Control of Microbial Growth

Control of Microbial Growth

Terms Related to Microbial Control

Sterilization

  • Definition: Destruction of ALL microbial life, including endospores and viruses.

  • Commercial sterilization: Specifically designed to eliminate Clostridium botulinum. Other non-pathogenic endospores may survive.

Antiseptic

  • Definition: Destruction of all vegetative cells.

  • Usage: Applied on animate surfaces.

  • Limitations: Does not destroy viruses and endospores; thus, it is not sterilization.

  • Disinfection: Generally refers to surfaces like table tops.

  • Antiseptic Examples: Used on hands and arms.

Germicide

  • Definition: A chemical agent that kills vegetative cells.

  • Specific Agents:

    • Bacteriocide: Does not necessarily kill endospores.

    • Fungicide: Kills fungi.

    • Virucide: Kills viruses.

    • Sporocide: Kills endospores.

  • Suffix Meaning: -cide indicates killing action.

Germistasis

  • Definition: A chemical agent that prevents vegetative cells from multiplying; does not kill.

  • Specific Agents:

    • Bacteriostatic: Prevents bacterial growth.

    • Fungistatic agent: Prevents fungal growth.

  • Suffix Meaning: -stasis indicates a state of maintaining or inhibiting growth.

Degermination

  • Definition: Removal of microorganisms by mechanical cleansing.

Sanitization

  • Definition: Reduction of numbers of organisms to levels deemed safe by public health agencies.

  • Primary Purpose: Mainly eliminates pathogens.

Deodorize

  • Definition: To destroy or mask offensive odors.

  • Key Note: Does not kill microorganisms.

Terminology of Microbial Control Table (Table 9.1)

Term

Definition

Examples

Comments

Antisepsis

Reduction in the number of microorganisms and viruses, especially potential pathogens, on living tissue.

Use of iodine or alcohol to prepare skin for injection.

Antiseptics often disinfectants at reduced strength.

Aseptic

An environment or procedure free of pathogenic contaminants.

Preparation of surgical field; flame sterilization.

Standardized aseptic techniques are followed routinely.

-cide

Suffix indicating destruction of a type of microbe.

Bactericide, fungicide, germicide, virucide.

Degerming

Removal of microbes by mechanical means.

Handwashing, alcohol swabbing.

Mechanical removal plays a key role.

Disinfection

Destruction of most microorganisms on nonliving tissue.

Use of phenolics, alcohols, aldehydes on equipment.

Primarily targets pathogens.

Pasteurization

Use of heat to destroy pathogens.

Pasteurized milk and fruit juices.

Brief heat treatment to minimize nutrient alteration.

Sanitization

Removal of pathogens from objects to meet public health standards.

Washing tableware in scalding water.

Standards vary among jurisdictions.

-stasis

Suffix indicating inhibition rather than complete destruction of microbes.

Bacteriostatic, fungistatic, virustatic.

Sterilization

Destruction of all microorganisms in or on an object.

Preparation of microbiological media, canned food.

Typically involves steam pressure or incineration.

Factors Affecting Microbial Control

  • Number of Organisms: Higher counts require more time for complete control.

  • Environmental Conditions: Factors such as pH and temperature can influence microbial control efficacy.

  • Time: Extended exposure increases chance of control.

  • Characteristics of Organisms:

    • Age of cells, structure of cell walls, and physiological states play a role in how susceptible organisms are to control methods.

Microbial Death Rate

  • Concept: Amplification of a chemical agent does not result in immediate death of all organisms.

  • Observation: The greater the number of organisms, the longer it takes to achieve total kill.

Pattern of Death

  • Observation: Microorganisms tend to die at a consistent rate over time following treatment.

  • Example: The decline of a population in a sequence:

    • 1,000,000 → 100,000 (90% in 1 minute)

    • 100,000 → 10,000 (90% in 1 minute)

    • 10,000 → 1,000 (90% in 1 minute)

    • 1,000 → 100 (90% in 1 minute)

    • 100 → 10 (90% in 1 minute)

    • 10 → 1 (90% in 1 minute)

    • 1 → 0

Mechanisms of Action for Control Agents

Affect Cell Membrane

  • Mechanism: Emulsification of lipid components alters permeability.

  • Consequence: Leakage of cellular contents leading to lysis.

  • Examples of Agents: Soaps, detergents, quaternary ammonium compounds, phenolic compounds.

Alteration of Colloid Nature of Proteins

  • Denaturation: Alters proteins' 3D shapes by affecting hydrogen bonds.

  • Examples of Agents: Heat, alcohols, and formaldehyde.

Interference with Nucleic Acids

  • Mechanism: Damage to DNA/RNA leading to breaks in molecules.

  • Methods: UV radiation, antibiotics interfere with nucleic acid production.

Enzyme Inhibition

  • Mechanism: Interference with enzyme functions by affecting receptor sites; can be competitive or non-competitive.

  • Examples: Heavy metals, oxidizing agents.

Interference with Protein Synthesis

  • Mechanism: Disruption of translation on ribosomes.

  • Types of Agents: Antibiotics, sulfa drugs.

