Microbiology

1. Antibiotic Sensitivity Testing

What is it?

• A method to see how effective antibiotics are against bacteria.

How is it done?

1. Spread bacteria on an agar plate.

2. Place small disks with antibiotics on the plate.

3. Incubate the plate.

4. Measure the clear zones (zones of inhibition) around the disks.

Why?

• To determine if bacteria are resistant, intermediate, or susceptible to the antibiotics.

Steps:

• Disk-Diffusion Method: Disks with antibiotics are placed on bacteria spread on agar.

• Clear Zones: Show the effectiveness of antibiotics.

Interpreting Results:

• Use a table to determine if the bacteria are resistant, intermediate, or susceptible based on the zone diameter.

Examples:

• Disk with antibiotic (blue) + Bacteria (white) → Clear zone (no bacteria growth)

2. Types of Media

Blood Agar

• What is it? Nutrient-rich agar with sheep's blood.

• Used for: Detecting hemolysis (destruction of red blood cells) by bacteria.

• Procedure:

  1. Swab throat and spread on blood agar.

  2. Incubate.

  3. Observe for hemolysis types:

     • Alpha (α) hemolysis: Partial, greenish zone.

     • Beta (β) hemolysis: Complete, clear zone.

     • Gamma (γ) hemolysis: No hemolysis.

Examples:

• Blood Agar + Bacteria (S. pneumoniae) → Greenish zone (alpha)

• Blood Agar + Bacteria (S. pyogenes) → Clear zone (beta)

• Blood Agar + Bacteria (Enterococcus) → No change (gamma)

Mannitol Salt Agar (MSA)

• What is it? Selective and differential medium.

• Selective for: Staphylococcus species (high salt tolerance).

• Differential for: Mannitol fermentation (turns yellow if fermented).

Examples:

• MSA + Staphylococcus aureus (ferments mannitol) → Yellow color

• MSA + Staphylococcus epidermidis (does not ferment mannitol) → No color change

MSB Agar

• What is it? Mitis-salivarius bacitracin agar.

• Used for: Detecting Streptococcus mutans (causes dental caries).

• Selective for: S. mutans (inhibits other oral bacteria).

Examples:

• MSB Agar + S. mutans → Growth (white colonies)

• MSB Agar + Enterococcus → Black colonies

3. Microbial Antagonism

What is it?

• Competition between normal microbiota and harmful bacteria.

How does it help?

• Normal bacteria protect us by outcompeting harmful bacteria for nutrients and space.

Examples:

• Normal microbiota in the mouth → Suppress growth of pathogens.

• Normal microbiota on the skin → Prevents colonization by harmful bacteria.

4. Key Bacteria and Tests

Streptococcus Species

• Types of Hemolysis:

  • Alpha (α): Partial, greenish zone (e.g., S. pneumoniae).

  • Beta (β): Complete, clear zone (e.g., S. pyogenes).

  • Gamma (γ): No hemolysis (e.g., Enterococcus).

Important Streptococcus Species:

• Streptococcus pneumoniae: Causes pneumonia, alpha-hemolytic, optochin-sensitive.

• Streptococcus pyogenes: Causes strep throat, beta-hemolytic, bacitracin-sensitive.

• Streptococcus mutans: Found in the mouth, causes dental caries, alpha-hemolytic.

Staphylococcus Species

• Key Differences:

  • Staphylococcus aureus: Coagulase positive, mannitol fermentation positive, golden yellow colonies.

  • Staphylococcus epidermidis: Coagulase negative, cannot ferment mannitol, white colonies.

Catalase Test

• Purpose: Differentiates Staphylococcus (positive) from Streptococcus (negative).

• Procedure:

  1. Add hydrogen peroxide (H2O2) to bacteria.

  2. Bubbles indicate positive result (catalase enzyme present).

Examples:

• Bacteria + H2O2 → Bubbles (Catalase positive - Staphylococcus)

• Bacteria + H2O2 → No bubbles (Catalase negative - Streptococcus)

Coagulase Test

• Purpose: Identifies Staphylococcus aureus (positive).

