Infection and disinfection
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Infection and Disinfection
1. Key Definitions
Infection is the invasion and multiplication of pathogenic microorganisms in body tissues. Infections can be:
- Community-acquired - present at or incubating before hospital admission
- Nosocomial (hospital-acquired) - acquired during hospital care, not present/incubating at admission. Infections occurring >48 hours after admission are usually classified as nosocomial. They include surgical wound infections, hepatitis B, and urinary tract infections.
- Opportunistic - caused by organisms normally innocuous (commensals) that become pathogenic when the host's immune defenses are compromised (e.g., in AIDS).
Sources: Park's Preventive & Social Medicine; Sherris Medical Microbiology 8e
2. The Microbial Killing Spectrum - Key Terms
| Term | Definition |
|---|---|
| Sterilization | Complete killing/removal of ALL living organisms (absolute term) |
| Disinfection | Destruction of pathogenic organisms - does NOT meet the criteria for sterilization |
| Antisepsis | Use of disinfecting agents (antiseptics) safe enough for body surfaces (skin, mucous membranes) |
| Pasteurization | Intermediate heat treatment that kills vegetative pathogens but not spores |
| Sanitization | Broad housekeeping term - somewhere between disinfection and general cleanliness |
| Asepsis | Working systems/procedures designed to prevent organisms from reaching a protected environment (e.g., the operating room) |
3. Kinetics of Microbial Killing
Killing by heat, radiation, or chemicals follows exponential kinetics - a fixed proportion of the population dies per unit time. On a log-scale graph, survivor counts fall linearly:
Panel A = homogeneous population (straight-line log kill). Panel B = mixed population containing spores - the curve flattens late, so extrapolating from the exponential phase underestimates total kill time.
- The kill rate increases exponentially with arithmetic rises in temperature or disinfectant concentration.
- Bacterial spores are far more resistant than vegetative forms - they can survive pasteurization and many chemical disinfectants.
Sherris Medical Microbiology 8e, p. 102
4. Methods of Sterilization
4a. Heat
| Method | Conditions | Notes |
|---|---|---|
| Autoclave (moist heat) | 121°C/15 psi for 15-20 min; "Flash" at 134°C for 3 min | Gold standard - kills all organisms including spores. Effectiveness requires pure saturated steam and no air pockets. |
| Pasteurization | 74°C for 3-5 sec, or 62°C for 30 min | Kills vegetative pathogens; spores survive |
| Boiling | 100°C | Kills most pathogens and some spores; not true sterilization |
| Incineration | Direct burning | Used for contaminated waste |
4b. Gas Sterilization
- Ethylene oxide (EtO): An alkylating agent - inactivates microorganisms by replacing labile hydrogen atoms in DNA. Used for heat-labile plastics, artificial heart valves, lensed instruments. Flammable and potentially explosive; requires prolonged post-treatment aeration.
- Formaldehyde vapor: Can decontaminate larger areas such as rooms without pressure.
4c. Radiation
- Ultraviolet (UV) light: Causes direct DNA damage. Limited by poor tissue/material penetration. Used for air irradiation in critical hospital sites and handling of hazardous organisms.
- Ionizing radiation (gamma/cathode rays): Damages DNA and generates free radicals/H₂O₂. Used industrially to sterilize pre-packaged surgical supplies (gloves, syringes, containers, food) because radiation penetrates packaging.
4d. Filtration
- Membrane filters (pore size 0.2 μm) remove bacteria from heat-sensitive liquids (e.g., serum). Not effective against viruses (too small).
Sherris Medical Microbiology 8e, pp. 103-107
5. Methods of Disinfection
5a. Physical Disinfection
- Pasteurization: 70°C for 30 min effective for plastic inhalation therapy equipment.
- Microwaves: Achieve near-boiling temperatures with moisture; used as an alternative to incineration for hospital waste. Not true sterilization - heat-resistant spores may survive.
5b. Chemical Disinfectants
All chemical disinfectants are inactivated by organic matter/dirt - surfaces must be cleaned first.
