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

•      
Terms such as biocidal, bactericidal, virucidal
and 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

Factors
affecting disinfection are:

1. Innate (natural) resistance of
   microorganisms
2. Microbial density
3. Disinfectant concentration and
   exposure time
4. Physical and chemical factors
1. Temperature
2. pH
3. Divalent cations
5. Presence of extraneous organic
   material

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