Topic 8: Control of Microorganisms by Physical and Chemical Agents

$Sterilization$$$Sterilization$$: destruction or removal of ALL viable organisms; usually used on inanimate objects

$Disinfection$$$Disinfection$$: killing, inhibition, or removal of pathogenic, vegetative organisms

$Disinfectants$$$Disinfectants$$: agents, usually chemical, used for disinfection; usually used on inanimate objects

$Chemotherapy$$$Chemotherapy$$: chemicals used internally to kill or inhibit growth of microorganisms within host tissues

$Sanitization$$$Sanitization$$: reduction of microbial population to levels deemed safe (based on public health standards)

$Antisepsis$$$Antisepsis$$: prevention of infection of living tissue by microorganisms

$Antiseptics$$$Antiseptics$$: chemical agents that kill or inhibit growth of microorganisms when applied to tissue

$Sepsis$$$Sepsis$$: microbial contamination

$Asepsis$$$Asepsis$$: absence of significant contamination

$Antimicrobial agents$$$Antimicrobial agents$$: agents that kill microorganisms or inhinit their growth

• $-cidal/-cide$$$-cidal/-cide$$: agents that kill

• $Germicide$$$Germicide$$: an agent that kills certain microorganisms

  • $Bactericide$$$Bactericide$$: an agent that kill bacteria; most do not kill endospores

  • $Viricide$$$Viricide$$: an agent that inactivates virsuses

  • $Fungicide$$$Fungicide$$: an agent that kills fungi

  • $Sporicide$$$Sporicide$$: an agent that kills bacterial endospores or fungal spores

• $-static$$$-static$$: agents that inhibit growth

Pattern of microbial death

• Microorganisms are not killed instantly

• Population death usually occurs exponentially

• Organisms can be in a viable but nonculturable (VBNC) conditon - may regain the ability to reproduce and cause infection

Conditions influencing the effectiveness of antimicrobial agent activity

• Population size

• Larger populations take longer to kill than smaller populations

• Population composition

• Microorganisms differ markedly in their sensitivity to antimicrobial agents

• Concentration or intensity of an antimicrobial agent

• Higher concentrations or intensities kill more rapidly (not linear relationship)

• Duration of exposure

• Longer exposure = more organisms killed

• Temperature

• Higher temperatures usually increased amount of killing

• Local environment

• Factors (pH, viscosity, concentration of organic matter) can impact effectiveness

• Organisms in bioflims are physically altered and less susceptible to many antimicrobial agents

Three main microbial control methods: $physical, chemical, and mechanical removal methods$$$physical, chemical, and mechanical removal methods$$

• Physical methods

• Heat

  • $Moist heat$$$Moist heat$$: kills microorganisms by coagulating (denaturing) their proteins, degrades nucleic acids, and disrupts membranes; more effective than dry heat

  • Autoclave sterilization - 121 °C at twice atmospheric pressure

  • $Boiling$$$Boiling$$: heat to 100 °C or more at sea level; kills vegetative forms of bacterial pathogens, most viruses, and fungi and their spores within 10 min or less.

    • Hepatitis virus can survive up to 30 min of boiling

    • Endospores can survive up to 20 hrs + of boiling

  • $Dry heat$$$Dry heat$$: kills by oxidation effects (ie bunsen burner)

  • $Direct flaming$$$Direct flaming$$: used to sterlize inoculating loops and needles - heat metal until it has a red glow

  • $Incineration$$$Incineration$$: effective way to sterlize disposable items (ex: paper cups, dressings) and biological waste

  • $Hot air sterlization$$$Hot air sterlization$$: place objects in an oven

  • Require 2 hrs at 170 °C for sterilization

  • Dry heat transfers heat less effectively to a cooler body than moist heat

• Low temperatures

  • Effect depends on microbe and treatment applied

  • $Refrigeration$$$Refrigeration$$: temperatures from 0 - 7 °C; bacteriostatic effect; reduces metabolic rate of most microbes, they cannot reproduce or produce toxins for some

  • $Freezing$$$Freezing$$: temperatures below 0 °C

  • $Flash freezing$$$Flash freezing$$: does not kill most microbes

  • $Slow freezing$$$Slow freezing$$: more harmful because ice crystals disrupt cell structure

  • Most parasites are killed by a few days of freezing

• Filtration

  • Reduces/removes microbial population or sterlizes solutions of heat-sensitive materials by removing microorganisms

