Microbiology Study Guide: Recombinant DNA, Microbial Control, Antimicrobial Drugs, and Epidemiology

Recombinant DNA Technology and Genetic Engineering

Vectors and Recombinant DNA

Recombinant DNA technology involves the intentional modification of the genetic material of organisms for practical purposes. A vector is a DNA molecule, such as a plasmid, virus, or transposon, that is used to deliver foreign genetic material into a cell.

• Recombinant Vector: A DNA construct containing a gene of interest (e.g., a human gene) inserted into a vector for delivery into a host cell.

• Example: Inserting the human insulin gene into a bacterial plasmid to produce insulin in bacteria.

Reverse Transcriptase and cDNA

Reverse transcriptase is an enzyme that synthesizes complementary DNA (cDNA) from an RNA template. This is crucial for expressing eukaryotic genes in prokaryotes, as it allows the production of intron-free DNA.

• Application: Production of human insulin in bacteria by converting mRNA to cDNA, which can be inserted into bacterial vectors.

Polymerase Chain Reaction (PCR)

PCR is a technique used to amplify specific DNA sequences, generating millions of copies from a small initial sample.

• Steps:

  1. Denaturation (94°C): Double-stranded DNA is separated into single strands.

  2. Priming (60°C): Primers anneal to the single-stranded DNA.

  3. Extension (72°C): DNA polymerase synthesizes new DNA strands.

• Thermus aquaticus: A bacterium whose heat-stable DNA polymerase (Taq polymerase) is essential for PCR.

Gel Electrophoresis

Gel electrophoresis separates DNA, RNA, or proteins based on size and charge by applying an electric field to a gel matrix.

• Applications: DNA fingerprinting, crime scene analysis, paternity testing.

Blotting Techniques

• Southern Blot: Detects specific DNA fragments.

• Northern Blot: Detects specific RNA fragments.

DNA Fingerprinting

DNA fingerprinting identifies individuals based on unique patterns in their DNA. Used in paternity testing and forensic investigations.

Key Definitions

• Recombinant DNA: DNA molecules formed by laboratory methods to bring together genetic material from multiple sources.

• Xenotransplantation: Transplantation of animal cells, tissues, or organs into humans (e.g., pig heart valves).

• Transgenic Organism: Organism with foreign genes inserted (GMO).

• DNA Ligase: Enzyme that joins DNA fragments together.

• Restriction Enzyme: Enzyme that cuts DNA at specific sequences.

• cDNA: Complementary DNA synthesized from mRNA.

• Mutagen: Agent that causes mutations in DNA.

• Antisense RNA: RNA molecule complementary to mRNA, inhibiting translation.

• DNA Polymerase: Enzyme that synthesizes DNA molecules.

• Splicing: Removal of introns from pre-mRNA.

• Plasmid: Small, circular DNA molecule in bacteria used as a vector.

• DNA Probe: Labeled DNA fragment used to detect specific sequences.

Controlling Microbial Growth in the Environment

Characteristics of Ideal Antimicrobial Agents

• Safe for humans and animals

• Inexpensive and shelf-stable

• Effective against a wide range of microbes

Physical Methods of Microbial Control

• Heat: Denatures proteins (e.g., autoclaving)

• Filtration: Removes microbes from liquids/air

• Radiation: Damages DNA (ionizing: X-rays, gamma rays; non-ionizing: UV light)

• Desiccation: Inhibits growth by drying

• Lyophilization: Freeze-drying to preserve microbes

Chemical Methods of Microbial Control

• Alcohols: Denature proteins, disrupt membranes (e.g., ethanol)

• Halogens: Oxidize cell components (e.g., bleach)

• Oxidizing Agents: Damage cellular components (e.g., hydrogen peroxide)

• Surfactants: Disrupt membranes (e.g., soaps)

Microbial Death Rate and Time

• Microbial Death Rate: Number of microbes killed per unit time

• Microbial Death Time: Time required to kill all microbes

• -static: Inhibits growth

• -cidal: Kills microbes

Inactivation of Resistant Microbes

• Protozoal Cysts: Require boiling or chemical agents

• Bacterial Endospores: Require autoclaving (high heat and pressure)

Biosafety Levels (BSL)

BSL	Description
1	Not known to cause disease in healthy humans
2	Moderately hazardous agents
3	High risk to humans
4	Cause fatal disease, no treatment

Relative Susceptibility of Microbes

Most Resistant	Most Susceptible
Prions, Bacterial Endospores	Gram-positive bacteria, Enveloped viruses

Tests for Efficacy of Antiseptics and Disinfectants

• Phenol Coefficient Test

• Use Dilution Test

• In-use Test

Key Definitions

• Antiseptic: Agent used on living tissue to inhibit or kill microbes (e.g., iodine)

• Disinfectant: Agent used on inanimate objects (e.g., bleach)

• Filtration: Removal of microbes by passing fluid through a filter

• Lyophilization: Freeze-drying for preservation

• Desiccation: Drying to inhibit growth

• Ionizing Radiation: Short wavelength, high energy (e.g., X-rays)

• Non-ionizing Radiation: Longer wavelength (e.g., UV light)

• Sanitize: Reduce microbial load on public objects

• Degerm: Remove microbes by scrubbing (e.g., handwashing)

• Aseptic: Free of contamination

Controlling Microbial Growth in the Body: Antimicrobial Drugs

Types of Antimicrobial Agents

• Antifungals: Treat fungal infections

• Antibacterials: Inhibit cell wall synthesis

• Anthelmintics: Treat parasitic worm infections

• Antivirals: Target viral life cycle (fewest agents due to host toxicity)

• Antiprotozoals: Treat protozoan infections

Kirby-Bauer Susceptibility Test

Measures the zone of inhibition around antibiotic disks to determine bacterial susceptibility.