Affect Cell Wall

  • Mechanism: Interference in cell wall production leading to disruption.

  • Examples of Agents: Penicillin, lysozyme.

Methods of Microbial Control

Physical Methods of Microbial Control

  • Methods: Include heat, low temperatures, desiccation, osmotic pressure, radiation, and electrostatic forces.

Heat

  • Overview: Most common method of microbial control.

  • Advantages: Effective, economical, and easy to control.

  • Mechanism: Kills by denaturing proteins.

Heat Resistance in Bacteria

  • Terminology:

    • Thermal Death Point (TDP): Lowest temperature needed to kill a liquid culture of bacteria in 10 minutes at pH 7.0.

    • Thermal Death Time (TDT): Time required to kill a liquid culture of bacteria at a specified temperature at pH 7.0.

    • Decimal Reduction Time (DRT): Time needed to kill 90% of bacteria at a specific temperature.

Dry Heat Sterilization

  • Incineration: Kills everything (e.g., flaming loop); generally destroys materials being sterilized.

  • Hot Air Sterilization: Example: oven at 350°F for 2 hours; paper does not ignite at this temperature.

Moist Heat

  • Advantages: Penetrates materials easier than dry heat.

  • Examples:

    • Boiling: Common practice; 212°F at sea level; kills vegetative forms in 10 minutes; does not kill endospores.

    • Fractional Sterilization: Sequential steps over three days involving boiling and incubation for endospore germination.

    • Autoclave: Most effective moist heat method, raising temperature through increased pressure (typically 15 minutes at 15 lbs/in² at 121°C, kills endospores).

    • Pasteurization: Mild heating to kill pathogenic organisms; various methods include LTLT, HTST, and UHT pasteurization.

Filtration

  • Definition: Passage of a liquid/gas through a mesh with pores small enough to retain bacteria; used for heat-sensitive materials.

  • Effectiveness: Viruses are filterable.

Low Temperatures

  • Purpose: Preservation; effects vary (some bacteria can grow, others die).

  • Observations: Germistatic for most organisms; rapid freezing reduces cell damage.

Desiccation

  • Definition: Bacteriostatic effect; organisms require water to grow and multiply.

  • Tolerances: Vary by organism (e.g., Neisseria gonorrhea dies after one hour of dryness, while Mycobacterium tuberculosis can survive for months).

Osmotic Pressure

  • Mechanism: Desiccation occurs in hypertonic environments (high salt/sugar concentrations).

  • Effect: Bacteriostatic; fungi are generally more tolerant.

Radiation

  • Types:

    • Ionizing Radiation: Short wavelength (e.g., x-rays, gamma rays); high penetration.

    • Non-ionizing Radiation: Longer wavelengths; includes UV light; germicidal properties.

    • Microwave Radiation: Effective through heat generation.

Electrostatic Forces

  • Mechanism: Generation of ions that destruct proteins; alters the 3D shape of enzymes.

  • Limitations: Hard to generate and control, resulting in minimal use.

Tables of Physical Methods of Microbial Control

  • Refer to Table 9.4 as a comprehensive comparison of methods, conditions, actions, and representative uses.

Chemical Methods of Microbial Control

Qualities of a Good Disinfectant

  • Key Requirements:

    • Quick acting.

    • Range of effectiveness across organisms.

    • Penetration capability.

    • Solubility in water.

    • Not inhibited by environmental conditions.

    • Stability: non-decomposed by light or heat.

    • Non-corrosive, non-toxic, inexpensive, plentiful, and safe for transport and manufacturing.

Methods for Evaluation

  • Phenol Coefficient: Uses phenol as a standard for evaluation; values greater than 1 indicate higher effectiveness.

  • Use Dilution Test: Tests effectiveness by adding bacteria to tubes with varying disinfectant concentrations and measuring growth or lack thereof.

  • Disk Diffusion Method: Test multiple products and concentrations on agar plates inoculated with organisms to determine effectiveness by zones of inhibition.

Categories of Chemical Agents

  • Phenolics: Initially used by Lister; derivatives like cresol found in Lysol.

  • Halogens: Effective against a wide array of organisms; includes iodine and chlorine.

  • Alcohols: Dissolve lipids and denature proteins; effective against bacteria and fungi but not viruses/endospores.

  • Heavy Metals: Exhibit oligodynamic action and denature proteins; examples include silver and copper.

  • Surfactants: Mechanically clean surfaces and disrupt membranes.

  • Quaternary Ammonium Compounds (Quats): Effective against gram-positive bacteria; includes mouthwashes.

  • Mineral and Organic Acids: Effective bactericides and food preservatives.

  • Alkaline Agents: Strong bases; generally more mechanical than effective.

  • Aldehydes: Include formaldehyde and glutaraldehyde; potent sterilizing agents.

  • Gaseous Chemosterilizers: Ethylene oxide is highly effective yet dangerous.

  • Oxidizing Agents: Highly reactive; include ozone and hydrogen peroxide; disrupt proteins and suppress anaerobes.

Tables of Chemical Methods of Microbial Control

  • Refer to Table 9.5 for a detailed comparison of chemical methods, their actions, levels of activity, and uses.