• Procedure:

  1. Mix bacteria with coagulase plasma.

  2. Clumps indicate positive result.

Examples:

• Bacteria + Coagulase plasma → Clumps (Coagulase positive - Staphylococcus aureus)

• Bacteria + Coagulase plasma → No clumps (Coagulase negative - Staphylococcus epidermidis)

5. Handwashing and Epidemiology

Handwashing Importance

• Why? Removes transient bacteria, reduces infections.

• Historical Example: Ignaz Semmelweis reduced puerperal fever by handwashing with chloride of lime.

Procedure:

1. Wash hands with soap and water.

2. Dry hands with a clean towel.

3. Use hand sanitizer for extra cleanliness.

Epidemiology

• What is it? Study of disease spread and control.

• Key Terms:

  • Pathogen: Disease-causing organism.

  • Host: Organism that harbors the pathogen.

  • Epidemic: Sudden increase in disease cases.

  • Pandemic: Worldwide epidemic.

  • Endemic: Constant presence of a disease.

Definitions and Terms:

• Pathogen → Disease-causing organism.

• Host → Organism harboring the pathogen.

• Communicable Disease → Spreads from one host to another (e.g., flu).

• Non-Communicable Disease → Does not spread (e.g., diabetes).

• Vector → Insects that carry pathogens (e.g., mosquitoes for malaria).

• Fomites → Inanimate objects that can carry disease (e.g., doorknobs).

Key Concepts:

• Microbial antagonism: Normal microbiota prevent pathogenic bacteria from colonizing.

• Handwashing: Reduces transient bacteria and lowers infection risk.

6. UV Radiation

What is it?

• Use of ultraviolet light to kill bacteria by damaging DNA.

Procedure:

1. Expose bacteria to UV light for different times.

2. Observe growth reduction.

Effects:

• UV light induces thymine dimers in DNA, leading to mutations.

• Light repair: Photolyases split dimers using visible light.

• Dark repair: Endonucleases remove dimers without light.

Important Terms and Concepts:

Antibiotic: Chemical that kills or inhibits the growth of bacteria. Antiseptic: Chemical used to reduce microbes on living tissues. Disinfectant: Chemical used to reduce microbes on inanimate objects. Minimum Inhibitory Concentration (MIC): Lowest concentration of an antibiotic that prevents visible growth of bacteria. Zone of Inhibition: Clear area around an antibiotic disk where bacteria cannot grow. Thymine Dimers: DNA damage caused by UV light, leading to mutations.

Handwashing Steps:

1. Wet hands with water.

2. Apply soap and lather well.

3. Scrub all surfaces for at least 20 seconds.

4. Rinse thoroughly with water.

5. Dry with a clean towel or air dry.

Epidemiology Steps:

1. Identify the index case (first patient).

2. Track secondary cases (subsequent patients).

3. Determine the mode of transmission (how the disease spreads).

Example Scenario: Determining Blood Type

1. Collect Blood Sample.

2. Add Anti-A Serum → Observe for clumping.

   • Clumping (agglutination) → Blood has A antigens.

3. Add Anti-B Serum → Observe for clumping.

   • Clumping → Blood has B antigens.

4. Add Anti-D Serum (Rh factor) → Observe for clumping.

   • Clumping → Rh positive.

Result Interpretation:

• No clumping in any test → O negative.

• Clumping with Anti-A only → A negative.

• Clumping with Anti-A and Anti-D → A positive.

• Clumping with Anti-B only → B negative.

• Clumping with Anti-B and Anti-D → B positive.

• Clumping with both Anti-A and Anti-B → AB negative.

• Clumping with all serums → AB positive.

Questions and Key Points from Transcription and PowerPoints

1. Why can't the catalase test be done directly on blood agar?

   • Blood agar contains catalase, which would give a false positive result.

2. Why is it important to test for catalase?

   • To differentiate between Staphylococcus (catalase positive) and Streptococcus (catalase negative).

3. What are the types of hemolysis?

   • Alpha (partial), Beta (complete), Gamma (none).

4. What does microbial antagonism mean?

   • Normal microbiota prevent pathogenic bacteria from colonizing.

