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The Control of Microbial Growth. Chapter 7. History. Use of drying and salting to preserve foods Ignaz Semmelweiss (1840)- aseptic techniques Joseph Lister (1867)- use of phenolics, aseptic surgery. The Terminology of Microbial Control.

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the control of microbial growth

The Control of Microbial Growth

Chapter 7

Dr. G. López-de-Victoria

history
History
  • Use of drying and salting to preserve foods
  • Ignaz Semmelweiss (1840)- aseptic techniques
  • Joseph Lister (1867)- use of phenolics, aseptic surgery

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the terminology of microbial control
The Terminology of Microbial Control
  • Sterilization is the process of destroying all microbial life on an object.
  • Commercial sterilization is heat treatment of canned foods to destroy C. botulinum endospores.
  • Disinfection is the process of reducing or inhibiting microbial growth on a non-living surface.
  • Antisepsis is the process of reducing or inhibiting microorganisms on living tissue.
  • Degerming removes microbes from a limited area.
  • Sanitization lowers microbial counts on utensils to safe public health levels.
  • The suffix –cide means to kill
  • The suffix – stat means to inhibit
  • Sepsis is bacterial contamination

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the rate of microbial death
The Rate of Microbial Death
  • Bacterial populations die at a constant rate when subjected to heat or antimicrobial agents.
  • The effectiveness of a treatment is affected by
    • The number of microbes- the higher the number the longer it takes to eliminate the population.
    • Environmental influences- presence of organic matter inhibits the action of the chemical. E.g. blood, vomit, feces.
    • Time of exposure
    • Microbial characteristics

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conditions influencing microbial control
Conditions influencing Microbial Control
  • Temperature
  • Types of Microorganism (see Fig. 7.11)
    • Gram positive
    • Gram negative
    • Pseudomonads
  • Physiological state
    • actively growing
    • endospores
  • Environment

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actions of microbial control agents
Actions of Microbial Control Agents
  • Alteration of membrane permeability
    • The susceptibility of the plasma membrane is due to its lipid and protein components.
    • Certain chemical control agents damage the plasma membrane by altering its permeability.

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actions of microbial control agents1
Actions of Microbial Control Agents
  • Damage to proteins and nucleic acids
    • Some microbial control agents damage cellular proteins by breaking hydrogen bonds and covalent bonds.
    • Other agents interfere with DNA and RNA replication and protein synthesis.

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physical methods of microbial control
Physical Methods of Microbial Control
  • Heat
    • Heat is frequently used to eliminate microorganisms.
    • Moist heat kills microbes by denaturing proteins (enzymes).
    • Dry heat kill organisms by oxidation.
    • Thermal Death Point (TDP)- is the lowest temperature at which all the microbes in a liquid culture will be killed in 10 minutes.

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heat cont
Heat (cont.)
  • Thermal Death Time(TDT) is the length of time required to kill all bacterial in a liquid culture at a given temperature.
  • Decimal Reduction Time (DRT) is the length of time required to kill 90% of a bacterial population at a given temperature; D value

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moist heat
Moist Heat
  • Boiling- 100 °C kills many vegetative cells and viruses within 10 minutes.
    • Hepatitis virus can survive 30 min boiling; endospores can survive 20 hr.
  • Autoclaving (steam under pressure) is the most effective method of moist heat sterilization. Steam must be directly in contact with the material to be sterilized.
    • The higher the pressure the higher the temperature.
    • Steam at a pressure of about 15 psi (121 °C) will kill all organisms and their endospores in about 15 min.

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moist heat cont
Moist Heat (cont.)
  • Pasteurization
    • In HTST pasteurization, a high temperature is used for a short time (72 °C for 15 seconds) to destroy pathogens without altering the flavor of food.
    • Ultra-high-temperature (UHT) treatment is used to sterilized dairy products (140 °C for 3 seconds).

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dry heat
Dry Heat
  • Methods of dry heat sterilization include
    • direct flaming- inoculating loops
    • incineration- to dispose of contaminated cups, dressings etc.
    • hot-air sterilization- oven; 170 °C for 2 hours
  • Dry heat kills by oxidation.

