Exam 3 Review Guide Medical Microbiology – BIOL215 Fall 2024
Exam 3 Review Guide - Medical Microbiology (BIOL215 Fall 2024)
1. D Value
Definition: The time required at a specific temperature to kill 90% of the microorganisms in a sample.
2. Ideal Agents for Microbial Control
Should be:
Inexpensive
Fast-acting
Stable during storage
Harmless to humans, animals, and objects
3. Biosafety Levels (BSL)
Four levels of safety in labs dealing with pathogens:
Biosafety Level 1 (BSL-1):
Handling pathogens that do not cause disease in healthy humans.
Biosafety Level 2 (BSL-2):
Handling moderately hazardous agents.
Biosafety Level 3 (BSL-3):
Handling microbes in safety cabinets.
Biosafety Level 4 (BSL-4):
Handling microbes that cause severe or fatal diseases.
4. Physical Methods of Microbial Control
4.1 Heat-Related Methods
Effects of high temperatures:
Denature proteins.
Interfere with the integrity of the cytoplasmic membrane and cell wall.
Disrupt structure and function of nucleic acids.
Thermal Death Point: The lowest temperature that kills all cells in a broth in 10 minutes.
Thermal Death Time: Time needed to sterilize a volume of liquid at a set temperature.
4.2 Moist Heat
Used to disinfect, sanitize, sterilize, and pasteurize.
Denatures proteins and destroys cytoplasmic membranes; more effective than dry heat.
Methods include:
Boiling
Autoclaving:
Conditions: 121°C, 15 psi, 15 minutes.
Sentinel strain for testing: Geobacillus stearothermophilus spores.
Pasteurization
Ultra-high-temperature sterilization:
Conditions: 140°C for 1 to 3 seconds, followed by rapid cooling.
Provides months-long shelf-life and better preserves taste and texture of food.
4.3 Dry Heat
Used for materials that cannot be sterilized with moist heat.
Denatures proteins and oxidizes metabolic and structural chemicals.
Requires higher temperatures for longer than moist heat.
Incineration is the ultimate means of sterilization.
4.4 Refrigeration and Freezing
Decreases microbial metabolism, growth, and reproduction.
Refrigeration halts growth of most pathogens; some microbes can multiply in refrigerated foods.
Slow freezing is more effective than quick freezing.
4.5 Desiccation and Lyophilization
Desiccation: Inhibits growth due to removal of water.
Lyophilization (freeze-drying): Used for long-term preservation of microbial cultures, prevents ice crystal formation.
4.6 Osmotic Pressure
High concentrations of salt or sugar inhibit microbial growth.
Cells in a hypertonic solution lose water.
Fungi have a greater ability than bacteria to survive in hypertonic environments.
4.7 Radiation
Ionizing Radiation:
Wavelengths shorter than 1 nm (electron beams, gamma rays, X-rays).
Ejects electrons from atoms to create ions, impacting molecular structure (e.g., DNA).
Effective at killing microbes but electron beams do not penetrate well; gamma rays require hours to kill.
X-rays not practical for microbial control.
Nonionizing Radiation:
Wavelengths greater than 1 nm; excites electrons, causing new covalent bonds that affect protein and nucleic acid structures.
UV light creates pyrimidine dimers in DNA; does not penetrate well, suitable for disinfecting surfaces.
5. Chemical Methods of Microbial Control
5.1 Phenol and Phenolics
Denature proteins and disrupt cell membranes.
Effective in presence of organic matter, remain active for prolonged periods.
Commonly used in healthcare, labs, and homes.
5.2 Alcohols
Intermediate-level disinfectants.
Denature proteins and disrupt cytoplasmic membranes.
More effective than soap in removing bacteria from hands.
Swabbing skin with alcohol before injection removes most microbes; not effective against fungal spores or bacterial endospores.
5.3 Halogens
Intermediate-level antimicrobial chemicals that damage enzymes through denaturation.
Widely used applications: iodine tablets, iodophors, chlorine treatment, bleach, and bromine disinfection.
5.4 Oxidizing Agents
Examples include peroxides, ozone, and peracetic acid.
Kill by oxidation of microbial enzymes.
Hydrogen peroxide can disinfect and sterilize surfaces but not effective for treating open wounds due to catalase activity.
Ozone treatment is utilized for drinking water.
Peracetic acid is an effective sporicide used for sterilizing equipment.
5.5 Surfactants
Reduce surface tension of solvents and can be classified into:
Soaps and detergents:
Soaps have hydrophilic and hydrophobic ends; good degerming agents but not antimicrobial.
Detergents are positively charged organic surfactants.