| Agent | Mechanism | Uses | Limitations |
|---|---|---|---|
| Alcohol (70-95%) | Protein denaturation | Skin decontamination before venipuncture | Inactive against spores and many viruses; 100% alcohol paradoxically fails (needs water) |
| Iodine / Iodophors | Iodination/oxidation of cellular components | Skin prep before surgery (povidone-iodine) | Tinctures cause staining; iodophors release iodine slowly to minimize this |
| Chlorine / Hypochlorite | Oxidation (hypochlorous acid) | Drinking water (<1 ppm), swimming pools, surface decontamination of spores/viruses (5% hypochlorite solution) | Rapidly inactivated by organic matter |
| Hydrogen peroxide | Oxidizes membrane lipids and other cell components | Contact lenses, medical device disinfection | Corrosive to some materials |
| Quaternary ammonium compounds ("Quats", e.g., benzalkonium chloride) | Disrupt cell membrane lipids - alter permeability, loss of essential cell components | Surface and skin disinfection | Inactive against spores and most viruses; inactivated by organic matter and cotton |
| Phenolics | Protein denaturation | Inanimate surfaces, laboratory bench disinfection | Too toxic for skin/tissue use |
| Chlorhexidine | Cationic - disrupts membrane; binds to skin for persistent effect | Routine hand and skin disinfectant | Neutralized by soaps and anionic detergents |
Sherris Medical Microbiology 8e, pp. 108-110
5c. Antiseptics vs. Disinfectants
- Antiseptics (e.g., alcohol, iodophors, chlorhexidine) - lower toxicity, safe on body surfaces, but usually less potent in killing organisms.
- Disinfectants (e.g., phenolics, strong hypochlorite) - higher potency but too toxic for body surfaces; used on inanimate objects/environments.
6. Spectrum of Resistance to Disinfectants
Most resistant → least resistant:
Bacterial spores > Mycobacteria (waxy coat) > Non-enveloped viruses > Vegetative bacteria > Enveloped viruses
This is why spores require autoclaving or sporicidal chemicals (e.g., glutaraldehyde, high-concentration chlorine), while routine vegetative bacteria are killed easily by most agents.
7. Hospital-Acquired (Nosocomial) Infections
Sources of HAI
- Endogenous - patient's own flora (e.g., gut bacteria post-surgery)
- Exogenous - from other patients, staff, environment, devices
The 4 Most Common Nosocomial Infections
| Infection | Typical Link |
|---|---|
| Urinary tract infection (UTI) | Indwelling urinary catheter |
| Surgical site infection (SSI) | Operative wound |
| Pneumonia (VAP/HAP) | Mechanical ventilation, aspiration |
| Primary bloodstream infection | Central venous catheter |
Surveillance Criteria (Simplified, WHO)
| Infection | Criterion |
|---|---|
| Surgical site | Purulent discharge/abscess/cellulitis within 1 month |
| Urinary | Positive culture ≥10⁵ bacteria/mL |
| Respiratory | Cough + purulent sputum + new CXR infiltrate (≥2 of 3) |
| Vascular catheter | Inflammation/lymphangitis/pus at insertion site |
| Septicaemia | Fever/rigors + ≥1 positive blood culture |
Park's Preventive & Social Medicine, p. 409
8. Prevention of Hospital-Acquired Infection
Standard (routine) precautions apply to ALL patients:
- Gloves for any contact with secretions/body fluids
- Gown, mask, and eye protection when splashing is anticipated
- Disposal of sharps safely
Specific preventive measures:
- Isolation - separate infectious patients from susceptibles
- Hand hygiene - the single most effective measure. WHO "Clean Care is Safer Care" (2009) guidelines; use soap+water or alcohol-based hand rub
- Dust control - wet dusting and vacuum cleaning rather than dry sweeping
- Disinfection of patient articles - equipment, urine, faeces, sputum properly disinfected
- Droplet control - masks, adequate bed spacing, ventilation, prevent overcrowding
- Barrier nursing - for highly infectious patients
- Infection control committee - hospital-level administrative oversight of policies
Device-specific proven measures (Table 2, Park's):
| Device/Infection | Effective | Not Effective |
|---|---|---|
| Urinary catheter | Limit catheter duration, aseptic insertion, closed drainage, antimicrobial-coated catheter | Systemic antibiotic prophylaxis, bladder irrigation |
| Surgical site | Proper surgical technique, clean operating environment | (Prophylactic antibiotics alone insufficient) |
Park's Preventive & Social Medicine, pp. 409-410
Summary Table: Sterilization vs. Disinfection vs. Antisepsis
| Feature | Sterilization | Disinfection | Antisepsis |
|---|---|---|---|
| Goal | Kill ALL organisms | Kill pathogens | Reduce pathogens on body surfaces |
| Kills spores? | Yes | Usually no | Usually no |
| Used on | Instruments, equipment | Inanimate surfaces/environment | Skin, mucous membranes |
| Examples | Autoclave, EtO gas, ionizing radiation | Hypochlorite, phenolics, glutaraldehyde | Chlorhexidine, iodophors, alcohol |
Primary sources: Sherris & Ryan's Medical Microbiology 8e (Ch. 3: Sterilization, Disinfection, and Infection Control); Park's Textbook of Preventive and Social Medicine (Nosocomial/Hospital-Acquired Infection sections)
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