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filtration
Filtration
  • Filtration is the passage of liquid or gas through a filter with pores small enough to retain microbes.
  • Microbes can be removed from air by high-efficiency particulate air filters (HEPA).
  • Membrane filters composed of nitrocellulose or cellulose acetate are commonly used to filter out bacteria, viruses, and even large proteins.

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low temperatures
Low Temperatures
  • The effectiveness of low temperatures depends on the particular microorganism and the intensity of the application.
  • Most microorganisms do not reproduce at ordinary refrigerator temperatures (0 - 7 °C); bacteriostatic.
  • Many microbes survive (but do not grow) at the subzero temperatures used to store foods.

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freeze drying
Freeze-Drying
  • Microbes are placed in a suspending medium and frozen quickly at temperatures between -52 and 95 °C.
  • Water is removed by vacuum (sublimation)- lyophilization.
  • Powder-like product can be reconstituted to bring culture back to viable conditions.

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desiccation
Desiccation
  • In the absence of water, microorganisms cannot grow but can remain viable.
  • Viruses and endospores can resist desiccation.
  • There is disruption of metabolism.
  • Method used in food preservation.

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osmotic pressure
Osmotic Pressure
  • Microorganisms in high concentrations of salts and sugars undergo plasmolysis.
  • Used as food preservation.
  • Mold and yeasts are more capable than bacteria of growing in materials with low moisture or high osmotic pressure.

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radiation
Radiation
  • The effects of radiation depend on its wavelength, intensity, and duration.
  • Ionizing radiation (gamma rays, X rays, and high-energy electron beams) has a high degree of penetration and exerts its effect primarily by ionizing water and forming highly reactive hydroxyl radicals.

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radiation1
Radiation
  • Ultraviolet (UV) radiation, a form of non-ionizing radiation, has a low degree of penetration and causes cell damage by making thymine dimers in DNA that interfere with DNA replication.
  • The most effective germicidal wavelength is 260 nm.
  • Microwaves can kill microbes indirectly as materials get hot.

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chemical methods of microbial control
Chemical Methods of Microbial Control
  • Chemical agents are used on living tissue (as antiseptics) and inanimate objects (as disinfectants).
  • Few chemical agents achieve sterility.

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principles of effective disinfection
Principles of Effective Disinfection
  • Properties of disinfectant
  • Concentration of disinfectant
  • Presence of organic matter
  • Degree of contact
  • Temperature

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evaluating a disinfectant
Evaluating a Disinfectant
  • In the use-dilution test, bacterial (S. choleraesuis, S. aureus, and P. aeruginosa) survival in the manufacturer’s recommended dilution of a disinfectant is determined.
  • Viruses, endospore-forming bacteria, mycobacteria, and fungi can be used in the use-dilution test.

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evaluating a disinfectant1
Evaluating a Disinfectant
  • In the disk-diffusion method, a disk of filter paper is soaked with a chemical and placed on an inoculated agar plate, a clear zone of inhibition indicated effectiveness.
  • Phenol coefficient- compares the effectiveness of a disinfectant against the effectiveness of phenol.

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types of disinfectant
Types of Disinfectant
  • Phenol and Phenolics
    • Phenolics exert their action by injuring the lipid-containing plasma membrane which results in leakage of cellular contents.
    • Mycobacteria are susceptible to phenolics due to their rich lipid content.
    • Cresols- O-phenylphenol, main ingredient in Lysol.
    • Bisphenols
      • Hexachlorophene, used in pHisoHex; effective against G+ cocci
      • Triclosan- soap, toothpaste, plastics kitchenware; G+ and fungi.

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types of disinfectant1
Types of Disinfectant
  • Biguanides
    • Chlorohexedine damages plasma membranes of vegetative cells.
    • Broad spectrum
    • Used for microbial control of skin and mucous membranes.
    • Used for surgical hand scrubs.
    • Effective against most vegetative bacteria and fungi.
    • Mycobateria, endospores, and protozoan cysts are not affected.

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types of disinfectant2
Types of Disinfectant
  • Halogens
    • Some halogens (iodine and chlorine) are used alone or as components of inorganic or organic solutions.
    • Iodine may combine with certain amino acids to inactivate enzymes and other cellular proteins.
    • Iodine is available as a tincture (in solution with alcohol) or as an iodophor (combined with an organic molecule) like in Betadine.

