Explain in details about disinfectant (microbiology and infection control subject bsc nursing 3rd sem)
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Disinfectants - Detailed Notes for BSc Nursing 3rd Semester (Microbiology & Infection Control)
1. Definition and Key Terms
Disinfection is the destruction of pathogenic microorganisms by processes that fail to meet the criteria for sterilization. The term is most commonly applied to liquid chemical agents (disinfectants) which usually have some degree of selectivity - bacterial spores, organisms with waxy coats (e.g., mycobacteria), and some viruses may show considerable resistance.
Key distinctions:
| Term | Meaning |
|---|---|
| Sterilization | Complete killing or removal of ALL living organisms (including spores) |
| Disinfection | Destruction of most pathogenic microorganisms - not necessarily spores |
| Antisepsis | Use of disinfecting agents (antiseptics) on body surfaces (skin, mucous membranes) to reduce pathogen numbers; lower toxicity than disinfectants |
| Sanitization | A less precise term - somewhere between disinfection and cleanliness; used in housekeeping and food preparation |
| Asepsis | Working systems designed to prevent microorganisms from reaching a protected environment |
Important for exams: Pasteurization is technically a form of disinfection - it destroys pathogenic organisms in liquids but does NOT sterilize (spores survive).
2. Levels of Disinfection
Disinfection is categorized into three levels:
| Level | Activity | Examples |
|---|---|---|
| High-level | Kills all microorganisms except high numbers of bacterial spores | Glutaraldehyde, hydrogen peroxide, peracetic acid, chlorine compounds, moist heat |
| Intermediate-level | Kills mycobacteria, vegetative bacteria, most viruses/fungi; NOT spores | Alcohols, iodophors, phenolic compounds |
| Low-level | Kills most vegetative bacteria, some fungi, enveloped viruses | Quaternary ammonium compounds |
When to use which level:
- High-level: For instruments used for invasive procedures that cannot be autoclaved (e.g., endoscopes)
- Intermediate-level: For surfaces and instruments where contamination with highly resistant organisms is unlikely
- Low-level: For non-critical items like blood pressure cuffs, electrodes, stethoscopes
3. Types of Disinfection (by Timing)
(a) Concurrent Disinfection - Application of disinfective measures as soon as possible after discharge of infectious material from a patient's body. Includes disinfection of urine, faeces, vomit, contaminated linen, dressings, gloves, etc., throughout the course of illness.
(b) Terminal Disinfection - Application of disinfective measures after the patient has been removed (by death, transfer to hospital, or cessation of being infectious). Now scarcely practiced; terminal cleaning with airing and sunning is considered adequate.
(c) Precurrent (Prophylactic) Disinfection - Preventive in nature. Examples: chlorination of water, pasteurization of milk, routine handwashing.
4. Properties of an Ideal Disinfectant
An ideal disinfectant should:
- Broad spectrum - wide antimicrobial activity against bacteria, fungi, viruses, spores
- Fast acting - produce rapid kill
- Not affected by environmental factors - active in the presence of organic matter (blood, pus, faeces); compatible with soaps and detergents
- Non-toxic - not harmful to the user or patient
- Surface compatible - should not corrode instruments/metallic surfaces; should not deteriorate cloth, rubber, or plastics
- Residual effect - leave an antimicrobial film on treated surfaces
- Easy to use - with clear label directions
- Odourless - pleasant or no odour to facilitate routine use
- Economical - not prohibitively expensive
- Solubility - soluble in water
- Stability - stable in concentrate and in use dilution
- Cleaner - good cleaning properties
- Environmentally friendly - should not damage the environment on disposal
No single disinfectant currently meets ALL these criteria.
5. Agents Used for Disinfection
A. Natural Agents
(1) Sunlight - Direct and continuous sunlight is destructive to many disease-producing organisms. UV rays (which do not penetrate glass) are lethal to bacteria and some viruses. Articles like linen, bedding, and furniture can be disinfected by exposure to direct sunlight for several hours.
