Microbial Growth and Control

Microbiology

Chapter 7: The Control of Microbial Growth

  • Terminology of Microbial Control

    • Sepsis: Refers to bacterial contamination.

    • Asepsis: Absence of significant contamination. Aseptic techniques are crucial in surgeries to prevent wound infections.

    • Sterilization: Process of removing and destroying all microbial life (e.g., using heat and pressure).

    • Commercial Sterilization: Specifically aimed at killing harmful endospores such as Clostridium botulinum in canning processes.

    • Disinfection: Process that destroys harmful microorganisms on inanimate objects.

    • Antisepsis: Involves the destruction of harmful microorganisms from living tissues.

    • Degerming: Mechanical removal of microbes from a small localized area.

    • Sanitization: Reducing microbial counts to safe levels on items such as utensils.

    • Biocide (germicide): Treatments that kill microbes.

    • Bacteriostasis: Inhibiting microbial growth without killing them.

  • Rate of Microbial Death

    • The effectiveness of microbial control treatments is influenced by several factors:

    • Number of microbes present.

    • Environmental conditions (presence of organic matter, temperature, biofilm formation).

    • Time duration of exposure to the treatment.

    • Unique characteristics of the microbes being treated.

  • Actions of Microbial Control Agents

    • Microbial control agents primarily affect cellular structures through:

    • Alteration of membrane permeability, leading to cell lysis.

    • Damage to proteins (enzymes), disrupting metabolic processes.

    • Damage to nucleic acids, affecting genetic material and replication.

  • Physical Methods of Microbial Control

    • Moist Heat: More effective than dry heat; denatures proteins. Types include:

    • Boiling: Kills vegetative cells but not spores.

    • Autoclaving: Uses pressurized steam (121°C at 15 psi for 15 min) to kill all organisms including endospores.

    • Pasteurization: Reduces spoilage organisms and pathogens; uses methods like High-temperature short-time (HTST), which involves heating to 72°C for 15 seconds but does not kill thermoduric organisms.

    • Dry Heat Sterilization: Kills microbes by oxidation methods such as flaming, incineration, and hot-air sterilization.

    • Filtration: Passes substances through a screen-like material to capture microbes, useful for heat-sensitive materials.

    • Low Temperatures: Refrigeration has a bacteriostatic effect, while deep-freezing and lyophilization preserve cultures by inhibiting metabolic processes.

    • Desiccation and Osmotic Pressure: Absence of water inhibits metabolism; osmotic pressure uses high concentrations of salts/sugars, creating a hypertonic environment that leads to plasmolysis.

  • Radiation as a Microbial Control Method

    • Ionizing Radiation: (X-rays, gamma rays, electron beams) ionizes water to form reactive hydroxyl radicals that can damage DNA.

    • Non-ionizing Radiation: (UV radiation at 260 nm) causes DNA damage via thymine dimers.

    • Microwaves: Primarily kill through heat rather than a direct antimicrobial effect.

  • Chemical Methods of Microbial Control

    • Effectiveness of disinfection depends on various factors:

    • Concentration of disinfectant.

    • Presence of organic matter.

    • pH of the environment.

    • Duration of exposure.

  • Use-Dilution Tests:

    • Metal cylinders are contaminated with test bacteria, dried, and submerged in disinfectants. Afterward, they are placed into culture media to assess survival.

  • Disk-Diffusion Method: Evaluates the efficacy of chemical agents by placing soaked filter paper disks on culture. The zone of inhibition indicates the effectiveness of the disinfectant.

  • Methods of Action and Uses of Chemical Disinfectants

    • Phenols and Phenolics: Disrupt plasma membranes causing leakage.

    • Bisphenols: Contain two phenolic groups (e.g., triclosan).

    • Biguanides: (e.g., chlorhexidine) used in surgical hand scrubs; disrupt plasma membranes.

    • Essential Oils (EOs): Natural antimicrobial agents derived from plants that possess strong activity against Gram-positive bacteria.

    • Halogens:

    • Iodine: Impairs protein synthesis and alters membrane functionality.

    • Chlorine: Oxidizing agent that deactivates cellular enzyme systems, with bleach as hypochlorous acid.

    • Alcohols: Denature proteins and dissolve lipids but are ineffective against endospores and non-enveloped viruses (e.g., ethanol).

    • Heavy Metals: Oligodynamic action denatures proteins (e.g., silver nitrate in preventing ophthalmia neonatorum).

  • Surface-Active Agents:

    • Soap: Functions primarily through degerming and emulsification of lipids.

    • Acid-anionic sanitizers: Interact with plasma membrane.

    • Quaternary ammonium compounds (quats): Bactericidal properties, denature proteins, and disrupt membranes.

  • Food Chemical Preservatives:

    • Sulfur Dioxide: Prevents wine spoilage.

    • Organic Acids: (sorbic, benzoic acid) inhibit metabolic processes, particularly effective in acidic foods.

    • Nitrites and Nitrates: Prevent endospore germination, often in meats.

  • Aldehydes: Inactivate proteins through cross-linking functional groups; used for specimen preservation (e.g., formaldehyde).

  • Antibiotics: (e.g., Nisin) proteins that inhibit other bacterial growth.

  • Chemical Sterilization Methods:

    • Gaseous Sterilants: Cause alkylation by replacing hydrogen with free radicals.

    • Used for heat-sensitive materials (e.g., ethylene oxide).

    • Plasma: Fourth state of matter that utilizes free radicals for microbial destruction; effective for tubular instruments.

    • Supercritical Fluids: CO2 in a mixed state with liquid and gas properties used for medical implants.

  • Peroxygens and Other Forms of Oxygen:

    • Used as oxidizing agents on contaminated surfaces and food packaging (e.g., ozone, hydrogen peroxide).

  • Microbial Characteristics Affecting Control:

    • The presence of endospores significantly impacts the control strategies; gram-negative bacteria exhibit stronger resistance to chemical biocides compared to gram-positive organisms due to their unique cell wall structure.