  • Used to reduce microbial populations in air

• Radiation

  • $Ionizing radiation$$$Ionizing radiation$$: gamma rays, x rays, electron beams, or higher energy rays; have short wavelengths (less than 1 nm)

  • Causes mutations in DNA

  • Used to sterlize pharmaceuticals, disposable medical supplies, food

  • Penetrates human tissues = may cause genetic mutation in humans

  • $Nonionizing radiation$$$Nonionizing radiation$$: ultraviolet light (UV); wavelength longer than 1nm

  • Damages DNA by producing thymine dimers, which cause mutations

  • Used to disinfect operating rooms, nurseries, cafeterias

  • Disadvantages: damages skin and eyes; doesn’t penetrate paper, glass, and cloth

  • $Microwave radiation$$$Microwave radiation$$: wavelength ranges from 1 mm to 1 m

  • May kill vegetative cells in moist food

  • Solid foods are unevenly penetrated by microwaves

  • Bacterial endospores are not damaged by microwave radiation because lack of water in the endospore

• Chemical methods

• Used to control microbes on body surfaces and inanimate objects; disinfectant substances do not sterlize, but reduce microbial population

• Qualities of a disinfectant

  • Acts rapidly

  • Attacks a wide range of microbes

  • High penetrability

  • Readily mixes with water

  • Not hampered by organic material

  • Does not stain, corrode or damage object being disinfected

  • Does not damage body tissue

• Phenols and phenolics

  • Phenol (carbolic acid) was first used by Lister as a disinfectant

  • Rarely used today because it is a skin irritant and has strong odor; used in some throat sprays and lozenges

  • Cresols: derived from coal tar (lysol)

  • Biphenols (phsiohex): effective against gram + staphylococci and streptococci; used in nursies but excessive use in infants may cause neurological damage

  • Destroy plasma membrane and denature proteins

  • Advantages: stable, persist for long times after applied and remain active in the presence of organic compounds

• Alcohols

  • Kill bacteria, fungi, but not endospores or naked viruses

  • Act by denaturing proteins and disrupting cell membranes

  • Evaporate, leaving no residue

  • Used to mechanically wipe microbes off skin before injections or blood drawing

  • Ethanol: drinking alcohol (optimum concentration 70%)

  • Isopropanol: rubbing alcohol (better disinfectant than ethanol - cheaper and less volatile)

• Halogens

  • “Bleach”

  • Any of five elements: fluorine, chlorine, bromine, iodine, and astatine

  • Iodine and chlorine are important antimicrobial agents

    • Iodine: Oxidized cell constituents and iodinates proteins; at high concentrations may kill spores; skin antiseptic.

    • Cons: skin damage, staining, and allergies

    • $Iodophor$$$Iodophor$$: iodine complexed with organic carrier

    • Chlorine: oxidizes cell constutients; important in disinfection of water supplies and swimming pools; used in dairy and food industries; effective household disinfectant; destroys vegetative bacteria and fungi but not spores

    • Can react with organic matter to form carcinogenic compounds (ammonia)

• Heavy metals

  • Ions of sliver, mercury, arsenic, zinc, and copper

  • Effective but usually toxic

  • Arsenic use in the past

  • Combine with and inactivate proteins; may also precipitate proteins

• Detergents

  • Have antimicrobial activity and are effective disinfectants, disrupt cell wall

  • $Amphipathic$$$Amphipathic$$: organic molecules with hydrophilic and hydrophobic ends

  • Act as wetting agents and emulsifiers

  • Cationic detergents are effective disinfectants that kill most bacteria by not Mycobacterium, tuberculosis, or endospores

  • Safe and easy to use but inactivated by hard water and soap

• Disinfectants

  • Aldehydes

  • most effective antimicrobials; inactivate proteins

  • Formaldehyde gas: commonly used as formalin to preserve biological specimens and inactivate viruses and bacteria in vaccines; irritates mucous membranes and has strong odor; used in mortuaries for embalming

  • Hydrogen peroxide (peroxygens)

  • Used as an antiseptic - toxic to cell; not good for open wounds because quickly broken down by catalase present in human cells; effective in disinfection of inanimate objects; used by food industry and to disinfect contact lenses

  • Sterilizing gas

  • Used to sterlize heat-sensitive materials

  • Microbicidal and sporicidal

  • Combine with and inactivate proteins

  • Ethylene Oxide Sterilizer