Sterilization and Disinfection

Sterility

Total absence of viable microorganisms as assessed by no growth on any culture medium

Sterilization

Complete elimination of all microbial life including spores, carried out by physical or chemical methods

Disinfection

Reduction of pathogenic organisms to a level at which they no longer constitute a risk; renders an object non-infectious

Decontamination

Removal of pathogenic microorganisms from objects so they are safe to handle, use or discard

Septic

Infection

Aseptic

Without infection

Antiseptic

Prevention of infection by inhibiting or destroying microbes

Who proposed the rational approach to S&D of patient-care items?

Earle Spaulding

Critical items (Spaulding classification)

Confer high risk of infection - should be sterile; includes surgical instruments, cardiac and urinary catheters, medical implants

Semi-critical items (Spaulding classification)

In contact with mucous membrane & non-intact skin; includes respiratory therapy & anaesthesia equipment, endoscopes; needs high level disinfection (e.g. Glutaraldehyde, H2O2, Chlorine)

Non-critical items (Spaulding classification)

Contact with intact skin but not mucous membrane, e.g. bed pans, furniture; disinfected with Phenolics, alcohols

Factors affecting kinetics of killing microorganisms

Medium/process employed, microbial load, antimicrobial agent, temperature, pH, presence of endospores

Two main categories of sterilization methods

Physical and Chemical

Physical methods of sterilization

Heat, Filtration, Radiation

Two types of heat sterilization

Moist heat and Dry heat

Temperature range for dry heat sterilization

160-180°C, exposure time up to 2 hours depending on temperature

Incineration

Method to dispose of things completely, e.g. body parts

Flaming (dry heat)

Used for needles, scalpels, forceps, etc.

Red heat

Sterilization using a Bunsen burner flame

Hot-air oven holding conditions

160°C for 1 hour holding time

Hot-air oven uses

Glassware, thermo-stable powder

Tests of efficiency for hot-air oven

Browne's tube (colour change), living spore suspension

Why is moist heat more effective than dry heat?

Steam under pressure generates high temperature for sterilization

Temperature range for moist heat sterilization

121-134°C

At what temperature do vegetative organisms die in a few minutes?

70°C

Autoclave

Device using pressurized steam in a chamber to destroy microbial forms, including both spores and vegetative forms

Standard autoclave sterilization conditions

1 kg/cm² (15 lbs/sq inch) pressure at 121°C for 15 minutes

Simple autoclave (wet saturated steam) limitation

Inefficient air removal, wet load, low temperature

Checks of autoclave function

Thermocouple, spore strip, Browne's tube

Three stages of steam jacketed autoclave operation

Load/pack items, air removal, steam administration and sterilization (steam enters, holding time, dry load, remove)

Flash autoclave conditions

Very high temperature (134°C for 3 minutes), used for unwrapped items so instruments are readily available for use

Bowie-Dick test

Tests for adequate air removal in autoclaves; uniform intensity of strips shows adequate removal

Uses of autoclaves in hospitals

Surgical instruments, dressings, culture media (all with autoclave tape attached to show efficiency)

Mechanism of sterilization by filtration

Removes microorganisms rather than destroying them, via sieving, adsorbing and trapping within a matrix

Filter types used for sterilization

Asbestos, sintered glass, cellulose membranes

Millipore membrane pore size

1μm or less; very fine, can hold back viruses and large proteins

Applications of cellulose filters in the laboratory

Removal of bacteria from viral culture media, sterilization of heat-sensitive substances (growth medium, serum, drugs), separating bacteria from toxins

HEPA filter

High Efficiency Particulate Air filter; removes up to 99.97% of particles greater than 0.3 micrometer in diameter

Applications of HEPA filters

Safety cabinets, laminar flow rooms (operation, isolation, clean rooms in industry e.g. pharmaceuticals)

Major target of radiation sterilization

Microbial DNA

Non-ionizing radiation sterilization

UV light; causes genetic damage, no penetration; applicable to surfaces e.g. safety cabinets

Ionizing radiation sterilization

Radioactive isotopes, highly penetrating, e.g. gamma rays

Industrial uses of radiation sterilization

Instruments (syringes, needles, gloves, IV lines, catheters), sterile containers, pharmaceutical products (vaccines), food

Chemical sterilization agents

Chemically reactive gases with biocidal activity, e.g. formaldehyde and ethylene oxide (alkylating agents)

Ethylene oxide properties

Colorless, odorless, flammable gas

Ethylene oxide exposure/aeration time

4-6 hour exposure time required, followed by aeration to remove absorbed gas

Disadvantages of ethylene oxide

High cost, hazardous, needs special equipment

Formaldehyde as a disinfectant/sterilant

Irritant, water-soluble gas; used for rooms, furniture, fabrics, blankets, wood, leather; formaldehyde/steam at 80°C for heat-sensitive equipment

Glutaraldehyde (Cidex)

2% alkaline solution, cidal for bacteria, fungi, viruses; used for heat-sensitive equipment like endoscopes (contain lenses)

Low temperature steam disinfection range

71-75°C

Pasteurization temperature range

55-75°C; removes vegetative organisms but not spores; used for milk, serum, vaccines

Boiling disinfection

Heat at 100°C; requires cleaning well before boiling; used for forceps, scissors, tubings

Tyndallization

Intermittent exposure at 20-45 minutes on 3 consecutive days; used for sugar-containing media

Conditions affecting chemical disinfection

Satisfactory contact (grease/protein interference), concentration, neutralization (by hard water/soap), stability, speed of action, range of action, cost

Indications for chemical disinfection

Skin and mucous membrane, instruments damaged by heat, decontamination of surfaces, making items safe to handle

Examples of hypochlorites

Household bleach, Parazone

Hypochlorite dilution for spilt blood

1:10

Hypochlorite dilution for decontamination

1:100

Hypochlorite dilution for wiping floors

1:1000

Main types of disinfectants

Phenolics, halogens, alcohols and aldehydes, detergents, metallic salts

Phenolics (e.g. lysol, cresol)

Strong smell; used for decontamination of environment, toilets; NOT for food preparation areas

Halogens as disinfectants

Inexpensive and fast acting, active against all organisms; high concentration is very toxic/corrosive; can lose potency after dilution (e.g. chlorine, iodine)

Tincture of iodine composition

2% iodine in 70% alcohol

Povidone iodine use

Surgical scrub

Alcohols as disinfectants

Evaporate quickly, leave a dry surface; e.g. ethyl alcohol, isopropyl alcohol; 70% in water is a skin disinfectant; used on surfaces, hands, thermometers

Formaldehyde disinfectant properties

Cidal for all organisms; potential carcinogen; used as an embalming agent

Glutaraldehyde disinfectant properties

Causes eye/skin irritation; high level disinfectant used for endoscopes, anaesthetic equipment

Hydrogen peroxide 3% use

Disinfecting soft contact lenses

Irgasan

Antiseptic used in soaps

Examples of detergent disinfectants

Cetavlon, cetrimide (note: some Gram-negative organisms like Pseudomonas can grow in it)

Examples of dye disinfectants

Crystal violet, brilliant green, acridine orange

Examples of metallic compound disinfectants

Mercuric chloride, silver nitrate