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Disinfection and Sterilization
Disinfection: killing of most microbial forms.
Disinfectant: a chemical substance used to kill microbes on
surfaces but too toxic to be applied directly to tissue.
Antisepsis: inhibit or eliminate microbes on skin or other living
tissue.
Sterilization: removal of life of every kind by physical or
chemical methods.
Sterilant: an agent or method used to remove or kill all microbes.
Septic: presence of pathogenic microbes in living tissue.
Aseptic: absence of pathogenic microbes.
Sterile: free of life of every kind.
Bacteriostatic: inhibiting bacterial multiplication. Bacteriostatic
action is reversible by removal or inactivation of agent.
Bactericidal: killing bacteria.
Modes of action of antimicrobial agents
1. Disruption of cell membrane or wall
2. Damage to DNA
Formation of pyrimidine dimer by UV irradiation
Single- or double-strand DNA break by ionizing radiation
DNA reactive chemicals, e.g. alkylating agents
3. Protein denaturation
4. Removal of free sulfhydryl groups
Formation of disulfide bond by oxidizing agents
Heavy metals combined with sulfhydryls
5. Chemical antagonism: interference with the normal
reaction between an enzyme and its substrate.
Methods of sterilization (I)
Methods of sterilization (II)
Methods of disinfection
Effectiveness of disinfectants is influenced by
1. nature of the item,
2. number and resilience of the contaminants,
3. amount of organic material present,
4. type and concentration of disinfectant, and
5. duration and temperature of exposure.
High-level disinfectants
Used for items involved in invasive procedures.
Intermediate-level disinfectants
Used for cleaning surface or instruments without bacterial
spores and highly resilient organism
Low-level disinfectants
Used to treat non-critical instruments and devices
Antiseptic agents
Reduce the number of microbes on skin
Triclosan (chlorinated aromatic), chlorhexidine,
parachlorometaxylenol (PCMX), and hexachlorophene
are commonly used for body or hand cleansing.
Physical methods
(moist heat, dry heat, filtration, radiation)
Heat (moist heat; dry heat)
The most common sterilizing methods used in hospitals,
suitable for most materials except those that are heatsensitive or consist of toxic or volatile chemicals.
Dry heat
Dry oven: 160 oC for 2 hrs or 171 oC for 1 hr. (use B.
subtilis spores to monitor the effectiveness)
Flaming; incineration
Moist heat
Boiling: boiling for 10 min. is sufficient for killing most
vegetative forms of bacteria. Killing of spores requires
much longer time. Addition of 2% Na2CO2 or 0.1%
NaOH may enhance destruction of spores and prevent
rusting of the metal wares.
Low temperature disinfection (Pasteurization): 6265 oC for 30 min. or 71.5 oC for 15 sec. This is mainly
used for disinfection of milk.
Autoclave: 121.5 oC for 15-20 min. in an autoclave.
Kill both the vegetative and spore forms of the bacteria.
Radiation
UV-light: UV-radiation causes
damage to DNA (requires direct
exposure).
Ionizing radiation (microwaves or grays).
Filtration
The pore size for filtering bacteria,
yeasts, and fungi is in the range of
0.22-0.45 mm (filtration membranes
are most popular for this purpose).
HEPA (high-efficiency particulate air)
filters
Chemical methods
Alcohols: protein denaturant. 70% aqueous solution of
ethanol and isopropanol are commonly used as skin
disinfectants. Inactivated by organic matter.
Oxidizing agents: inactivate cells by oxidizing free
sulfhydryl group, e.g., ozone, peracetic acid, iodine,
iodophore (similar to alcohols in range of activity; acts
rapidly; frequently used with alcohol for disinfecting the
skin), H2O2 (3-6%), H2O2 vapor (30%, 55-60 oC), and
chlorine compounds: hypochlorite, and chlorine etc.
Plasma gas sterilization: H2O2 vapors treated with
microwave or radio energy to produce reactive free
radicals; no toxic byproducts. An efficient sterilization
for dry surfaces.
Phenolics: phenol and phenolic compounds (e.g. lysol)
lyse the cell membrane and denature proteins at 12% (aqueous solution). Effective for mycobacteria.
Hexachlorophene, Trichlosan, and PCMX are also
phenolics.
Alkylating agents
Formalin (37% aqueous solution of formaldehyde);
Glutaraldehyde. Both are sporicidal. Toxic for viable
tissues. Inactivated by organic matters.
Ethylene oxide gas (made nonexplosive by mixing
with CO2 or a fluorocarbon): a reliable disinfectant for
dry surfaces. Gas must be dissipated by aeration for
16 h before the item can be used.
Detergents: surface-active
agents that disrupt the cell
membranes.
Anionic detergents: e.g.
soaps, and bile salts.
Cationic detergents: e.g.,
the quaternary ammonium
compounds, are highly
bactericidal for most
gram-positive and
negative bacteria in the
absence of contaminating
organic matter.
Sites of action of chemical agents
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B. stearothermophilus spores