Disinfection Concept and Definitions

Disinfection Concept and Definitions

Learning objectives

• Difference between sterilization and disinfection

• Factors affecting sterilization

• Factors affecting choice of a disinfectant

Intended learning objectives

At the end of this lecture, the student will be able to:

• Explain disinfection and the related terms

• Explain the different factors affecting disinfection

• List the factors affecting choice of an antimicrobial agent

Introduction

• Disinfectants, antiseptics and preservatives are chemicals that have the ability to destroy or inhibit the growth of microorganisms and that are used for this purpose

Disinfection, antisepsis and preservation

• Disinfection is the process of removing microorganisms, including potentially pathogenic ones, from the surfaces of inanimate objects

• Antisepsis is defined as destruction or inhibition of microorganisms on living tissues having the effect of limiting or preventing the harmful results of infection

• Preservatives are included in pharmaceutical preparations to prevent microbial spoilage of the product and to minimize the risk of the consumer acquiring an infection when the preparation is administered

Terminology

Kill	Inhibit growth
“-cidal”	“-static”
Bactericidal	Bacteriostatic
Kills bacteria	Inhibits the growth of bacteria

• Terms such as biocidal, bactericidal, virucidaland fungicidal describe a killing activity,

• Bacteriostatic and fungistatic refer to inhibition of growth of the organism

• Minimum inhibitory concentration (MIC): refers to the minimum concentration of an antimicrobial agent that inhibits growth of the microorganism under test

• Minimum bactericidal concentration (MBC): refers to the minimum concentration of an antimicrobial agent that kills the microorganism (bacteria) under test

Factors affecting the antimicrobial activity of disinfectants

Innate (natural) resistance of microorganisms

• Microorganisms vary greatly in their resistance to chemicals - germicides and sterilization processes.

• To destroy the most resistant types of microorganisms, the user needs to employ exposure times and a concentration of germicide needed to achieve complete destruction.

• Prions, bacterial spores possess the highest innate resistance to chemical germicides.

Microbial density and location:

• The larger the number of microorganisms present, the longer it takes a disinfectant to complete killing of all cells

• Researchers also have shown that aggregated or clumped cells are more difficult to inactivate than mono-dispersed cells.

E.g Using identical test conditions, it has been shown that 10 spores of the anthrax bacillus (Bacillus anthracis) were destroyed in 30 minutes, while it took 3 hours to kill 100 000 (10⁵) spores.

Microbial density and location:

• The location of microorganisms also must be considered when factors affecting the efficacy of germicides are assessed.

• Medical instruments with multiple pieces must be disassembled and equipment such as endoscopes that have crevices, joints, and channels are more difficult to disinfect than are flat- surface equipment because penetration of the disinfectant of all parts of the equipment is more difficult.

Disinfectant concentration and exposure time

• The more concentrated the disinfectant, the greater its efficacy and the shorter the time necessary to achieve microbial kill.

• In general, longer contact times are more effective than shorter contact times.

Physical and chemical factors

1. Temperature

The cidal activity of most disinfectants increases as the temperature rises

2. pH

Changes in pH may affect the potency of the agent and its ability to combine with cell surface sites

3. Divalent cations

Divalent cations (e.g. Mg2+, Ca2+) present in hard water may also interact with the microbial cell surface and block disinfectant adsorption sites necessary for activity

Presence of extraneous organic and inorganic material

• Cidal activity of many antimicrobial agents is seriously impaired under ‘dirty’ conditions

• Example: halogen disinfectant reacts with the organic matter to form inactive complexes

• Organic material may adhere to the microbial cell surface and block adsorption sites necessary for disinfectant activity

Presence of extraneous organic material

• Organic matter in the form of serum, blood, pus, or fecal or lubricant material can interfere with the antimicrobial activity of disinfectants in at least two ways.

• Mainly, interference occurs by a chemical reaction between the germicide and the organic matter resulting in a complex that is less germicidal or non-germicidal, leaving less of the active germicide available for attacking

• Alternately, organic material can protect microorganisms from attack by acting as a physical barrier.

• Inorganic contaminants of microorganisms to all sterilization processes results from occlusion in salt crystals.

• This further emphasizes the importance of meticulous cleaning of medical devices before any sterilization or disinfection procedure because both organic and inorganic soils are easily removed by washing.

Biofilms

• Microorganisms may be protected from disinfectants by production of thick masses of cells and extracellular materials, or biofilms.

• Biofilms are microbial communities that are tightly attached to surfaces and cannot be easily removed.

• These can be resistant to disinfectants by multiple mechanisms, including physical characteristics of older biofilms, genotypic variation of the bacteria, microbial production of neutralizing enzymes, and physiologic gradients within the biofilm (e.g., pH).

Factors affecting choice of antimicrobial agent

Choice of the most appropriate antimicrobial compound for a particular purpose depends on:

• Properties of the chemical agent

• Microbiological challenge

• Intended application

• Environmental factors

• Toxicity of the agent.

Summary

• Disinfection is the process of removing microorganisms, including potentially pathogenic ones, from the surfaces of inanimate objects

• Terms such as biocidal, bactericidal, virucidal and fungicidal describe a killing activity,

• Bacteriostatic and fungistatic refer to inhibition of growth of the organism

• MIC and MBC are the minimum concentrations of an antimicrobial that inhibit or kill the microbes respectively