Chemical Means of Disinfecting
Chemical Means of Disinfecting
Overview of Chemical Disinfection
Chemicals are employed to control organisms on both living and inanimate surfaces.
Most cleaners primarily control microbial growth rather than achieve complete sterility.
The effectiveness of cleaning agents depends on various factors.
Evaluating Disinfectants
For FDA approval, all disinfectants must be proven effective.
Use-Dilution Test is a standard method to determine effectiveness:
Process:
Metal or glass cylinders are dipped into standardized bacterial cultures in liquid media.
After drying, the rings are immersed in the disinfectant prepared according to manufacturer recommendations.
Rings are kept in contact with the disinfectant for 10 minutes at 20°C.
The effectiveness is measured by placing rings back into media and checking for bacterial growth.
Variants of this test are conducted for viruses and endospores.
Use-Dilution Test Variations
A common variation involves performing serial dilutions of the disinfectant before testing, which helps identify the maximum dilution that is effective.
This information is often included on the packaging for industrial cleaners.
Disk Diffusion Method
An educational method for testing disinfectants:
Filter paper disks are soaked in the disinfectant.
Bacteria are spread uniformly over an agar plate, and the disks are placed on top.
After incubation, the presence of a zone of inhibition (area around the disks with no bacterial growth) is assessed.
Larger zones indicate a more effective disinfectant.
Overview of Disinfectants
Phenol:
Identified as carbolic acid, first used to control infections in surgical theaters (inspired by Joseph Lister).
Not widely used due to odor and skin irritation; exceptions include throat sprays (effective only at concentrations above 1%) for antimicrobial properties.
Phenolic Compounds:
Derivatives of phenol modified to reduce irritation and odor.
O-phenylphenol is commonly found in Lysol products and serves as an effective disinfectant.
Mechanism: disrupts the plasma membrane, leading to leakage of cytoplasmic contents, is effective against lipid-rich Mycobacterium spp., and is stable in organic matter, making it suitable for disinfecting biological fluids like pus and feces.
Bisphenols:
Composed of two phenolic compounds connected by a chemical bridge.
Examples:
Hexachlorophene: Found in pHisoHex, effective against Staphylococcus and Streptococcus; used as a hospital antimicrobial lotion.
Triclosan: Previously common in hand soaps; inhibits fatty acid biosynthesis but associated with antibiotic resistance and endocrine disruption; banned from hand soap in 2016.
General mechanism is believed to be plasma membrane disruption; effective against gram-positive bacteria.
Biguanides
Biguanides are broad-spectrum with high affinity for skin and mucus.
Commonly combined with alcohol for pre-operative scrubs (e.g., Chlorhexidine, used in Avagard).
Mechanism involves disruption of the plasma membrane and is effective against most vegetative bacteria and yeast, but not against endospores, cysts, or most viruses.
Halogens
Referring to elements in the 7th row of the periodic table: primarily chlorine and iodine.
Both have advantages of acting alone or in presence of other substances, with applications such as water treatment.
Iodine:
Available as tincture (aqueous iodine in alcohol) or iodophore (iodine complexed with organic molecules).
Tinctures act quickly but stain; iodophores are slower acting, long-lasting, and used for water sterilization.
Mechanism: inhibits protein functions and alters cell membrane.
Chlorine:
Used as a gas or in combinations like sodium hypochlorite (bleach).
Effective mechanisms in water:
$Cl2 + H2O \rightleftharpoons H^+ + Cl^- + HOCl$
$HOCl \rightleftharpoons H^+ + OCl^-$
Results in hypochlorite ions that penetrate cells and disrupt cellular compartments; acts as a strong oxidizer.
Practical Application of Bleach
Used for water disinfection post-emergency (example: September 11th).
Recommended to disinfect water by adding 2 drops per liter (increased to 4 drops if water is cloudy) and allowing it to sit for 30 minutes.
Water should have a slight bleach smell but is considered microbiologically safe.
Alcohol
Effectively kills bacteria and fungi but ineffective against endospores and non-enveloped viruses.
Functions by denaturing proteins and disrupting membranes; evaporates fast, no residue when applied to intact skin, but actual antimicrobial activity can take minutes.
Preferred alcohols: Isopropanol and ethanol (effective concentrations between 60-95%).
Alcohol-based hand sanitizers typically contain 60-65% alcohol claiming 99.9% efficacy, but effectiveness is variable, especially against endospores.
Heavy Metals
Certain heavy metals exhibit antimicrobial properties through a mechanism known as oligodynamic action:
Trace metal amounts interact negatively with bacteria, causing protein denaturation and lipid dissolution.
Examples include copper, silver, and mercury, historically used in infant eye drops to prevent gonococcal infections.
Soap and Detergent
Not antimicrobial on their own; often enhanced with antimicrobials.
Function as emulsifying agents, interacting with oils on skin to lift and remove debris (grease, dirt, bacteria).
Acid-Anionic Sanitizers
Used in the dairy and food processing industry for their wide spectrum of activity and non-corrosiveness.
Comprises combinations of phosphoric acid and a soap/detergent; mechanism unknown but believed to involve enzyme inactivation or disruption.
Quaternary Ammonium Compounds (Quats)
Effective as cleaning and antimicrobial agents; positive charge aids in cleaning.
Mechanism includes enzyme inhibition, protein denaturation, and membrane disruption; very effective against gram-positive bacteria but not effective against Mycobacteria, endospores, or Pseudomonas.
Food Preservatives
Chemicals added to prevent spoilage; often organic acids or their salts (e.g., sodium benzoate, sorbic acid, calcium propionate).
Commonly used preservatives include sodium nitrate and nitrite, which are used in meats to inhibit Clostridium botulinum growth.
Aldehydes
Formaldehyde and glutaraldehyde are the primary antimicrobials, mostly used for preserving tissue specimens and sterilizing instruments.
Mechanism involves protein denaturation through crosslinking, effective against most microbes but not prions.
Gas Sterilizers
Compounds like ethylene oxide are toxic gases used for sterilization without heat, suitable for heat-sensitive items.
Major drawback includes long exposure times and carcinogenic properties.
Ethylene Oxide
Most common gas sterilizer, functions via alkylation affecting proteins and nucleic acids.
Highly penetrative, effective for large items such as mattresses in a hospital setting.
Plasma Sterilization
Utilizes electrically excited gas for sterilizing surgical instruments, effectively targeting materials with small diameters.
The process is efficient but costly due to the technology required.
Peroxygens
Includes ozone, hydrogen peroxide, and peracetic acid; function by oxidizing microbial cellular components.
They effectively sterilize inanimate objects but are less suitable for wound care due to catalase present in host cells.
Summary of Disinfection Agents
A range of disinfectants exists, each with specific uses depending on the type of organisms targeted and the cleaning task at hand.
Caution is advised in following the manufacturer's guidelines to ensure effectiveness.
Key Considerations in Disinfection
The type of contamination influences selection of disinfectants.
Organism type affects the choice and efficacy of agents.
Contact time is critical, as effectiveness is rarely instantaneous.
Prions
Conventional methods of disinfection are largely ineffective against prions.
Current approaches include incineration of infected materials or using specific enzyme cleaners capable of degrading prions.
Microbial Resistance Hierarchy
Most Resistant:
Prions
Endospores of bacteria
Mycobacteria
Cysts of protozoa
Vegetative protozoa
Gram-negative bacteria
Fungi (and most fungal spores)
Viruses without envelopes
Gram-positive bacteria
Viruses with lipid envelopes
Least Resistant: