Sterilization and disinfection microbiology
Biocides : a general term describing a chemical agent, usually broad-spectrum, that
inactivates microorganisms .
Sterilization :A physical or chemical process that completely destroys or removes all microbial
life, including spores.
Disinfectants : Products or biocides used to kill microorganisms on inanimate objects or
surfaces. Disinfectants can be sporostatic but are not necessarily sporicidal.
Septic : Characterized by the presence of pathogenic microbes in living tissue.
Antiseptic : A biocide or product that destroys or inhibits the growth of microorganisms in or
on living tissue.
Aseptic : Characterized by the absence of pathogenic microbes.
Preservation :The prevention of multiplication of microorganisms in formulated products,
including pharmaceuticals and foods.
Antibiotics : Naturally components or synthetic organic compounds which inhibit or destroy
selective bacteria, generally at low concentrations.
Bacteriostatic : A specific term referring to the property by which a biocide is able to inhibit
bacterial multiplication; multiplication resumes upon removal of the agent. (The terms
"fungistatic" and "sporostatic" refer to biocides that inhibit the growth of fungi and spores,
respectively.)
Bactericidal : A specific term referring to the property by which a biocide is able to kill
bacteria. Bactericidal action differs from bacteriostasis only in being irreversible; ie, the
"killed" organism can no longer reproduce, even after being removed from contact with the
agent.
Modes of Action
Damage to DNA
A number of antimicrobial agents act by damaging DNA; these include ionizing radiations,
ultraviolet light, and DNA-reactive chemicals. Among the last category are alkylating agents
and other compounds that react covalently with purine and pyrimidine bases to form DNA
adducts or interstrand cross-links. Radiations damage DNA in several ways: Ultraviolet light,
for example, induces cross-linking between adjacent pyrimidines on one or the other of the
two polynucleotide strands, forming pyrimidine dimers; ionizing radiations produce breaks in
single and/or double strands.
Protein Denaturation
Proteins exist in a folded, three-dimensional state determined by intramolecular covalent
disulfide linkages and a number of noncovalent linkages such as ionic, hydrophobic, and
hydrogen bonds. This state is called the tertiary structure of the protein; it is readily disrupted
by a number of physical or chemical agents, causing the protein to become nonfunctional. The
disruption of the tertiary structure of a protein is called protein denaturation.
Disruption of Cell Membrane or Wall
The cell membrane acts as a selective barrier, allowing some solutes to pass through and
excluding others. Many compounds are actively transported through the membrane,
becoming concentrated within the cell. The membrane is also the site of enzymes involved in
the biosynthesis of components of the cell envelope. Substances that concentrate at the cell
surface may alter the physical and chemical properties of the membrane, preventing its
normal functions and therefore killing or inhibiting the cell.
The cell wall acts as a corseting structure, protecting the cell against osmotic lysis. Thus,
agents that destroy the wall (eg, lysozyme) or prevent its normal synthesis (eg, penicillin) may
bring about lysis of the cell.
Physical Agents
1. dry heat
2. moist heat
3. boiling
4. pasteurization
Application of heat is the simplest means of sterilizing materials, provided the material is itself
resistant to heat damage. A temperature of 100°C will kill all but spore forms of bacteria
within 2–3 minutes in laboratory-scale cultures; a temperature of 121°C for 15 minutes is
utilized to kill spores. Steam is generally used, both because bacteria are more quickly killed
when moist and because steam provides a means for distributing heat to all parts of the
sterilizing vessel. Steam must be kept at a pressure of 15 lb/sq in above atmospheric pressure
to obtain a temperature of 121°C; autoclaves or pressure cookers are used for this purpose.
For sterilizing materials that must remain dry, circulating hot air electric ovens are available;
since heat is less effective on dry material, it is customary to apply a temperature of 160–
170°C for 1 hour or more.
2-Radiation
Ultraviolet light and ionizing radiations have various applications as sterilizing agents.
3- Filitration : used for heat-labile material example sera and sugars, but some viruses and
small bacteria can pass through filter pores
Chemical Agents :There are several compounds used as a antimicrobial (disifectants)
agents :
Alcohols : Ethyl alcohol, isopropyl alcohol, etc. exhibit rapid, broad-spectrum antimicrobial
activity against vegetative bacteria, viruses, and fungi but are not sporicidal. Activity is
optimal when they are diluted to a concentration of 60–95% with water.
Aldehydes : Glutaraldehyde is used for low-temperature disinfection and sterilization of
endoscopes and surgical equipment. It is normally used as a 2% solution to achieve
sporicidal activity. Formaldehyde is bactericidal, sporicidal, and virucidal.
Biguanides : Chlorhexidine is widely used in hand washing and oral products and as a
disinfectant and preservative.
Halogen-Releasing Agents
The most important types of chlorine-releasing agents are sodium hypochlorite, chlorine
dioxide, etc. which are oxidizing agents that destroy the cellular activity of proteins.
Hypochlorous acid is the active compound responsible for the bactericidal and virucidal
effect of these compounds. At higher concentrations, these compounds are sporicidal.
Iodine is rapidly bactericidal, fungicidal, tuberculocidal, virucidal, and sporicidal.
Heavy Metal Derivatives
Silver sulfadiazine , a combination of two antibacterial agents, Ag
+
and sulfadiazine , has a
broad spectrum of activity. Binding to cell components such as DNA may be responsible for
its inhibitory properties.
Peroxygens :Hydrogen peroxide has broad-spectrum activity against viruses, bacteria,
yeasts, and bacterial spores. Sporicidal activity requires higher concentrations (10–30%) of
H
2
O
2
and longer contact times.
Phenols :Phenols and many phenolic compounds have antiseptic, disinfectant, or
preservative properties.