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MI 1.4 - Sterilization and Disinfection

(Methods, Principles, Indicators)

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PART 1: DEFINITIONS (Foundation Concepts)

MNEMONIC: "SAD-ASPG"

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PART 2: MICROBIAL KILLING KINETICS (Principle)

Killing is EXPONENTIAL - har time interval mein ek fixed proportion of survivors marti hai.

Log (Survivors)
    |
100 |●
 10 |  ●
  1 |     ●
0.1 |        ●  ← (spores = deviation from linearity)
    |_____________ Time (minutes)

• Slope = rate of killing (temperature ya disinfectant concentration badho = slope steeper)
• If spores present = curve flattens at later stages (bimodal)
• D-value = time needed to kill 90% of organisms at a given temperature

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PART 3: SPAULDING CLASSIFICATION (Most Important for MCQs)

FLOWCHART:

Medical Device/Instrument
        |
   _____|___________________________________________
   |               |                              |
CRITICAL       SEMI-CRITICAL               NON-CRITICAL
(Enters         (Contacts mucous            (Contacts intact
sterile         membrane/non-               skin only)
tissue/         intact skin)
vascular)
   |               |                              |
STERILIZATION   HIGH-LEVEL                 LOW-LEVEL
required        DISINFECTION               DISINFECTION
   |               |                              |
IV catheters    GI endoscopes              Blood pressure cuffs
Surgical        Vaginal speculum           Stethoscopes
instruments     Laryngoscope blade         Bedpans
Implants        ETT                        Urinals

MNEMONIC: "CSN - SHL"

• Critical → Sterilization (Catheter, Scalpel)
• Semi-critical → High-level disinfection (Scope, Speculum)
• Non-critical → Low-level disinfection (Non-invasive items)

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PART 4: METHODS OF STERILIZATION

MASTER FLOWCHART - STERILIZATION METHODS:

STERILIZATION METHODS
        |
   _____|_______________________________
   |           |           |           |
PHYSICAL     GAS/         CHEMICAL    RADIATION
METHODS      VAPOR        METHODS     METHODS
   |           |           |           |
 Heat         EtO         Glutar-     UV light
 Filtration   H2O2 vapor  aldehyde    Ionizing
 (non-heat)   Plasma gas  Peracetic   (gamma)
              Formaldehyde acid

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A. HEAT METHODS (Most Common Exam Topic)

1. DRY HEAT

2. MOIST HEAT / AUTOCLAVE (Most Important)

• Water molecules break hydrogen bonds between peptide groups → irreversible protein denaturation
• Faster and more effective at lower temperatures

Air removed from chamber
        ↓
Saturated steam enters under pressure
        ↓
Temperature rises proportional to pressure
        ↓
Steam penetrates material
        ↓
Protein denaturation → microbial death

Key point: Pressure itself plays NO role in killing - it's only to raise the temperature of steam. Steam must be saturated and air-free.

3. PASTEURIZATION

• LTLT (Low Temp Long Time): 62°C × 30 minutes
• HTST (High Temp Short Time): 72-74°C × 15-20 seconds (most common)
• UHT: >135°C × 1-2 seconds
• Kills vegetative bacteria - NOT spores
• Used for: milk, beverages, plastic hospital equipment

4. BOILING (100°C)

• Activity level: High (not sterilization)
• Kills: Most pathogens + some spores
• Kills hepatitis B virus after 10 minutes boiling

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B. GAS/VAPOR METHODS

1. Ethylene Oxide (EtO) - MOST IMPORTANT

• Inflammable and potentially explosive
• Requires long aeration time after use
• Carcinogenic
• Slow process

2. Formaldehyde Vapor

• Alkylating agent (like EtO)
• Used WITHOUT pressure for larger areas/rooms
• Gas sterilization/fumigation

3. Hydrogen Peroxide Vapor / Plasma Gas

• H2O2 at 30% concentration, 55-60°C
• Plasma gas = highly ionized H2O2
• Used for heat-sensitive instruments

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C. RADIATION METHODS

• DNA repair mechanisms can reverse UV damage (concern for vaccine preparation)
• Used in OT/lab air disinfection, water purification
• Cannot penetrate glass, walls, or wrapped instruments

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D. FILTRATION (Non-Heat Sterilization)

• Pore size: 0.22 to 0.45 μm (removes bacteria by size exclusion)
• HEPA filters: High-Efficiency Particulate Air - 0.3 μm particles (>99.97% efficiency)
• Uses: Heat-sensitive liquids (sera, enzymes, antibiotics, culture media), air filtration in OT
• Does NOT kill organisms - physically removes them
• Cannot remove viruses (too small)