Mechanisms of Action of Antimicrobial Drugs

• Selective Toxicity: Drug is more toxic to pathogen than host

• Mechanisms:

  • Inhibition of cell wall synthesis

  • Inhibition of protein synthesis

  • Disruption of cytoplasmic membrane

  • Inhibition of DNA/RNA synthesis

Classes of Antimicrobials

• Beta-lactams: Inhibit cell wall synthesis (e.g., penicillin)

• Sulfonamides: Inhibit folic acid synthesis

• Antisense Nucleic Acids: Block translation by binding mRNA

Antibiotic Resistance

• Beta-lactamase: Enzyme that deactivates beta-lactam antibiotics

• R-plasmids: Plasmids carrying resistance genes

• Mechanisms:

  • Enzyme production (drug destruction)

  • Efflux pumps (drug removal)

  • Cell membrane changes (prevent drug entry)

  • Target modification (reduce drug binding)

  • Altered metabolic pathways

  • Biofilm formation

  • Decoy proteins

Natural Selection and Resistance

Antibiotic use selects for resistant bacteria, leading to populations that are harder to treat.

Semi-synthetic vs. Synthetic Drugs

• Semi-synthetic: Modified natural antibiotics, often more stable

• Synthetic: Fully man-made, designed for specific targets

Broad vs. Narrow Spectrum Antibiotics

• Broad Spectrum: Effective against many types of microbes

• Narrow Spectrum: Effective against a few specific microbes

Infection, Infectious Diseases, and Epidemiology

Types of Symbiosis

Type	Effect on Organisms
Mutualism	Both benefit
Commensalism	One benefits, other unaffected
Amensalism	One harmed, other unaffected
Parasitism	One benefits, one harmed

Opportunistic Pathogens

• Cause disease when host defenses are compromised or when introduced to unusual sites

• Normal microbiota can become opportunistic under stress, immune suppression, or changes in microbiota

Signs vs. Symptoms

• Signs: Objective, observable (e.g., rash, swelling, vomiting)

• Symptoms: Subjective, felt by patient (e.g., pain, headache, nausea)

Epidemiological Terms

Term	Definition	Example
Endemic	Stable frequency in area	Common cold
Sporadic	Few scattered cases	Typhoid fever
Epidemic	Higher than normal frequency	West Nile virus
Pandemic	Global epidemic	COVID-19
Index case	First case in area	First COVID-19 patient
Prevalence	Total cases (old + new)	All current HIV cases
Incidence	New cases	New flu cases this month
Epidemiology	Study of disease occurrence and transmission	John Snow's cholera study

John Snow and Cholera

John Snow traced the 1854 cholera outbreak in London to a contaminated water pump, founding modern epidemiology.

Nosocomial Infections and Related Terms

• Nosocomial Infections: Acquired in healthcare settings

• Etiology: Study of disease causes

• Hand Hygiene: Critical for infection control

• Pathogenicity: Ability to cause disease

• Virulence: Degree of pathogenicity

• Virulence Factors: Traits that enhance pathogenicity

Modes of Disease Transmission

• Contact Transmission:

  • Direct: Person-to-person (e.g., kissing)

  • Indirect: Via fomites (e.g., doorknobs)

  • Droplet: Coughing, sneezing

• Vehicle Transmission:

  • Airborne: Dust, aerosols

  • Waterborne: Contaminated water

  • Foodborne: Undercooked food

  • Bodily fluids: Blood, saliva

• Vector Transmission:

  • Biological: Arthropods as hosts (e.g., mosquitoes)

  • Mechanical: Arthropods as carriers (e.g., flies)

Portals of Entry and Exit

• Entry: Skin, mucous membranes, placenta

• Exit: Mouth, nose, urethra

Axenic Environments

• Axenic: Free of other living organisms (e.g., blood, lungs, kidneys)

Acquisition of Normal Flora

• Babies acquire normal flora during birth and from the environment

Stages of Infectious Disease

1. Incubation Period: Time between infection and first symptoms

2. Prodromal Period: Mild, early symptoms

3. Illness: Most severe symptoms

4. Decline: Symptoms subside, recovery begins

5. Convalescence: Recovery and tissue repair

Endotoxins vs. Exotoxins

Type	Source	Example
Endotoxin	Gram-negative bacteria (Lipid A)	E. coli
Exotoxin	Secreted proteins	Botulinum toxin

Probiotics

• Live microbes that confer health benefits when administered in adequate amounts

Contributions of Key Scientists

• John Snow: Epidemiology, traced cholera outbreak

• Domagk: Discovered sulfanilamide

• Ehrlich: Developed chemotherapy ("magic bullets")

• Fleming: Discovered penicillin

• Waksman: Discovered antibiotics

Additional Key Terms

• Transient Microbiota: Temporary residents

• Resident Microbiota: Permanent residents

• Pathogen: Disease-causing organism

• Iatrogenic: Disease caused by medical intervention

• Reservoir: Source of pathogen

• Microbial Antagonism: Competition between microbes

• Synergism: Combined effect greater than sum

• Selective Toxicity: Drug targets pathogen, not host

• Cross-resistance: Resistance to similar drugs

• Fomite: Nonliving object transmitting disease