5. What is the function of penicillin?

   • Penicillin is an antibiotic that kills bacteria by inhibiting cell wall synthesis.

6. What are thymine dimers?

   • DNA damage caused by UV light, leading to mutations.

7. What is the minimum inhibitory concentration (MIC)?

   • The lowest concentration of an antibiotic that prevents visible growth of bacteria.

8. How do you measure the effectiveness of antibiotics using the disk-diffusion method?

   • Measure the clear zones (zones of inhibition) around the antibiotic disks on the agar plate.

9. Why is handwashing important in healthcare settings?

   • Reduces the spread of infections and removes transient bacteria.

10. What are the key steps in epidemiology?

    • Identify the index case, track secondary cases, and determine the mode of transmission.

Scientists and Discoveries

1. Lazzaro Spallanzani

   • Discovered: Heat sterilization to kill microorganisms.

   • When: 1765

   • Importance: Helped disprove spontaneous generation and showed that microorganisms cause decay.

2. Louis Pasteur

   • Discovered: Germ theory of disease, pasteurization, and vaccines for rabies and anthrax.

   • When: 1860s-1880s

   • Importance: Proved that microorganisms cause disease and developed methods to control their spread.

3. Joseph Lister

   • Discovered: Antiseptic surgery using carbolic acid.

   • When: 1865

   • Importance: Reduced infections in surgeries by killing bacteria on surgical instruments and wounds.

4. Alexander Fleming

   • Discovered: Penicillin

   • When: 1928

   • Importance: First antibiotic that could treat bacterial infections, leading to a revolution in medical treatment.

5. Ignaz Semmelweis

   • Discovered: Importance of handwashing in preventing puerperal fever.

   • When: 1847

   • Importance: Reduced mortality rates in maternity wards by advocating handwashing with chlorinated lime solutions.

6. Elie Metchnikoff

   • Discovered: Phagocytosis by white blood cells.

   • When: 1882

   • Importance: Foundational work in immunology showing how the body defends itself against pathogens.

7. Emil von Behring

   • Discovered: Diphtheria antitoxin.

   • When: 1890

   • Importance: Developed the first effective treatment for diphtheria, saving countless lives.

Differentiating Bacterial Species

1. Staphylococcus aureus

   • Tests: Coagulase positive, mannitol fermentation positive, catalase positive.

   • Media: Yellow colonies on MSA, golden colonies on LB agar.

2. Staphylococcus epidermidis

   • Tests: Coagulase negative, cannot ferment mannitol, catalase positive.

   • Media: White colonies on MSA and LB agar.

3. Streptococcus pneumoniae

   • Tests: Alpha-hemolytic, optochin sensitive, bile solubility positive.

   • Media: Greenish zone on blood agar.

4. Streptococcus pyogenes

   • Tests: Beta-hemolytic, bacitracin sensitive.

   • Media: Clear zone on blood agar.

5. Streptococcus mutans

   • Tests: Alpha-hemolytic, grows on MSB agar.

   • Media: White colonies on MSB agar.

6. Streptococcus salivarius

   • Tests: Alpha-hemolytic, produces dextran from sucrose.

   • Media: Produces gummy colonies on sucrose blood agar.

7. Streptococcus sanguinis

   • Tests: Alpha-hemolytic, cannot produce dextran.

   • Media: Clear colonies on sucrose blood agar.

   • Lab 11: Ultraviolet Radiation

1. What is radiation?

   • Energy that travels through space.

2. Relationship between wavelength and energy?

   • Shorter wavelength → Higher energy.

3. Types of electromagnetic radiation?

   • Microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays.

   • Differ by wavelength and energy: shorter wavelength → higher energy.

4. Ionizing radiation?

   • High-energy radiation that removes tightly bound electrons from atoms, causing DNA damage.

5. Non-ionizing radiation?

   • Lower energy, essential for cellular processes, causes DNA damage differently than ionizing radiation (e.g., UV light).

6. Ultraviolet radiation?

   • Three forms: UVA, UVB, UVC.

   • UVA: Longest wavelength, least energy, causes aging.

   • UVB: Medium wavelength, causes sunburn.

