Control of Microbial growth

The Control of Microbial Growth

Recommended Reading

  • Chapter 13: Control of Microbial Growth

  • Chapter 14: Antimicrobial Drugs

Microbial Environments

  • Most environments (e.g., cars, homes) are not sterile.

  • A study analyzing 11 locations in 18 cars revealed:

    • Center Console: Most microbes found (506 CFUs) due to spills from drinks.

    • Frequent contact sites exhibited high microbial concentrations.

Terminology of Microbial Control

  • Sterilization: Complete elimination of all forms of microbial life, including spores and viruses.

  • Disinfection: Reduction of microbial load to a safe level, typically on inanimate objects using chemicals or heat.

    • Sanitization: Aimed at reducing microbial load on items to public health levels.

  • Biocide/Germicide: Agents that kill microbes.

  • Bacteriostasis: Inhibition of microbial growth without killing.

  • Antisepsis: Reduction of microbial load on living tissues using antimicrobial agents.

  • Degerming: Physical removal of microbes from a limited area (e.g., skin).

  • Asepsis: Absence of significant contamination; emphasizes the prevention of microbial infection.

    • Aseptic techniques in surgery and labs help prevent microbial contamination.

Common Protocols for Control of Microbial Growth

Disinfection Protocols for Inanimate Items

  • Disinfection: Reduces microbial load via heat or chemicals.

    • Common agents: Chlorine bleach, phenols (e.g., Lysol), glutaraldehyde.

  • Sanitization: Lowers microbial count to safe levels.

    • Common applications: Commercial dishwashing, restroom cleaning.

  • Sterilization: Eliminates all microbes.

    • Instruments used: Autoclave (pressure steam), radiation.

Treatment Protocols for Living Tissue

  • Antisepsis: Reduction of microbes using antimicrobials.

    • Agents: Boric acid, isopropyl alcohol, hydrogen peroxide, iodine.

  • Degerming: Physical removal combined with mild chemicals.

    • Common practice: Handwashing with soap or antiseptic wipes.

Microbial Control Efficacy

  • Effectiveness depends on:

    • Number of microbes.

    • Environmental factors (organic matter presence, temperature, biofilms).

    • Time of exposure to control agent.

    • Characteristics of the microbe itself, including resistance.

Microbial Death Curves

  • Microbial death rate is logarithmic and best represented using a semilog plot.

  • D-value: Time needed to kill 90% of the microbial population.

    • Illustrated through semi-log and scalar plots of viable microbes over time.

Thermal Death Rate Curves

  • Thermal Death Point (TDP): Lowest temperature to kill all cells in 10 minutes.

  • Thermal Death Time (TDT): Time required to kill all microorganisms at a given temperature.

  • Decimal Reduction Time (DRT): Time in minutes to kill 90% of a microbial population at a specific temperature.

Heat as a Control Method

Moist Heat Techniques

  • Moist Heat sterilizes by denaturing proteins.

    • Autoclave: Uses steam under pressure; must ensure steam contacts the item’s surface.

  • Dry Heat: Kills through high temperatures for extended times (e.g., flaming, incineration).

Pasteurization

  • Reduces spoilage organisms/pathogens, not sterilization.

    • Examples include:

    • 63°C for 30 minutes (batch pasteurization).

    • 72°C for 15 seconds (high-temperature short time).

    • 140°C for less than 1 second (ultra-high temperature).

Filtration and Radiation in Microbial Control

Filtration

  • HEPA filters: Remove microbes over 0.3 micrometers; membrane filters for smaller size.

Radiation Methods

  • Ionizing Radiation (X-rays, gamma rays): Damages DNA and can ionize water releasing reactive hydroxyl radicals.

  • Non-ionizing Radiation (UV light): Specifically, UV-B light causing thymine dimers that damage DNA.

Chemical Methods of Microbial Control

Disinfectants and Antiseptics

  • Chemicals are selected based on:

    • Target microbe (susceptibility).

    • Cleaning requirement level (degree of sterility).

    • Integrity of the treated object.

    • Safety, cost, and ease of use.

Examples of Chemical Efficacy

  • Disk diffusion assay: Tests the effectiveness of agents against specific microbes by measuring zones of inhibition.

Classes of Chemical Agents

  • Phenolics: Denature proteins, disrupt cell membranes.

  • Heavy Metals: Such as Silver (Ag), Mercury (Hg), Copper (Cu) for their oligodynamic action.

  • Halogens: Disinfectants like iodine and chlorine, with various forms (e.g., tinctures, iodophors).

  • Alcohols: Ethanol and isopropanol, effective in concentrations requiring water for efficacy.

  • Surfactants: Remove microbes; do not kill but disrupt adhesion.

  • Alkylating Agents: Effective but carcinogenic, damage nucleic acids.

  • Peroxygens: Generate free radicals that attack cellular components.

  • Food Preservatives: Such as benzoic and sorbic acids, inhibit growth of spoilage organisms.

Historical Perspective on Antimicrobials

  • Penicillin: Mass production began in the 1940s and significantly reduced infections during WWII.

  • Discoveries by Key Figures:

    • Alexander Fleming: Discovered penicillin.

    • Howard Florey & Ernst Chain: Developed production methods.

    • Selman Waksman: Explored antibacterial properties of soil bacteria (actinomycetes).

Antibiotic Mechanisms

Classes of Antimicrobials

  • Beta-lactam antibiotics: Inhibit cell wall synthesis leading to cell lysis.

  • Other classes target various bacterial processes including protein synthesis and metabolic pathways.

Antibiotic Resistance

  • Resistance can develop through the acquisition of beta-lactamase genes, which cleave the beta-lactam ring.