Control of Microbial Growth and Viruses: Study Guide

Control of Microbial Growth

Terminology in Microbial Control

Understanding the terminology used in microbial control is essential for distinguishing between different methods and their effectiveness.

• Sterilization: The complete destruction or removal of all forms of microbial life, including endospores. Used for surgical instruments, culture media, etc.

• Disinfection: The elimination of most pathogenic microorganisms (except bacterial endospores) on inanimate objects.

• Antisepsis: The destruction of pathogens on living tissue (e.g., skin) using chemical agents safe for tissue.

• De-germing: Mechanical removal of microbes from a limited area, such as handwashing or swabbing skin with alcohol before injection.

• Sanitation: Lowering microbial counts on eating utensils to safe public health levels.

• -cidal: Suffix meaning "killing" (e.g., bactericidal = kills bacteria).

• -static: Suffix meaning "inhibiting growth" (e.g., bacteriostatic = inhibits bacterial growth).

Factors Affecting Antimicrobial Treatment

Several factors influence the effectiveness of antimicrobial treatments:

• Number of microbes: More microbes require longer treatment times.

• Microbial characteristics: Endospores, mycobacteria, and Gram-negative bacteria are more resistant.

• Environment: Presence of organic matter, temperature, and biofilms can affect efficacy.

• Time of exposure: Longer exposure increases effectiveness.

Microbial death occurs at a constant rate; not all cells die instantly. Death is logarithmic.

Physical Methods of Microbial Control

Physical methods are commonly used to control microbial growth in various settings.

• Temperature:

  • Moist heat sterilization (autoclaving): Uses steam under pressure (121°C, 15 min) to kill all organisms and endospores.

  • Pasteurization: Reduces spoilage organisms and pathogens in food. Methods include:

    • Batch pasteurization: 63°C for 30 min

    • Continuous (High-Temperature Short-Time, HTST): 72°C for 15 sec

    • Ultra-pasteurization (Ultra-High Temperature, UHT): 140°C for 4 sec

  • Cold (refrigeration, freezing): 4°C slows growth (bacteriostatic), freezing preserves but does not kill most microbes.

• Osmotic Pressure: High concentrations of salt or sugar cause plasmolysis, inhibiting microbial growth (used in food preservation).

• Desiccation: Drying removes water, inhibiting metabolism; not all microbes are equally sensitive.

• Filtration: Physically removes microbes from liquids or air (e.g., HEPA filters, membrane filters).

• Radiation:

  • Ionizing radiation (gamma rays): Penetrates deeply, damages DNA, sterilizes medical supplies and food.

  • Non-ionizing radiation (UV): Causes thymine dimers in DNA, used for surface sterilization; poor penetration.

Chemical Methods of Microbial Control

Chemical agents target cell membranes, proteins, or other cellular components.

• Phenol and Phenolics: Disrupt plasma membranes, denature proteins; used in disinfectants (e.g., Lysol).

• Bisphenols: Disrupt membranes; hexachlorophene (effective against Staphylococcus aureus), triclosan (found in soaps, toothpaste).

• Biguanides: Chlorhexidine used as surgical scrub; disrupts membranes.

• Halogens: Chlorine (as hypochlorite) and iodine are oxidizing agents; used in water treatment and antiseptics.

• Alcohols: Ethanol and isopropanol (60–95%) denature proteins and dissolve lipids; not effective against endospores.

• Heavy Metals: Mercury (thimerosal), silver (silver nitrate for newborns' eyes), copper (algaecide); denature proteins.

• Surfactants: Soaps and detergents mechanically remove microbes; quaternary ammonium compounds (quats) disrupt membranes.

• Chemical Preservatives: Organic acids (inhibit metabolism), nitrates (prevent endospore germination in meats).

• Aldehydes: Formaldehyde, glutaraldehyde, and ethylene oxide are sterilizing agents; cross-link proteins and nucleic acids.

Example: Silver nitrate was historically used in newborns' eyes to prevent Neisseria gonorrhoeae infection; now, antibiotics like erythromycin are used.

Viruses

General Characteristics of Viruses

Viruses are acellular infectious agents with unique properties.

• Contain a single type of nucleic acid (DNA or RNA, single- or double-stranded).

• Lack most cellular structures; are obligate intracellular parasites—require living host cells to replicate.

• Each virus has a narrow host range, often limited to closely related species.

Viral Structure

• Capsid: Protein coat made of subunits called capsomeres.

• Envelope: Lipid membrane derived from host cell, with viral proteins embedded; not all viruses have envelopes.

• Nucleic Acid: Either DNA or RNA, single- or double-stranded.