(2) Air - Exposure to open air (airing) acts by drying or evaporation of moisture, which is lethal to most bacteria. However, natural agents cannot be totally depended upon due to their variability.
B. Physical Agents
(1) Burning/Incineration - Excellent method for contaminated dressings, rags, swabs, and faeces. Should be done in an incinerator, not in open air.
(2) Hot Air (Dry Heat) - Used for glassware, syringes, swabs, dressings, oils, vaseline, and sharp instruments. Done in a hot air oven at 160-180°C for at least 1 hour (to kill spores). Drawback: no penetrating power; not suitable for bulky articles; destroys plastic and rubber.
(3) Boiling - Boiling for 5-10 minutes (rolling boil) kills vegetative bacteria but NOT spores or viruses. Suitable for small instruments, rubber goods, linen. To ensure spore destruction, temperatures above 100°C are required (achieved only by autoclaving).
(4) UV Radiation - Wavelength range 210-328 nm; maximum bactericidal effect at 240-280 nm. Mercury vapour lamps emit radiation at 253.7 nm. Used for disinfecting drinking water, air, titanium implants, and contact lenses. Bacteria and viruses are more easily killed than bacterial spores.
(5) Microwave - Used for soft contact lenses, dental instruments, dentures, milk, and urinary catheters. Frequency: 2450 MHz. Mechanism: production of heat by intermolecular friction of water molecules. Completely inactivates bacterial cultures, mycobacteria, viruses, and even G. stearothermophilus spores within 60 seconds to 5 minutes.
(6) Pasteurization - Not sterilization; destroys all pathogenic microorganisms but NOT spores. Time-temperature: 70°C for 30 minutes (or 74°C for 3-5 seconds for milk).
(7) Flushing and Washer Disinfectors - Automated closed equipment that clean and disinfect bedpans, urinals, surgical instruments, anaesthesia tubes. Clean by flushing with warm water ± detergent, then disinfect with hot water or steam.
(8) Ozone - Used as a drinking water disinfectant. Produced when O₂ is energized and split into monatomic O⁻ molecules, which react with surrounding O₂ to form O₃ (ozone).
C. Chemical Agents
1. Phenol and Related Compounds
| Agent | Key Properties | Use |
|---|---|---|
| Phenol (carbolic acid) | Standard reference for comparing germicidal activity; not an effective disinfectant on its own | Reference standard |
| Crude phenol | Mixture of phenol and cresol; dark oily liquid; effective against Gram+ and Gram- bacteria | General disinfection |
| Cresol | 3-10x more powerful than phenol; NOT more toxic; effective against spores, acid-fast bacteria | 5-10% for faeces/urine; 5% for mopping floors |
| Lysol | Saponified cresol (cresol emulsified with soap); 50-60% cresol; very powerful | 2% for disinfection of faeces |
| Chlorhexidine (Hibitane) | Highly active against Gram+ organisms; inactivated by soaps/detergents | 0.5% alcoholic/aqueous solution for handwashing; 1% cream for burns |
| Hexachlorophane | Active against Gram+; slow but cumulative effect on skin; compatible with soaps | Incorporated in soap preparations |
| Dettol (chloroxylenol) | Relatively non-toxic; active against streptococci; less effective against some Gram- bacteria | 5% for instruments and plastic equipment (contact 15 min) |
Mechanism: Phenolic compounds disrupt lipid-containing membranes, causing leakage of cellular contents.
2. Quaternary Ammonium Compounds (QACs)
- Cetrimide (Cetavlon): Actively bactericidal against vegetative Gram+ organisms; less effective against Gram- organisms. Used in 1-2% strength.
- Savlon: Combination of cetavlon + hibitane. Plastic appliances: 20 minutes in normal strength savlon. Clinical thermometers: disinfected in Savlon 1-in-6 in spirit in under 3 minutes.
- Benzalkonium chloride: Widely used QAC.
Mechanism: QACs denature/disrupt cell membranes.
Level: Low-level disinfectants (do not kill mycobacteria, spores, or non-enveloped viruses).
