Microbial Growth and Control

Chapter 7 The Control of Microbial Growth

Terminology of Microbial Control

  • Sterilization: complete removal/destruction of all microbial life, achieved through heat and pressure

  • Disinfection: elimination of harmful microorganisms on inanimate objects

  • Antisepsis: destroying harmful microorganisms from living tissues

  • Degerming: mechanical removal of microbes from a localized area

  • Sanitization: reduction of microbial counts on utensils to safe levels

  • Biocide (Germicide): treatments that effectively kill microbial life

  • Bacteriostasis: inhibiting the growth of bacteria without necessarily killing them

  • Sepsis: refers to bacterial contamination

  • Asepsis: absence of significant microbial contamination

Factors Impacting Rate of Microbial Death

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

    • Number of microbes present

    • Environmental conditions (e.g., organic matter, temperature, presence of biofilms)

    • Duration of the exposure to the control agent

    • Characteristics of the microorganisms being treated

Mechanism of Action of Microbial Control Agents

  • Actions of microbial control agents include:

    • Alteration of cell membrane permeability

    • Damage to proteins (enzymes), which are vital for cellular functions

    • Damage to nucleic acids, affecting replication and transcription

Physical Methods of Microbial Control

Heat Treatment

  • Heat denatures proteins, causing cell death:

    • Thermal Death Point (TDP): the lowest temperature at which all cells are killed in a defined period (10 min)

    • Thermal Death Time (TDT): minimum time required to kill all bacteria at a specific temperature

    • Decimal Reduction Time (DRT): the time required to kill 90% of a specific population at a given temperature

Moist Heat Sterilization

  • Moist heat denatures proteins by coagulation:

    • Autoclave: sterilization using steam under pressure, typically at 121°C for 15 minutes at 15 psi, effective against all organisms including endospores

    • Pasteurization: reduces spoilage organisms and pathogens (e.g., HTST method at 72°C for 15 seconds)

Dry Heat Sterilization

  • Dry heat kills microbes through oxidation:

    • Methods include flaming, incineration, and hot-air sterilization

Filtration

  • Filtration allows passage of substances through a screen-like material for heat-sensitive materials:

    • HEPA filters: remove particles > 0.3 μm

    • Membrane filters: can filter out microbes > 0.22 μm, and even 0.01 μm for viruses

Low Temperatures

  • Low temperatures exert a bacteriostatic effect which can inhibit growth:

    • Methods include refrigeration, deep-freezing, and lyophilization (freeze-drying)

Osmotic Pressure

  • Osmotic pressure utilizes high concentrations of salts/sugars to create a hypertonic environment, leading to plasmolysis and hindered microbial metabolism

Radiation Methods

  • Ionizing Radiation (e.g., X-rays, gamma rays): ionizes water to create reactive hydroxyl radicals, leading to DNA damage

  • Nonionizing Radiation (e.g., UV light at 260 nm): causes damage by creating thymine dimers in DNA

Chemical Methods of Microbial Control

Effective Disinfectants

  • Principles of effective disinfection include:

    • Concentration of disinfectant

    • Presence of organic matter

    • pH of the environment

    • Time of exposure

Use-Dilution Tests

  • Test cylinders dipped in bacteria are dried and then exposed to disinfectant to measure survival in culture media

Disk-Diffusion Method

  • Evaluates the efficacy of chemical agents by soaking filter paper disks in a chemical and observing zones of inhibition on cultures

Classes of Chemical Agents

  • Phenols and Phenolics: injure lipids of plasma membranes; common example includes triclosan

  • Bisphenols: two phenolic groups; used in disinfectants

  • Biguanides: e.g., chlorhexidine used for surgical hand scrubs; disrupts plasma membranes

  • Essential Oils: plant extracts such as peppermint oil show antimicrobial activity

  • Halogens:

    • Iodine: denatures proteins

    • Chlorine: oxidizing agent, denatures cellular enzymes

  • Alcohols: denatures proteins and dissolves lipids but may not kill endospores

  • Heavy Metals: exert oligodynamic action; e.g., silver nitrate, which prevents infection in newborns

  • Surface-Active Agents: can be soaps, quaternary ammonium compounds that disrupt membranes

Food Preservatives

  • Sulfur Dioxide: prevents spoilage in wine

  • Organic Acids: inhibit metabolism in acidic foods

  • Nitrites and Nitrates: prevent germination of endospores

Aldehydes

  • Compounds that inactivate proteins through cross-linking; formaldehyde and glutaraldehyde used for sterilization of instruments

Antibiotics

  • Bacteriocins: proteins that inhibit growth of similar or closely related bacterial strains, e.g., nisin used in cheese preservation

Chemical Sterilization

  • Gaseous Sterilants (e.g., ethylene oxide): replace hydrogen atoms in microbial cells, leading to cross-linking with nucelic acids or proteins

  • Plasma: electrically charged gas used for sterilization of tubular instruments

  • Supercritical Fluids: liquid CO2 used in medical implants

Peroxygens and Forms of Oxygen

  • Oxidizing agents used for sanitizing surfaces and food packaging include O3, H2O2, and peracetic acid

Microbial Characteristics Affecting Control

  • The type of microbe significantly influences control strategies; for example:

    • Presence of endospores and gram-negative bacteria which show more resilience against chemical biocides than gram-positive bacteria.

Chapter 7 Summary

  • Recap of definitions associated with microbial control, treatment effectiveness factors, actions of microbial control agents on cellular structures, comparison of heat treatments, suppression methods, principles of effective disinfection, chemical disinfectant action, and how microbial type affects growth control.