimers, leading to errors in DNA replication.

Horizontal Gene Transfer in Bacteria

• Transformation: Uptake of naked DNA from the environment.

• Transduction: Transfer of DNA via bacteriophages (viruses that infect bacteria).

• Conjugation: Direct transfer of DNA between bacteria via a pilus.

Chapter 9: Controlling Microbial Growth in the Environment

Definitions and Practical Uses

• Sterilization: Complete removal or destruction of all microbes, including endospores.

• Disinfection: Destruction of most microbes on non-living surfaces.

• Antisepsis: Destruction of microbes on living tissue.

• Degerming: Mechanical removal of microbes (e.g., handwashing).

• Sanitization: Lowering microbial counts to safe public health levels.

• Pasteurization: Mild heat to kill pathogens in food and beverages.

Microbial Control Agents

• -static agents: Inhibit growth (e.g., bacteriostatic).

• -cidal agents: Kill microbes (e.g., bactericidal).

Microbial Death Rate

• Describes the rate at which a microbial population is destroyed.

• Often follows a logarithmic decline.

Equation: $\text{D-value} = \text{Time required to kill 90\% of microbes at a given condition}$

Factors in Selecting Control Methods

• Nature of the site, susceptibility of microbes, environmental conditions, cost, and safety.

Microbial Resistance

• Most resistant: Prions, bacterial endospores, mycobacteria.

• Most susceptible: Enveloped viruses, Gram-positive bacteria.

Biosafety Levels

Level	Example Microbes	Safety Measures
BSL-1	Non-pathogenic E. coli	Standard microbiological practices
BSL-2	Staphylococcus aureus	Lab coats, gloves, biohazard signs
BSL-3	Mycobacterium tuberculosis	Controlled access, biosafety cabinets
BSL-4	Ebola virus	Full-body suits, isolated facilities

Physical Methods of Microbial Control

• Heat (moist and dry), filtration, radiation (UV, ionizing), desiccation, osmotic pressure.

UV Light Disinfection

• UV light damages DNA, used to disinfect surfaces and some foods.

Chemical Methods of Microbial Control

• Alcohols, halogens, oxidizing agents, surfactants, heavy metals, aldehydes, phenolics, gases, enzymes.

• Limitation: Many disinfectants are inactivated by organic matter.

Chapter 10: Antimicrobial Drugs

Principle of Selective Toxicity

• Antimicrobial drugs should harm pathogens without harming the host.

Mechanisms of Action

• Inhibition of cell wall synthesis (e.g., penicillins)

• Inhibition of protein synthesis (e.g., tetracyclines)

• Disruption of cytoplasmic membrane (e.g., polymyxins)

• Inhibition of nucleic acid synthesis (e.g., quinolones)

• Inhibition of metabolic pathways (e.g., sulfonamides)

• Inhibition of pathogen attachment/entry (e.g., arildone)

Drug Spectrum and Resistance

• Broad-spectrum drugs: Affect many types of microbes; higher risk of superinfection and resistance.

• Narrow-spectrum drugs: Target specific microbes; less disruption to normal flora.

Antimicrobial Susceptibility Testing

Test	Description
Kirby-Bauer	Disk diffusion; measures zone of inhibition
Etest	Gradient strip; determines MIC
MIC	Minimum inhibitory concentration; lowest drug concentration that inhibits growth
MBC	Minimum bactericidal concentration; lowest drug concentration that kills bacteria

Routes of Administration

• Oral, intramuscular, intravenous; each has advantages and disadvantages regarding absorption, convenience, and side effects.

Side Effects of Antimicrobial Therapy

• Toxicity, allergies, disruption of normal microbiota.

Development of Resistance

• Populations of resistant microbes arise through mutation and horizontal gene transfer.

• R plasmids: Carry resistance genes; can be transferred between bacteria.

• Seven mechanisms: Enzyme destruction, target alteration, decreased uptake, increased efflux, metabolic bypass, biofilm formation, cross-resistance.

• Biofilms: Protect bacteria from drugs and immune responses.

• Retarding resistance: Use drugs appropriately, combination therapy (synergism), limit use, develop new drugs.

Chapter 11: Characterizing and Classifying Prokaryotes

Morphology and Arrangement

• Shapes: Cocci (spherical), bacilli (rod-shaped), spirilla (spiral), etc.

• Arrangements: Chains, clusters, pairs, etc.

Bacterial Endospores

• Formed by some bacteria (e.g., Bacillus, Clostridium) for survival in harsh conditions.

• Highly resistant to heat, chemicals, and radiation.

Prokaryotic Reproduction

• Binary fission (most common), budding, snapping division.

Domain Archaea

• Distinct from Bacteria; unique membrane lipids, lack peptidoglycan.

• Extremophiles: Thermophiles (heat-loving), halophiles (salt-loving).

• Methanogens: Produce methane, important in carbon cycling and sewage treatment.

Major Bacterial Phyla

• Bacillota (Firmicutes): Gram-positive, endospore formers.

• Mycoplasmatota: Lack cell walls.

• Actinomycetota: Filamentous, antibiotic producers.

• Pseudomonadota (Proteobacteria): Diverse, Gram-negative.

• Chlamydiota: Obligate intracellular pathogens.

• Spirochaetota: Spiral-shaped, motile.

• Bacteroidota: Anaerobic, common in gut flora.

Chapter 12: Characterizing and Classifying Eukaryotes

Complexity of Eukaryotic Reproduction

• Multiple chromosomes, mitosis and meiosis, sexual and asexual cycles, alternation of generations.

Mitosis Phases

• Prophase, metaphase, anaphase, telophase; involve chromosomes, chromatids, centromeres, and spindle fibers.

Haploid vs. Diploid

• Haploid (n): One set of chromosomes.

• Diploid (2n): Two sets of chromosomes.

Protozoa

• Unicellular, lack cell walls, motile at some stage.

• Most reproduce asexually by binary fission; some sexually.

Fungi

• Cell walls of chitin, non-photosynthetic, absorb nutrients.

• Beneficial as decomposers, antibiotics, food production.

• Dimorphic: Can exist as yeast or mold forms.

Vectors and Disease Transmission

• Vectors (e.g., fleas, lice, flies, mosquitoes, kissing bugs) transmit diseases such as plague, typhus, malaria, Chagas disease.

• Changing climate and travel contribute to spread in new locations.

Chapter 13: Viruses, Viroids, and Prions

Viruses

• Non-cellular, obligate intracellular parasites.

• Contain nucleic acid (DNA or RNA), protein capsid, sometimes envelope.

• Bacteriophage: Viruses that infect bacteria.

• Host specificity determined by surface proteins.

Viral Structure

• Capsid: Protein shell protecting nucleic acid.

• Envelope: Lipid membrane derived from host; enveloped vs. naked viruses.

Viral Replication Cycles

• Lytic cycle: Attachment, entry, synthesis, assembly, release (lysis).

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

• Animal viruses may be released by budding (enveloped) or lysis (naked).

Viral Protein Synthesis and Baltimore Classification

• Seven classes based on nucleic acid type and replication strategy.

• All viruses must produce mRNA for protein synthesis.

Lifelike vs. Non-lifelike Aspects

• Lifelike: Replicate, evolve.

• Non-lifelike: No metabolism, not cellular, require host machinery.

Prions

• Infectious proteins causing neurodegenerative diseases.

• Replicate by inducing misfolding of normal proteins.

• Diseases: Creutzfeldt-Jakob disease, kuru, scrapie, bovine spongiform encephalopathy (mad cow disease).