   • UVC: Shortest wavelength, most energy, used for sterilization.

7. UV radiation and DNA damage?

   • Causes thymine dimers, leading to mutations.

8. Light repair?

   • Enzyme: Photolyase, activated by visible light, splits thymine dimers.

9. Dark repair?

   • Enzymes: Endonucleases, DNA polymerase, ligase.

   • Does not require light, removes and replaces damaged DNA.

10. Limitation of UV radiation?

    • Cannot penetrate plastic or glass, must have direct exposure.

11. XL-1 Blue II cells?

    • UV sensitive strain, shows more damage compared to other bacteria.

Lab 12: Antiseptics and Disinfectants

1. Disinfectants vs. Antiseptics?

   • Disinfectants: Used on inanimate objects.

   • Antiseptics: Used on living tissues.

   • Goal: Reduce or eliminate microbial load.

2. Bacteriostatic vs. Bactericidal?

   • Bacteriostatic: Inhibits growth.

   • Bactericidal: Kills bacteria.

3. Are disinfectants and antiseptics bacteriostatic or bactericidal?

   • Can be either, depends on concentration and contact time.

4. Single chemical agent for all microorganisms?

   • No, effectiveness varies by species and environment.

5. Definition of DRT (Decimal Reduction Time)?

   • Time needed to kill 90% of bacteria at a specific concentration.

6. DRT value example:

   • Different species and concentrations have unique DRT values.

7. Calculate time needed to kill X bacteria:

   • Use given DRT value for specific bacteria and antiseptic/disinfectant.

8. Determining most effective antiseptic/disinfectant:

   • Compare DRT values for effectiveness.

9. Standard test name:

   • Use-Dilution Test.

Lab 13: Antimicrobial Drugs

1. Pasteur's discovery?

   • Identified antibiosis.

2. Antibiosis?

   • Inhibition of one microorganism by another.

3. Antibiotic?

   • Substance produced by microbes to kill or inhibit other microbes.

4. Antibiotic vs. Disinfectant?

   • Antibiotics target specific bacterial processes; disinfectants are non-specific.

5. Penicillin discovery:

   • Alexander Fleming, 1928, produced by Penicillium mold, effective against Gram-positive bacteria.

6. Streptomycin discovery:

   • Selman Waksman, 1943, produced by Streptomyces, effective against tuberculosis.

7. Other antibiotic producers?

   • Actinobacteria, fungi.

8. Antibiotic problem today?

   • Antibiotic resistance.

9. Finding the best antimicrobial drug?

   • Perform susceptibility testing.

10. Disk diffusion method?

    • Antibiotic disks on agar plate, measure inhibition zones.

11. Zone of inhibition?

    • Clear area around antibiotic disk.

12. Factors controlling inhibition zone size?

    • Antibiotic concentration, diffusion rate, bacterial growth rate.

13. Kirby-Bauer test?

    • Standardized disk diffusion method, not suitable for antiseptics/disinfectants.

14. Kirby-Bauer media:

    • Mueller-Hinton agar, standardized ingredients for consistency.

15. Determining susceptibility:

    • Compare inhibition zone size to standard chart.

Lab 14: Effectiveness of Hand Scrubbing

1. Skin microbiome at birth?

   • Sterile until birth.

2. Skin microbiome constant?

   • Changes over time.

3. Normal microbiota vs. Transient microbiota?

   • Normal: Permanent residents.

   • Transient: Temporary visitors.

4. Founder of handwashing?

   • Ignaz Semmelweis.

5. Discovery of handwashing:

   • 1847, reduced puerperal fever in maternity wards.

6. Puerperal fever?

   • Postpartum infection, also called childbed fever.

7. Chloride of lime:

   • Calcium hypochlorite, pH around 11.

8. CDC statement on handwashing:

   • Essential for preventing infections.

9. Effects of not washing hands:

   • Increased infection risk.

10. Components of skin:

    • Sebum, keratin, make it difficult to remove bacteria with water alone.

11. Handwashing removes all bacteria?

    • No, reduces transient bacteria, not all.