Quaternary ammonium compounds (quats):
Low-level disinfectants; disrupt cellular membranes; ideal for numerous medical and industrial applications.
5.6 Heavy Metals
Heavy-metal ions denature proteins and are low-level bacteriostatic and fungistatic agents.
Examples: 1% silver nitrate (formerly used to prevent blindness from Neisseria gonorrhoeae), thimerosal (vaccine preservative), copper (algal growth control).
5.7 Aldehydes
Compounds containing terminal -CHO.
Cross-link functional groups to denature proteins and inactivate nucleic acids.
Examples: Glutaraldehyde (disinfects and sterilizes), formalin (embalming agent).
5.8 Gaseous Agents
Microbicidal and sporicidal gases used in closed chambers to sterilize items.
They denature proteins and DNA by cross-linking functional groups.
Disadvantages: often highly explosive, extremely poisonous, and potentially carcinogenic.
5.9 Enzymes
Antimicrobial enzymes act against microorganisms.
Example: human tears contain lysozyme which digests peptidoglycan cell walls of bacteria.
Applications:
Lysozyme used to reduce bacteria in cheese.
Prionzyme can remove prions from medical instruments.
6. Methods for Evaluating Disinfectants and Antiseptics
6.1 Use-Dilution Test
Metal cylinders dipped into broth cultures of bacteria.
Contaminated cylinders immersed in dilution of disinfectant.
Cylinders removed, washed, and placed into fresh medium.
Most effective agents prevent growth at highest dilution; current standard in the U.S.
6.2 Kelsey-Sykes Capacity Test
Alternative assessment approved by the European Union.
Bacterial suspensions are added to the chemical being tested.
Samples removed at predetermined times and incubated.
Lack of bacterial reproduction indicates minimum time for disinfectant effectiveness.
7. Drugs
7.1 Definitions
Drug: Chemicals that affect physiology in any manner.
Chemotherapeutic agents: Drugs that act against diseases.
Antimicrobial agents (antimicrobials): Drugs that treat infections.
7.2 Historical Figures
Paul Ehrlich: Proposed the concept of "magic bullets"; discovered arsenic compounds that killed microbes (e.g., syphilis).
Alexander Fleming: Discovered penicillin from Penicillium.
Gerhard Domagk: Discovered sulfanilamide.
Selman Waksman: Coined the term antibiotics.
7.3 Types of Antibiotics
Natural antibiotics: Found in nature.
Semisynthetics: Chemically altered antibiotics for improved efficacy or ease of use.
Synthetics: Antimicrobials entirely synthesized in a lab.
7.4 Mechanisms of Antibiotic Action
Inhibition of Cell Wall Synthesis:
Prevents cross-linkage of N-acetylmuramic acid (NAM) subunits, making bacterial cells susceptible to lysis.
Beta-lactams are prominent agents (contain beta-lactam rings):
Bind to enzymes responsible for cross-linking NAM subunits.
Isoniazid and ethambutol: Disrupt mycolic acid formation in mycobacterial species (effective against tuberculosis).
Polymyxin: Disrupts cytoplasmic membranes of Gram-negative bacteria but is toxic to human kidneys.
Disruption of Cytoplasmic Membranes:
Some drugs form channels in cytoplasmic membranes damaging their integrity.
Example drugs: Nystatin and amphotericin B target ergosterol in fungal membranes.
Drugs selectively damage microorganisms; humans are somewhat susceptible due to cholesterols' similarity.
Anti-fungals: Azoles and allylamines inhibit ergosterol synthesis.
Protein Synthesis Inhibitors:
Interfere with prokaryotic ribosomes (70S).
Can selectively target translation; mammalian mitochondria have 70S ribosomes (potentially harmful).
Examples of inhibitors:
Oxazolidinone: Linezolid
Tetracycline: Doxycycline
Macrolides: Azithromycin, Erythromycin
Chloramphenicol: Chloromycetin
Aminoglycosides: Amikacin, Kanamycin
Mupirocin: Binds to isoleucyl-tRNA synthetase (prevents loading of isoleucine only in Gram-positive bacteria).
Inhibition of Metabolic Pathways:
Effective due to differences between pathogen and host metabolism.
Atovaquone: Interferes with electron transport in protozoa and fungi.
Heavy metals: Inactivate enzymes.
Agents that disrupt tubulin polymerization and glucose uptake in many protozoa and parasitic worms.
Drugs that block viral activation or other metabolic antagonists.
Sulfonamides: Inhibit folic acid synthesis in bacteria.
Inhibition of Nucleic Acid Synthesis:
Quinolones and fluoroquinolones: Act against prokaryotic DNA gyrase.
