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Control of Microbial Growth: Study Guide and Review

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Chapter 7 – Control of Microbial Growth

Core Terms

This section introduces essential terminology for understanding how microbial growth is controlled in laboratory, clinical, and industrial settings.

  • Sterilization: Destruction/removal of all microbial life, including endospores.

  • Disinfection: Destruction of vegetative pathogens on inanimate surfaces (not endospores).

  • Antisepsis: Disinfection of living tissue.

  • Degerming: Mechanical removal (e.g., alcohol swab).

  • Sanitization: Lowering microbial counts to public health standards.

  • Bacteriocide/Virucide: Agents that kill bacteria/viruses.

  • Bacteriostasis: Inhibits growth; resumes when agent removed.

  • Asepsis: Absence of contamination.

Microbial Death Rate Curve: Death is logarithmic; constant percentage dies per unit time.

Cellular Targets: Membrane damage, protein denaturation, nucleic acid damage.

Physical Control Methods

Physical methods are widely used to control microbial populations by altering environmental conditions.

  • Moist Heat: Boiling (kills vegetative cells), autoclaving (sterilization), pasteurization (reduces pathogens).

  • Dry Heat: Hot air oven, incineration.

  • Filtration: For heat-sensitive materials.

  • Low Temperature: Bacteriostatic.

  • High Pressure: Denatures proteins.

  • Desiccation: Drying inhibits growth.

  • Osmotic Pressure: High salts/sugar cause plasmolysis.

  • Radiation: Ionizing (X-rays), Nonionizing (UV disinfects surfaces).

Chemical Control

Chemical agents are used to disinfect, sterilize, or preserve materials and surfaces.

  • Disinfectant Evaluation: Use-Dilution Test, Disk Diffusion.

  • Classes: Alcohols, Halogens, Phenolics, QACs, Biguanides, Aldehydes, Peroxygens, Heavy Metals, Gaseous agents.

  • Advantages of Glutaraldehyde: Broad spectrum, sporicidal, used for heat-sensitive tools.

Relative Resistance: Prions > Endospores > Mycobacteria > Cysts > Non-enveloped viruses > Fungi > Vegetative cells > Enveloped viruses.

Method

Target

Example

Autoclaving

All microbes, endospores

121°C, 15 psi, 15 min

Filtration

Heat-sensitive liquids

Serum, antibiotics

UV Radiation

DNA (thymine dimers)

Surface disinfection

Alcohols

Membrane disruption

Ethanol, isopropanol

Halogens

Protein denaturation

Iodine, chlorine

QACs

Membrane disruption

Benzalkonium chloride

Antimicrobial Agents

History & Discovery

Antimicrobial agents revolutionized medicine by enabling targeted treatment of infectious diseases.

  • Paul Ehrlich: "Magic bullet" Salvarsan for syphilis.

  • Alexander Fleming: Discovered penicillin.

  • Natural antibiotic producers: Streptomyces, Bacillus, Penicillium, Cephalosporium.

Mechanisms of Action

Antibiotics target specific cellular processes to inhibit or kill microbes.

  1. Cell wall synthesis: β-lactams, vancomycin, bacitracin.

  2. Protein synthesis: Aminoglycosides, tetracyclines, chloramphenicol, macrolides.

  3. Cell membrane: Polymyxin B, bacitracin, neomycin.

  4. Nucleic acids: Rifamycins, fluoroquinolones.

  5. Antimetabolites: Sulfonamides, trimethoprim.

Resistance Mechanisms

  • Enzymatic inactivation (e.g., β-lactamases)

  • Target alteration

  • Efflux pumps

  • Bypass pathways

Challenges: Selective toxicity is harder for viruses, fungi, protozoa, helminths.

Principles of Disease and Pathogenicity

Virulence Factors

Pathogens possess specific traits that enable them to infect hosts and cause disease.

  • Entry: Mucous membranes, skin, parenteral routes. Some require preferred portal.

  • Dose: ID50/LD50 — lower = more virulent.

  • Adherence: Adhesins, biofilms.

  • Defenses: Capsules, M proteins, enzymes (coagulase, collagenase, hyaluronidase), antigenic variation.

  • Toxins: Exotoxins (proteins, potent), endotoxins (LPS).

  • Exit: Usually same route as entry.

  • Virulence genes: Identified by modified Koch’s postulates.

Review Questions

Sample Questions

Use these questions to test your understanding of microbial control and pathogenicity.

  1. Define sterilization vs disinfection.

  2. Why does microbial death follow a log curve?

  3. Which method kills endospores reliably?

  4. Give a heat-sensitive item that should be filtered.

  5. UV light kills by forming what DNA lesion?

  6. Name two factors that reduce disinfectant effectiveness.

  7. In the use-dilution test, how is survival detected?

  8. Match: Alcohols / Aldehydes / QACs / Halogens.

  9. Arrange resistance: endospores, enveloped viruses, mycobacteria.

  10. Broad vs narrow spectrum—one proof.

  11. Clavulanic acid helps which class and how?

  12. Why are INH and ethambutol specific for Mycobacterium?

  13. Which ribosomal subunit is targeted by tetracyclines?

  14. Rifamycins vs fluoroquinolones—targets?

  15. Sulfonamides inhibit which pathway?

  16. Two mechanisms of antibiotic resistance.

  17. Define preferred portal of entry with example.

  18. What defines a lower ID50?

  19. Contrast exotoxin and endotoxin.

  20. Two pathogen enzymes aiding invasion.

Key Equations

  • Microbial Death Rate:

Where = number of survivors at time , = initial number, = death rate constant.

  • ID50 and LD50: Dose required to infect/kill 50% of test population.

Additional info:

  • Some content inferred for completeness, such as the table summarizing physical and chemical methods, and the equation for microbial death rate.

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