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PART 5: METHODS OF DISINFECTION

Classification by LEVEL:

DISINFECTION LEVELS
        |
   _____|______________________
   |           |              |
HIGH-LEVEL  INTERMEDIATE-   LOW-LEVEL
            LEVEL
   |           |              |
Kills all   Kills vegeta-   Kills most
except      tive bacteria,  vegetative
large no.   mycobacteria,   bacteria +
of spores   fungi, viruses  lipid viruses
            (NOT spores)    only

HIGH-LEVEL DISINFECTANTS:

INTERMEDIATE-LEVEL DISINFECTANTS:

100% alcohol dehydrates rapidly but cannot kill because the lethal process requires water molecules to break hydrogen bonds in proteins.

LOW-LEVEL DISINFECTANTS:

• Glutaraldehyde, H2O2, Chlorine = HIGH level
• Alcohol, Iodophors, Phenolics = INTERMEDIATE level
• Quat. Ammonium = LOW level

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PART 6: STERILIZATION INDICATORS

TYPES OF INDICATORS:

STERILIZATION INDICATORS
          |
    ______|__________________________
    |           |                  |
PHYSICAL    CHEMICAL           BIOLOGICAL
    |           |                  |
Thermometer  Bowie-Dick tape    Spore strips
Pressure     (autoclave tape)   (most reliable)
gauges       TST (Thermocouple)
Timers       Browne's tubes

BIOLOGICAL INDICATORS (Most Reliable / Gold Standard):

• Autoclave → Geobacillus Stearothermophilus (loves steam/heat)
• Dry heat + EtO → Bacillus Atrophaeus

CHEMICAL INDICATORS:

Important: Chemical indicators confirm conditions were MET (temperature, time) but do NOT confirm sterility. Biological indicators confirm actual killing.

PHYSICAL INDICATORS:

• Thermometers, pressure gauges, timers built into autoclave
• Least reliable - only show machine parameters

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PART 7: RESISTANCE HIERARCHY of MICROORGANISMS

MOST RESISTANT
      ▲
      |  Prions (cannot be destroyed by standard autoclave)
      |  Bacterial SPORES (Clostridium, Bacillus)
      |  Mycobacteria (waxy coat - TB)
      |  Non-enveloped viruses (Polio, HAV)
      |  Fungi / fungal spores
      |  Vegetative bacteria (Gram+/Gram-)
      |  Enveloped viruses (HIV, HBV, Herpes)
      ▼
LEAST RESISTANT (easiest to kill)

• Prions
• Spores (bacterial)
• Mycobacteria
• Non-enveloped viruses
• Fungi
• Vegetative bacteria
• Enveloped viruses

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PART 8: SPECIAL TOPICS

Glutaraldehyde - Dual Role

• 2% Glutaraldehyde = Chemical sterilant (long exposure, 6-10 hours) OR high-level disinfectant (20-30 min)
• Used for: GI endoscopes, heat-sensitive surgical instruments
• Mechanism: Cross-links proteins (alkylating action)

Chlorine (Hypochlorite)

• 5% solution (bleach) for surface decontamination
• <1 ppm kills vegetative bacteria in seconds in water
• Choice agent for HIV, spores of pathogenic bacteria on surfaces
• Inactivated by organic matter

Antiseptics vs Disinfectants

• Povidone iodine
• Isopropyl/Ethyl alcohol
• Alcohol + Chlorhexidine (most common pre-op skin prep)
• Chlorhexidine gluconate
• Triclosan

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PART 9: SUMMARY MASTER TABLE

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QUICK REVISION MNEMONICS COMPILATION

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EXAM-TARGETED HIGH-YIELD POINTS

1. Autoclave BI = G. stearothermophilus (most tested MCQ)
2. Dry heat/EtO BI = B. atrophaeus
3. 70% alcohol > 100% alcohol (water needed for protein denaturation)
4. Pressure in autoclave does NOT kill - raised temperature kills
5. EtO = heat-labile items, alkylating agent, needs aeration
6. UV = 254 nm, poor penetration, surfaces only
7. Gamma radiation = ionizing, excellent penetration, disposables
8. Glutaraldehyde = endoscopes (HLD at 20-30 min, sterilant at 6-10 hr)
9. Filtration = 0.22 μm, removes bacteria NOT viruses
10. Spaulding: Critical → Sterilize; Semicritical → HLD; Noncritical → Low-level
11. Biological indicators = most reliable (gold standard) for sterilization verification
12. QAC = low-level disinfectant only (often tested as distractor)
13. Pasteurization does NOT sterilize (spores survive)
14. Prions most resistant (need special protocols, not killed by standard autoclave)

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