3. Halogens and Their Compounds
a. Chlorine and Chlorine Compounds
Potent bactericidal, fungicidal, sporicidal, tuberculocidal, and virucidal agents.
| Compound | Details |
|---|---|
| Bleaching powder (CaOCl₂) | ~33% available chlorine; 1-3% kills most organisms; 5% solution for faeces/urine (allow 1 hour) |
| Sodium hypochlorite (liquid bleach) | 5.25-6.15% household bleach; broad spectrum; inexpensive; fast acting; removes biofilms |
| Chlorine tablets (Halazone) | Good for disinfecting small quantities of water |
| Chloramine-T, sodium dichloroisocyanurate | Retain chlorine longer; more prolonged bactericidal effect than hypochlorites |
| Superoxidized water | Contains hypochlorous acid (~144 mg/L); inexpensive; end product is water (environment-friendly) |
Mechanism: Strong oxidizing agents (precise mechanism not fully defined).
b. Iodine
- Tincture of iodine: Used as antiseptic on skin; readily available and quick in action.
- Iodophores (e.g., Povidone-iodine / Betadine): Iodine complexed with polyvinylpyrrolidone (PVP). Provides sustained-release reservoir of iodine. Non-irritant; does not stain skin. Used as antiseptic and for disinfecting blood culture bottles and medical equipment.
Mechanism: Iodine compounds precipitate proteins and oxidize essential enzymes.
4. Alcohols
- Ethyl alcohol (industrial methylated spirit): Most commonly used for skin disinfection and handwashing.
- Isopropyl alcohol: Also effective.
- Key facts:
- Pure alcohol has NO disinfection power.
- Diluted to 60-90% vol/vol: potent bactericidal, fungicidal, virucidal, and tuberculocidal.
- Does NOT destroy bacterial spores.
- Activity decreases rapidly below 50%.
- 70% alcohol kills all types of non-sporing bacteria within seconds.
- Activity disappears as alcohol dries off.
- Inflammable - must be stored in cool areas.
- Most effective skin antiseptics: alcoholic solutions of chlorhexidine and iodine.
5. Formaldehyde (Formalin)
- Highly toxic and irritant gas.
- Mechanism: Precipitates and destroys protein.
- Spectrum: Effective against vegetative bacteria, fungi, many viruses; only slowly effective against bacterial spores and acid-fast bacteria.
- 2-3% solution (20-30 ml of 40% formalin per litre water) for spraying rooms, walls, furniture.
- Gas form: Used for room disinfection; most effective at high temperature and relative humidity of 80-90%. Also used for blankets, books, and valuable articles that cannot be boiled.
- Does not injure fabrics or metals.
6. Oxidizing Agents
| Agent | Details |
|---|---|
| Potassium permanganate | Purple-black crystalline powder; strong oxidizing action (colours everything it touches); used for aquariums, swimming pools, disinfecting fruits/vegetables |
| Hydrogen peroxide | Bactericidal, virucidal, sporicidal, fungicidal. 0.5% (accelerated) - bactericidal/virucidal in 1 min; mycobactericidal/fungicidal in 5 min. 3% solution: antiseptic and wound cleaning |
| Peracetic acid | Produced by reacting H₂O₂ with acetic acid. Not deactivated by catalase/peroxidase. Inactivates Gram+ and Gram- bacteria, fungi, yeast in <5 min at <100 ppm. Breaks down to environment-friendly residue (acetic acid + water) |
7. Metals as Microbicides (Oligodynamic Action)
- Heavy metals like silver have anti-infective activity.
- Uses: prophylaxis of neonatal conjunctivitis, topical therapy for burn wounds, bonding to indwelling catheters.
- Silver and zinc ions (zeolite ceramic coating) can inactivate bacteria on stainless steel surfaces.
- Silver, iron, and copper can be used for environmental control and disinfection of water or reusable medical devices.
8. Lime
- Cheapest of all disinfectants.
- Used as fresh quick lime or 10-20% aqueous suspension ("milk of lime").
- Faeces and urine: disinfected by mixing with 10-20% aqueous lime suspension; allow 2 hours.