12. Benefits of hot water, soap, scrubbing:

    • More effective at removing bacteria.

13. Limitations of hand sanitizer:

    • Does not remove dirt, less effective on some bacteria.

14. Surgical scrub:

    • Removes transient microbiota, not all normal microbiota.

15. Sterile skin after surgical scrub?

    • No, some normal microbiota remain.

Lab 15: Epidemiology

1. Pathogen and host?

   • Pathogen: Disease-causing organism.

   • Host: Organism harboring the pathogen.

2. Communicable vs. Non-communicable diseases:

   • Communicable: Spread from person to person (e.g., flu).

   • Non-communicable: Not spread (e.g., diabetes).

3. Epidemiology definition:

   • Study of disease spread, control, and prevention.

4. Do diseases change?

   • Yes, due to evolution and environmental factors.

5. Endemic, epidemic, pandemic:

   • Endemic: Constant in a population.

   • Epidemic: Sudden increase.

   • Pandemic: Worldwide epidemic.

6. Index case?

   • First identified case in an outbreak.

7. Case definition?

   • Set of criteria for diagnosing a disease.

8. Direct contact transmission?

   • Direct physical contact.

9. Methods of transmission:

   • Droplet, fomite, aerosol, vector, mechanical, biological.

10. Droplet, fomite, aerosol transmission:

    • Droplet: Large particles, short distance.

    • Fomite: Inanimate objects.

    • Aerosol: Small particles, long distance.

11. Vector?

    • Organism that transmits pathogens (e.g., mosquito).

12. Mechanical transmission?

    • Pathogen carried on surface of vector.

13. Typhoid fever:

    • Bacterial infection, transmitted by contaminated food/water.

14. Biological transmission:

    • Pathogen develops within vector (e.g., malaria).

15. Reservoirs:

    • Sources of pathogens, maintain disease.

16. Carrier?

    • Individual harboring pathogen without symptoms.

17. Epidemiologist's job:

    • Study disease patterns, control outbreaks.

18. Incidence:

    • Number of new cases in a specific time period.

19. Epidemic curve:

    • Graph showing number of cases over time.

20. Index case, primary case, secondary case:

    • Index: First case.

    • Primary: Original case.

    • Secondary: Infected by primary case.

Lab 16: Agglutination Reactions

1. Innate immunity:

   • Discovered by Elie Metchnikoff, involves phagocytosis by white blood cells.

2. Adaptive immunity:

   • Discovered by Emil von Behring, involves specific response to pathogens, includes antibodies.

3. Antigens:

   • Substances that trigger immune response.

4. Antibodies:

   • Proteins that bind to antigens, neutralize pathogens.

5. Antibody-antigen specificity:

   • Antibodies bind to specific antigens.

6. Agglutination reactions:

   • Clumping of particles, requires antigens and antibodies.

7. Bacterial agglutination reaction:

   • Detects bacterial cell antigens.

8. Hemagglutination:

   • Clumping of red blood cells.

9. Blood components:

   • Plasma, red blood cells, white blood cells, platelets.

10. Blood plasma vs. serum:

    • Plasma: Liquid part of blood.

    • Serum: Plasma without clotting factors.

11. Rh factor:

    • D antigen on red blood cells.

12. Blood types:

    • A, B, AB, O with positive or negative Rh factor.

13. O- blood:

    • Universal donor.

14. AB+ blood:

    • Universal acceptor.

15. Determining blood type:

    • Hemagglutination test with anti-sera.

16. Blood transfusions:

    • Match antigens and antibodies to prevent reactions.

17. Rh factor in pregnancy:

    • Rh- mother can develop antibodies against Rh+ fetus.

Lab 17: Bacteria of the Skin

1. Skin inhospitable for microbes:

   • Dry, acidic, high salt concentration.

2. Sebum:

   • Oily substance, produced by sebaceous glands.

3. Keratin:

   • Protein in skin cells.

4. Normal microbiota requirements:

   • Adapted to dry, salty, acidic conditions.

5. Transient microbiota:

   • Found on skin surface.

6. Identifying all skin bacteria:

   • No, some cannot be cultured.

