Chapter 7 : Microbial Control and Disinfection Notes

Control of Microbial Growth

Historical Context

• Scientific Control of Microbial Growth (Last 100 years): Evolved from Pasteur’s work suggesting microbes as disease causatives.

• Key Figures:

• Ignaz Semmelweis & Joseph Lister introduced founding practices to prevent microbial contamination.

• Used chloride of lime for handwashing and aseptic techniques for surgery.

• Impact of Nosocomial Infections: Before such practices, nosocomial infections resulted in high mortality rates (10%-25%). Surgical hygiene was often neglected, leading to severe outcomes.

• Modern Practices: Handwashing remains critical in preventing pathogens like noroviruses, with extensive developments in disinfection methods.

Terminology of Microbial Control

• Disinfection: Destruction of vegetative pathogens without achieving sterility.

• Antisepsis: Disinfection applied to living tissue.

• Sterilization: Total removal/destruction of all microbial life.

• Methods:

• Chemical Agents: Disinfectants (inert surfaces) vs. antiseptics (living tissue).

• Mechanisms: Disinfectants often called '-cide' (e.g., bactericide) for killing powers, and '-stasis' (e.g., bacteriostasis) for inhibiting growth.

The Rate of Microbial Death

• Constant Death Rate: Microbial populations die logarithmically; the more microbes present, the longer it takes to eliminate them.

• Factors Influencing Effectiveness:

• Microbial Load: Higher load, longer sterilization required.

• Environmental Influences: Organic matter presence inhibits control methods.

• Time of Exposure: Longer exposure typically necessary for resistant forms.

Actions of Microbial Control Agents

1. Alteration of Membrane Permeability: Membrane disruption leads to leakage of cellular contents.

2. Damage to Proteins and Nucleic Acids: Microbial control agents can denature essential proteins and affect replication processes.

Physical Methods of Microbial Control

• Heat Sterilization: Most common method, effective at destroying endospores using moist heat (e.g., autoclaving).

• Moist Heat:

  • Boiling: Effective against most pathogens within 10 minutes but ineffective against endospores.

  • Autoclaving: Best for sterilization (121°C at 15 psi for 15 min).

• Dry Heat: Effective for sterilizing lab tools at higher temps (170°C for 2 hours).

• Filtration: Removes microbes from liquids and gases (e.g., HEPA filters).

• Low Temperatures: Refrigeration halts bacterial growth without killing organisms.

• Chemical Methods:

• Alcohols: Effective against bacteria and fungi; denature proteins.

• Halogens: Effective disinfectants (e.g., iodine, chlorine).

• Aldehydes: Disinfects by cross-linking with proteins (e.g., glutaraldehyde).

Chemical Agents in Microbial Control

• Types of Disinfectants:

• Phenolics: Disrupt plasma membranes; effective in organics.

• Alcohols: Widely used for disinfection; optimal concentration at 60%-95%.

• Heavy Metals: Exhibit oligodynamic action (e.g., silver, mercury).

• Quaternary Ammonium Compounds (Quats): Effective against gram-positive bacteria, but ineffective against mycobacteria and endospores.

Resistance of Microbes

• Gram-negative vs. Gram-positive:

• Gram-negative bacteria are typically more resistant to biocides due to their outer membrane.

• Factors: Endospores, mycobacteria, and biofilms present heightened resistance to these agents.

Case Study: Norovirus Outbreak

• Spread likely due to environmental contamination (high transmission efficiency via surfaces such as computer mice).

• Importance of effective cleaning protocols with disinfectants, focusing on high-risk areas.

• Proper handwashing and surface disinfection protocols are key to controlling outbreaks.

Summary of Key Methods and Agents

• Physical Methods: Heat, Filtration, Cold, Radiation.

• Chemical Agents: Alcohols, Halogens, Heavy Metals, Quats, Aldehydes, Organic Acids.

• Factors to consider: Organic matter, temperature, pH, exposure time.

• Preparation for Tests: Understand terms and methodologies like use-dilution and disk-diffusion to evaluate disinfectant efficiency.

By employing this structured knowledge of microbial control, students prepare for scientific rigor in understanding both historical contexts and modern solutions in microbiology.

Definitions:

1. Sepsis: A life-threatening condition resulting from the body's response to infection, leading to tissue damage and organ failure.

2. Asepsis: The absence of pathogenic microorganisms, preventing contamination during medical procedures.

3. Antisepsis: The process of disinfection applied to living tissue to eliminate pathogens without harming the host.

4. Sterilization: The complete destruction or removal of all forms of microbial life, including spores.

5. Disinfection: The process that eliminates most pathogens but not necessarily all microbial life, particularly resistant bacterial spores.

6. Degerming: The removal of transient microbes from a surface, often through mechanical scrubbing (e.g., handwashing).

7. Sanitization: The process of reducing the microbial load on surfaces or objects to safe levels.

8. Germicide: A chemical agent that kills pathogens.

9. Bacteriostasis: The inhibition of bacterial growth without necessarily killing the bacteria.

Factors Influencing Effectiveness of Antimicrobial Agents:

• Microbial Load: Higher loads may require longer exposure for effective eradication.

• Environmental Influences: Presence of organic matter can inhibit the action of antimicrobial agents.

• Time of Exposure: Longer exposure times typically improve effectiveness.

• Temperature and pH: Optimal conditions can enhance antimicrobial action.

• Nature of the Microbe: Different microorganisms (e.g., gram-positive vs. gram-negative) have varying resistance levels.

Physical Agents of Microbial Control:

1. Heat: Kills microorganisms by denaturing proteins.

• Moist Heat: Effective (e.g., autoclaving) for destroying spores and all microbes.

• Dry Heat: Sterilizes lab tools at higher temperatures.

1. Radiation: Disrupts microbial DNA; UV light is used for surface sterilization.

2. Filtration: Removes microorganisms from liquids and gases, including HEPA filters.

3. Mechanical Scrubbing: Physically removes microbes from surfaces, enhancing antisepsis.

4. Refrigeration: Slows microbial growth without killing, effectively prolonging the shelf life of foods.

5. Osmotic Pressure: High concentrations of solutes can cause microbial dehydration and inhibit growth.

Chemical Agents of Microbial Control:

1. Ethylene Oxide: A gas used for sterilization of heat-sensitive items; works by alkylating proteins and nucleic acids.

2. Aldehydes (e.g., glutaraldehyde): Disinfect by cross-linking proteins, damaging their functional groups.

3. Phenolics: Effective against bacteria by disrupting cell membranes and denaturing proteins; remains active in the presence of organic matter.

4. Quaternary Ammonium Compounds (Quats): Disrupt microbial membranes and are effective against gram-positive bacteria.

5. Chlorine: Oxidizes proteins and nucleic acids, commonly used for disinfection in water treatment.

6. Iodine: An antiseptic that disrupts protein synthesis and microbial cell membranes.

7. Alcohols: Denature proteins and disrupt membranes; effective against most bacteria and fungi.

8. Heavy Metals (e.g., silver): Have oligodynamic action; can disrupt microbial metabolism by inactivating enzymes.