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types of disinfectant3
Types of Disinfectant
  • The germicidal action of chlorine is based on the formation of hypochlorous acid when chlorine is added to water. It is an excellent oxidizing agent.
  • Chlorine is used as a disinfectant in gaseous form (Cl2) or in the form of a compound, such as calcium hypochlorite, sodium hypochlorite, sodium dichloroisocyanurate, and chloramines.

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types of disinfectant4
Types of Disinfectant
  • Alcohol exert their action by denaturing proteins and dissolving lipids.
  • In tinctures, they enhance the effectiveness of other antimicrobial agents.
  • Aqueous ethanol (60-90%) and isopropanol are used as disinfectants.
  • Not effective against spores or non-enveloped viruses.

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heavy metals and their compounds
Heavy Metals and their Compounds
  • Silver, mercury, copper, and zinc are used as germicidals.
  • They exert their antimicrobial action through oligodynamic action. When heavy metal ions combine with sulfhydryl (-SH) groups, proteins are denatured.
  • Ex. 1% Silver nitrate solution, mercuric chloride, copper sulfate (algicide)

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surface active agents
Surface-active agents
  • Soaps and acid anionic detergents
  • The agents decrease the surface tension among molecules of a liquid; soaps and detergents are examples.
  • Soaps have limited germicidal action but assist in the removal of microorganisms through scrubbing.
  • Acid-anionic detergents are used to clean dairy equipment.

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quaternary ammonium compounds
Quaternary Ammonium Compounds
  • Quats are cationic detergents attached to NH4+.
  • By disrupting the plasma membranes, they allow cytoplasmic constituents to leak out of the cell.
  • Quats are most effective against Gram-positive bacteria.
  • They do not kill endospores or mycobacteria.
  • Examples
    • Zephiran (benzalkonium chloride)
    • Cepacol (cetylpyridinium chloride)
  • Pseudomonads are highly resistant, can even live in quats.

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fig 7 9 the ammonium ion and a quaternary ammonium compound benzalkonium chloride zephiran
Fig. 7.9 The ammonium ion and a quaternary ammonium compound, benzalkonium chloride (Zephiran).

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chemical food preservatives
Chemical Food Preservatives
  • Organic Acids and Derivatives
    • Inhibit metabolism
    • Effective mostly against mold
    • Interferes with mold metabolism or the integrity of the plasma membrane
    • Examples include sorbic acid and benzoic acid; calcium propionate used as a fungistat in bread
    • Widely used in foods and cosmetic industry.
  • Nitrate and nitrite salts prevent germination of Clostridium botulinum endospores in meats.

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slide50
Antibiotics
    • Nisin and Natamycin are antibiotics used to preserve foods, especially cheese.
  • Aldehydes
    • Aldehydes such as formaldehyde and glutaraldehyde (Gidex) exert their antimicrobial effect by inactivating proteins.
    • They are among the most effective chemical disinfectants.

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slide52
Gaseous Chemosterilizers
    • Chemicals that sterilize in a closed chamber
    • Ethylene oxide is the gas most frequently used for sterilization.
    • Other examples include propylene oxide and beta-propiolactone.
    • It penetrates most materials and kills all microorganisms by protein denaturation.
    • It requires long exposure and is toxic and explosive when in pure form.

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peroxygens oxidizing agents
Peroxygens (Oxidizing Agents)
  • Ozone, peroxide, and peracetic acid are used as antimicrobial agents.
  • They exert their effect by oxidizing molecules inside cells.
  • Used on contaminated surfaces; some deep wounds, in which they are very effective against oxygen-sensitive anaerobes.

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microbial characteristics and microbial control
Microbial Characteristics and Microbial Control
  • Gram-negative bacteria are generally more resistant than gram-positive bacteria to disinfectants and antiseptics.
  • Mycobacteria, endospores, and protozoan cysts and oocysts are very resistant to disinfectants and antiseptics.
  • Non-enveloped viruses are generally more resistant than non-enveloped viruses to disinfectants and antiseptics.

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