7. Common skin microbiota genera:

   • Staphylococcus, Corynebacterium, Propionibacterium.

8. Propionibacterium:

   • Found in hair follicles, produce propionic acid.

9. Staphylococcus epidermidis colonies:

   • White colonies on agar.

10. Staphylococcus aureus colonies:

    • Golden yellow, produce pigment for protection.

11. Coagulase:

    • Enzyme clots blood, protects bacteria.

12. Catalase:

    • Enzyme breaks down hydrogen peroxide.

13. S. epidermidis:

    • Coagulase-negative, catalase-positive.

14. S. aureus:

    • Coagulase-positive, catalase-positive.

15. MSA fermentation:

    • S. aureus turns yellow, S. epidermidis no color change.

16. Catalase test:

    • Staphylococcus positive, Streptococcus negative.

17. Distinguishing S. aureus and S. epidermidis:

    • Coagulase test, colony morphology, MSA fermentation.

Lab 18: Bacteria of the Respiratory Tract

1. Upper respiratory tract:

   • Nose, throat; heavily contaminated due to exposure to air.

2. Lower respiratory tract:

   • Lungs; mostly sterile due to ciliary action and immune defenses.

3. Normal microbiota of lower respiratory tract:

   • Prevotella, Veillonella, Streptococcus, Pseudomonas.

4. Bacteria in throat:

   • Warm, moist environment supports growth.

5. Normal microbiota of throat:

   • Streptococcus, Haemophilus, Neisseria, Moraxella.

6. Bacterial infection in throat:

   • Minimized by normal microbiota competition.

7. Predominant throat microorganisms:

   • Streptococcus spp.

8. Identifying Streptococcus spp.:

   • Hemolysis and Lancefield grouping.

9. Lancefield grouping system:

   • Based on carbohydrate antigens on bacterial cell walls.

10. Hemolytic reactions:

    • Alpha, beta, gamma; tested on blood agar.

11. Blood-enriched agar:

    • Contains blood and nutrients; differential medium.

12. Catalase test on blood agar:

    • No, false positive due to catalase in blood.

13. Forms of hemolysis:

    • Alpha: greenish, beta: clear, gamma: no change.

14. Pathogenic streptococci hemolysis:

    • Mostly beta-hemolytic.

15. Gram stain and catalase test:

    • Streptococci: Gram-positive, catalase-negative.

16. Streptococcus pneumoniae:

    • Alpha-hemolytic, optochin sensitive.

17. Optochin sensitivity test:

    • Disk on agar, measure zone of inhibition.

18. Optochin-sensitive zone:

    • ≥14mm for S. pneumoniae.

19. Bile solubility test:

    • Bacteria lysed by bile salts.

20. Distinguishing S. pneumoniae:

    • Optochin sensitivity, bile solubility.

21. Lancefield group A streptococci:

    • Beta-hemolytic, cause most streptococcal infections.

22. Bacitracin sensitivity test:

    • Disk on agar, measure zone of inhibition.

23. Distinguishing S. pyogenes:

    • Bacitracin sensitivity.

24. Simplest test for S. pyogenes vs. S. pneumoniae:

    • Hemolysis type.

Lab 19: Bacteria of the Mouth

1. Source of transient mouth bacteria:

   • Food, air, contact with objects.

2. Normal microbiota of mouth:

   • Streptococcus mutans, Streptococcus sanguinis, Streptococcus salivarius.

3. Plaque-forming disaccharide:

   • Sucrose.

4. Dextran:

   • Made from sucrose by S. mutans and S. sanguinis.

5. Levan:

   • Made from sucrose by S. salivarius.

6. Dental plaque:

   • Biofilm on teeth.

7. Formation of dental caries:

   • Acid from bacteria erodes enamel.

8. Carbohydrates and plaque:

   • Sucrose forms plaque; other sugars do not.

9. Accelerating dental caries:

   • Frequent sugar intake.

10. Sucrose blood medium:

    • Shows bacterial growth and dextran production.

11. MSB agar:

    • Selective for S. mutans.

12. Calculating bacteria in saliva:

    • CFU/mL = (Number of colonies) / (Volume plated).