• Shapes: Helical (rod-shaped), polyhedral (icosahedral), complex (e.g., bacteriophage with head and tail).

• Viral Enzymes: Some viruses (e.g., retroviruses) carry enzymes like reverse transcriptase for replication.

Growing Animal Viruses

• Historically required whole animals.

• Embryonated chicken eggs: Support growth of many viruses; used in vaccine production.

• Tissue cell culture: Viruses grown in cultured cells; cytopathic effects (CPE) are areas of cell death.

Bacterial Viruses (Bacteriophage)

• Viruses that infect bacteria; studied using plaque assays.

• Plaque: Clear area on a bacterial lawn where phage has lysed cells.

Viral Replication Cycle

The replication cycle of viruses involves several key steps:

1. Attachment: Virus binds to specific receptors on host cell surface.

2. Penetration and Uncoating: Virus enters cell (fusion or endocytosis); capsid is removed to release genome.

3. Biosynthesis: Viral genome directs host machinery to synthesize viral components (nucleic acids and proteins).

4. Assembly/Maturation: Viral parts self-assemble into new virions.

5. Release: Non-enveloped viruses cause cell lysis; enveloped viruses bud from cell, acquiring envelope.

Types of Viral Infections

• Acute: Rapid onset, short duration (e.g., influenza).

• Latent: Virus remains dormant in host, can reactivate (e.g., herpes simplex).

• Persistent: Virus remains in host, producing low levels of virus over long periods (e.g., HIV).

Viruses and Cancer

• Some viruses can cause cancer by integrating into host DNA and disrupting normal cell regulation (oncogenic viruses).

Prions

Prions are infectious proteins that cause neurodegenerative diseases.

• Misfolded proteins that induce normal proteins to misfold.

• Cause spongiform encephalopathies (e.g., mad cow disease, kuru, Creutzfeldt-Jakob disease).

Review Questions (with Academic Context)

1. Sterilization vs. Disinfection: Sterilization destroys all forms of microbial life, including spores; disinfection eliminates most pathogens but not necessarily spores.

2. Disinfectant vs. Antiseptic: Disinfectants are used on inanimate objects; antiseptics are safe for use on living tissue.

3. Pasteurization Methods:

   • Batch: 63°C for 30 min

   • Continuous (HTST): 72°C for 15 sec

   • Ultra (UHT): 140°C for 4 sec

4. Phenolics: Chemical compounds derived from phenol; disrupt cell membranes and denature proteins.

5. Hexachlorophene and Triclosan: Hexachlorophene was used in hospital nurseries; triclosan is found in soaps and toothpaste.

6. Hexachlorophene Discontinuation: Discontinued in newborn nurseries due to neurotoxicity concerns.

7. Silver Nitrate in Newborns: Used to prevent gonococcal eye infections; now replaced by antibiotics like erythromycin.

8. Silver and Copper: Silver used in wound dressings and catheters; copper used in water systems and surfaces to reduce microbial growth.

9. Ethylene Oxide: Gaseous sterilant used for heat-sensitive medical equipment; alkylates proteins and DNA.

10. Viral Structures:

    • Naked virus: nucleic acid + capsid

    • Enveloped virus: nucleic acid + capsid + lipid envelope

    • Complex virus: additional structures (e.g., bacteriophage head and tail)

11. Six Steps of Virus Replication: Attachment, penetration, uncoating, biosynthesis, assembly, release.

12. Attachment/Adsorption: Determined by specific interactions between viral proteins and host cell receptors.

13. Latent Virus: Virus remains dormant in host cell, can reactivate (e.g., herpesviruses).

14. Naming Influenza A Strains: Based on hemagglutinin (H) and neuraminidase (N) surface proteins (e.g., H1N1).

Table: Comparison of Physical and Chemical Methods of Microbial Control

Method	Example	Mechanism	Application
Autoclaving	121°C, 15 min	Denatures proteins, destroys membranes	Sterilizing media, instruments
Pasteurization	72°C, 15 sec (HTST)	Reduces pathogens	Milk, juices
Filtration	HEPA, membrane filters	Physical removal	Heat-sensitive liquids, air
Alcohols	70% ethanol	Denatures proteins, dissolves lipids	Skin antisepsis
Halogens	Chlorine, iodine	Oxidation	Water, surfaces, wounds
Heavy Metals	Silver nitrate	Denatures proteins	Newborn eyes, dressings
Aldehydes	Glutaraldehyde	Cross-links proteins	Equipment sterilization

Additional info: Academic context and definitions were expanded for clarity and completeness. Table entries inferred from standard microbiology sources.