- Lime wash: Used for treating walls.
- As a deodorant: sprinkled in cattle sheds, stables, public urinals and latrines.
9. Ethylene Oxide (EtO)
- Used for sterilizing heat-sensitive articles at 55-60°C.
- Kills bacteria, spores (e.g., tetanus spores), and viruses.
- Explosive in pure form - mixed with CO₂ (12%) for safety.
- Addition of water vapour (relative humidity 33%) increases efficiency.
- Used for fabrics, plastic equipment, cardiac catheters, books.
- Drawback: Process is difficult to control - discouraged when alternatives are available.
- Effectiveness depends on: concentration of gas, relative humidity, exposure time, and temperature. Best at elevated temperature, high EtO concentration, and 30% relative humidity.
10. Glutaraldehyde
- Used for high-level disinfection and sterilization (depending on concentration and duration).
- Used for endoscopes, surgical instruments that cannot be autoclaved.
6. Spaulding Classification (Instrument Categories)
This is the basis for selecting disinfection levels in clinical settings:
| Category | Definition | Examples | Required Level |
|---|---|---|---|
| Critical | Enters sterile tissue or vascular system | Surgical instruments, needles, catheters | Sterilization |
| Semi-critical | Contacts mucous membranes or non-intact skin | Endoscopes, laryngoscopes, respiratory equipment | High-level disinfection |
| Non-critical | Contacts intact skin only | BP cuffs, stethoscopes, electrodes | Low-level disinfection |
7. Factors Affecting Disinfection Efficacy
- Concentration - Higher concentration generally increases efficacy (within limits).
- Time of exposure - Longer contact time increases kill.
- Temperature - Higher temperature generally increases effectiveness.
- Organic matter - Blood, pus, faeces can inactivate many disinfectants (especially QACs and chlorine compounds).
- pH - Activity of most disinfectants is pH-dependent.
- Nature of microorganism - Spores, mycobacteria, and non-enveloped viruses are most resistant.
- Number of organisms - Higher microbial load requires more potent/prolonged treatment.
- Physical nature of item - Penetration into materials (cracks, biofilms) affects efficacy.
8. Resistance Spectrum of Microorganisms (Most to Least Resistant)
Most resistant → Least resistant:
Prions → Bacterial spores → Mycobacteria → Non-enveloped viruses → Fungi → Gram-negative bacteria → Gram-positive bacteria → Enveloped viruses (most susceptible)
9. Summary Table of Common Chemical Disinfectants
| Disinfectant | Level | Mechanism | Key Uses | Limitations |
|---|---|---|---|---|
| Glutaraldehyde | High | Alkylates proteins | Endoscopes, heat-sensitive instruments | Toxic vapours, long contact time |
| Hydrogen peroxide | High/Intermediate | Oxidation | Hospital surface disinfection, wound care | Corrosive to some metals |
| Sodium hypochlorite | High/Intermediate | Oxidation | Water, surfaces, blood spills | Inactivated by organic matter, corrosive |
| Iodophors (Betadine) | Intermediate | Protein precipitation, oxidation | Skin antisepsis, blood culture bottles | Staining, thyroid effects |
| Alcohols (70%) | Intermediate | Protein denaturation, membrane disruption | Skin prep, thermometers | No spore activity, flammable |
| Phenolics (Lysol) | Intermediate | Membrane disruption | Floors, drains, faeces/urine | Toxic, not for food areas |
| Chlorhexidine | Intermediate | Membrane disruption | Handwashing, burn wounds | Inactivated by soaps |
| QACs (Savlon) | Low | Membrane disruption | Non-critical surfaces, thermometers | No activity against mycobacteria/spores |
| Formaldehyde | High (gas) | Protein precipitation | Room fumigation, heat-sensitive items | Highly toxic, carcinogenic |
Sources: Park's Textbook of Preventive and Social Medicine; Sherris & Ryan's Medical Microbiology, 8th Edition; Medical Microbiology, 9th Edition (Murray et al.)
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