8. Mechanisms of Resistance to Antibiotics
At least seven mechanisms include:
Produce enzymatic destruction/deactivation of the drug.
Slow or prevent drug entry into the cell.
Alter target of the drug for less effective binding.
Altering their metabolic chemistry.
Active pumping of antimicrobial drugs out of the cell.
Bacteria in biofilms can resist antimicrobials.
Mycobacterium tuberculosis produces MfpA protein that binds DNA gyrase, preventing fluoroquinolone effectiveness.
8.1 Multiple Resistance and Cross Resistance
Pathogen can acquire resistance to multiple drugs; common with R plasmids exchange.
Develops in hospitals and nursing homes due to constant drug use, eliminating sensitive cells.
Cross-resistance occurs when drugs share similar structures.
8.2 Retarding Resistance
Strategies:
Maintain high drug concentrations for sufficient time.
Inhibit pathogens to enable the immune system to eliminate them.
Use combinations of antimicrobial agents; synergism occurs when one drug enhances a second drug's effect.
Antagonism occurs when drugs interfere with each other.
Limit antimicrobial use to necessary cases.
Develop new drug variations (second- and third-generation drugs).
Search for new antibiotics, semisynthetics, and synthetics.
Bacteriocins: Design drugs that complement the shape of microbial proteins to inhibit them.
9. Microbiome of Humans
Definition: Organisms that colonize the body’s surfaces without usually causing disease; referred to as normal microbiota, normal flora, or indigenous microbiota.
9.1 Types of Microbiota
Resident Microbiota:
Cells establish permanently; long-term residence.
Transient Microbiota:
Remain in the body temporarily; influenced by competition and body defenses.
10. Opportunistic Pathogens
Definition: Normal microbiota that can cause disease under certain circumstances.
10.1 Conditions for Opportunistic Pathogen Activation
Introduction of normal microbiota into unusual body sites.
Immune suppression.
Changes in normal microbiota.
Stressful conditions.
11. Reservoirs of Infection
11.1 Types of Reservoirs
Animal Reservoirs: Zoonoses - diseases that spread naturally from animals to humans.
Transmission routes include direct contact, eating animals, or bloodsucking arthropods.
Human Carriers: Asymptomatic infected individuals can infect others; some may develop illness while others remain healthy. - Example: Typhoid Mary.
Nonliving Reservoirs: Soil, water, and food can serve as infection reservoirs, often contaminated by feces or urine.
12. Portals of Entry
Sites through which pathogens enter the body include:
Skin
Mucous membranes
Placenta
Entry via the parenteral route circumvents usual portals, through injection.
13. Conditions Affecting Embryo or Fetus
13.1 Pathogens and Effects
Protozoan: Toxoplasma gondii: Toxoplasmosis
Effects include abortion, epilepsy, encephalitis, mental retardation, etc.
Bacteria: Treponema pallidum: Syphilis
Effects include abortion, congenital syphilis.
DNA Viruses: Cytomegalovirus: Generally asymptomatic but can cause deafness, microcephaly, etc.
RNA Viruses: Lentivirus (HIV): Causes AIDS and immunosuppression.
14. Virulence Factors of Infectious Agents
14.1 Definitions
Pathogenicity: Ability of a microorganism to cause disease.
Virulence: Degree of pathogenicity.
14.2 Contributing Virulence Factors
Adhesion factors: Enable pathogens to attach to host tissues.
Biofilms: Provide protective environments for pathogens.
Extracellular enzymes: Assist in invasion and degradation of tissues.
Toxins: Harm tissues or trigger harmful immune responses.
Exotoxins: E.g., botulinum toxin, diphtheria toxin.
Endotoxins: E.g., lipopolysaccharide (LPS) of Gram-negative bacteria.
Antiphagocytic factors: Prevent phagocytosis, allowing pathogens to evade immune response.
Examples include bacterial capsules and leukocidins.
15. Stages of Infectious Disease
15.1 The Five Stages
Incubation period: Time from infection to first symptoms.
Prodromal period: Early, mild symptoms.
Illness: Disease is most severe; full-blown symptoms.
Decline: Symptoms begin to subside.
Convalescence: Recovery period.
16. Modes of Infectious Disease Transmission
Transmission occurs from a reservoir or portal of exit to another host's portal of entry.
16.1 Contact Transmission
Direct contact: Body contact between hosts.
Indirect contact: Pathogens spread by fomites.
Droplet transmission: Spread via droplets of mucus.
16.2 Vehicle Transmission
Airborne transmission: Pathogens travel >1 m via aerosols.
Waterborne transmission: Important for gastrointestinal diseases via fecal-oral route.
Foodborne transmission: Spread via inadequately processed or prepared food.
Bodily fluid transmission: Pathogens in blood, urine, saliva; prevent contact with mucous membranes.
16.3 Vector Transmission
Biological vectors: Serve as hosts for some pathogen life cycles (e.g., arthropods).
Mechanical vectors: Passively transmit pathogens present on their bodies.
17. Disease Frequency
Measures include:
Incidence: Number of new cases within a given area over a specific period.
Prevalence: Total number of cases within a given area over a specific period.
18. Hospital Epidemiology (Healthcare-Associated Infections)
18.1 Types of Infections
Exogenous: Acquired from healthcare environment.
Endogenous: Arises from normal microbiota within the patient.
Iatrogenic: Results from modern medical procedures.
Superinfections: Result from disruption of resident microbiota allowing other microbes to thrive.
19. Role of Public Health Agencies
Public health efforts to limit disease transmission include:
Enforcing cleanliness of water and food supplies.
Reducing disease vectors and reservoirs.
Establishing immunization schedules.
Locating and treating exposed individuals.
Enacting isolation and quarantine measures.
20. Immune System Overview
20.1 Types of Immunity
Innate Immunity:
First line of defense; non-specific, immediate response.
Adaptive Immunity:
Specific response; involves activation of lymphocytes (B and T cells).
20.2 Anatomical and Humoral Components of the Immune System
Innate (Non-specific):
Skin, mucous membranes, various immune cells (neutrophils, macrophages).
Adaptive (Specific):
Antibodies produced by B cells, T cell variations (CD4+, CD8+).
21. Components of the Innate Immune System
21.1 Innate Arm Components
Recognition occurs through preformed effectors; quick response.
Components include:
Anatomical defenses: Skin, mucous membranes.
Humoral: Complement system, cytokines.
Cellular: Innate immune cells (e.g., NK cells, macrophages).
22. Adaptive Immune System Components
22.1 Cellular Components
T Cells:
Include helper T cells (CD4+) and cytotoxic T cells (CD8+).
B Cells:
Produce antibodies and undergo class switching.
22.2 Activation of Adaptive Immune Response
T cells recognize antigens via TCR, presented on MHC molecules.
B cells undergo activation through T cell interaction (T-dependent activation).
Memory Cells: Formed after initial exposure, provide quicker response upon re-exposure.
23. Mechanisms of Antibody Action
23.1 Antibody Functions
Varied functions include:
Activation of complement system.
Neutralization of toxins and pathogens.
Opsonization: Marking pathogens for destruction by phagocytes.
Agglutination: Clumping of pathogens to enhance clearance.
Antibody-dependent cellular cytotoxicity (ADCC): Marking infected cells for destruction by immune cells.
23.2 Immunoglobulin Classes
IgA: Found in mucosal areas, secreted as a dimer.
IgD: Membrane-bound, role in B cell activation.
IgE: Associated with allergic responses and bound to mast cells.
IgG: Most abundant in serum; can cross placenta.
IgM: First produced; secreted as a pentamer.
24. Vaccination and Immunization
24.1 Types of Vaccines
Disabled/pathogenic: Inactivated agents (e.g., formaldehyde).
Live attenuated strains: Cause mild, self-limiting infections.
Toxoid vaccines: Stimulate active immunity via modified toxins.
Subunit and recombinant vaccines: Contain parts of pathogens (e.g., proteins).
DNA/RNA vaccines: Introduce genetic material to produce antigens.
24.2 Passive and Active Immunity
Active Immunity: Acquired through infection or vaccination.
Passive Immunity: Acquired via direct antibody transfer (e.g., maternal antibodies).
25. Diagnostic Methods
25.1 ELISAs
Enzyme-Linked Immunosorbent Assay: Detects antigens or antibodies.
Types include Direct, Indirect, Sandwich, and Competitive ELISA.
Advantages:
Sensitive, can quantify, easy to perform, and test many samples.
26. Essay Preparation Topics
Similarities and differences between TCR and Immunoglobulins.
Steps from bone marrow to naive T cell development.
Mechanisms of T cell activation.
Principal B cell activation mechanisms.
Structural and functional properties of immunoglobulins.
Role of clonal expansion in the immune system.
27. Conclusion
Understanding these topics provides a comprehensive base for medical microbiology, emphasizing the importance of microbial interactions, immune responses, and disease control methods.
Further exploration of the human microbiome and immune system intricacies can lead to improved therapeutic and prevention strategies for infections and diseases.