Queensland Government
Queensland Health
Health Professionals > Endoscope Reprocessing

2.2 Factors Impacting on Sterilisation & Disinfection

The activity of biocides against microorganisms depends on a number of factors. Some of these are intrinsic qualities of the microorganism whereas others depend on the chemical and external physical environment. An awareness of these factors should lead to an improved ability to carry out sterilisation and disinfection processes.


It has been shown that the more organisms present the longer it takes to kill them all. There is an exponential relationship between the number of organisms killed and the time taken to kill them.

This is one reason for the scrupulous manual cleaning of instruments prior to sterilisation or disinfection. By reducing the number of microorganisms present the exposure time required to kill the entire microbial load is reduced. A log five reduction or more in the number of organisms present can be achieved by scrupulous cleaning.

For more information on cleaning see the sections about pre-cleaning and cleaning in the reprocessing module.
If the microorganisms are in a place that the biocide can't reach then the efficacy of the biocide will obviously be compromised. It is critical that all surfaces are in contact with the biocide. A biocide can't kill microorganisms on a surface if that surface is not in contact with the biocide. This means that instruments with multiple pieces must be disassembled to ensure the penetration of the biocide to all of the parts.

Equipment with crevices, joints and channels such as endoscopes require particular attention to be paid to ensuring all air is expelled from internal spaces and the equipment completely immersed for the entire exposure period.


Microorganisms vary greatly in their resistance to chemical biocides. All sterilisation and disinfection strategies in terms of concentration and exposure times are designed to destroy the most resistant of these microorganisms.

Below is a table that indicates the varying resistance to chemical biocides of different types of organisms. It also shows the relative activity of different levels of disinfection.

Reistance to biocides

Pseudomonas aeruginosa has the most intrinsic resistance to biocides of all the vegetative bacteria.

Go to Toptop of page


If everything else is constant then the more concentrated the biocide the greater its efficacy and the shorter the time necessary to kill all the microorganisms. Not all biocides are similarly affected by concentration adjustments. In other words, halving the concentration may double the exposure time for some biocides but quadruple it for others.

It is particularly important to ensure that disinfectants do not become diluted with excess water remaining on endoscopes after rinsing, as the activity of the disinfection process will be significantly compromised. Thus it is important that water is purged from all channels after cleaning and before disinfecting and to regularly monitor the concentration of biocide when it is being reused.

 See the biocide monitoring section of the Quality Assurance module for more information about this.
 The potency of a biocide affects the exposure time required to achieve the same level of microbial kill.


Several physical and chemical factors influence the efficacy of biocides. These include temperature, pH, relative humidity and water hardness.

Generally the activity of most biocides increases as the temperature increases. There is a point at which the chemical degrades if heated too much.

An increase in pH improves the antimicrobial activity of some agents as with glutaraldehyde, but decreases the activity of others such as hypochlorites. This effect is caused either by alteration of the germicidal molecule or the cell surface of the microorganism.

Relative humidity influences the activity of gaseous agents such as ethylene oxide.

Water hardness reduces the rate of kill of some biocides because divalent cations such as magnesium and calcium interact with soap to form insoluble precipitates.

Go to Toptop of page


Organic matter such as serum, blood, pus or faecal material may interfere with the activity of biocides in at least two ways.

First, a chemical reaction between the biocide and the organic matter may result in a complex that is less germicidal or non-germicidal leaving less of the active agent to attack the microorganisms.

Secondly, organic material may protect microorganisms from attack by acting as a physical barrier. This is another reason for the meticulous cleaning of objects before any sterilisation or disinfection procedure.

For more information on cleaning see the sections about pre-cleaning and cleaning in the reprocessing module.



To adequately sterilise or disinfect an item it must be exposed to the appropriate concentration of biocide for a certain minimum contact time. All surfaces of the item must come in contact with the biocide for that period of time.

This means that for endoscopic equipment the biocide must be introduced into all lumens and channels. Presence of air pockets and incomplete immersion in the biocide means items will not be reliably or completely processed.

The minimum contact time for sterilisation or disinfection will be affected by the interaction of all the factors discussed here. The times that are required to be used are discussed in the section on disinfecting and sterilising agents.



The production of thick masses of cells and extracellular materials or biofilms can protect microorganisms from the cidal action of biocides. Biofilms are microbial masses attached to surfaces that are bathed with liquids. A biocide must saturate or penetrate the biofilm matrix before it can kill the microorganisms within it.


Interactive divider

Biofilms can form on surfaces of endoscopy equipment and in the tubing of automated washer/disinfectors as well as on water filters, housings and pipes thus protecting the embedded organisms from exposure to biocides and serving as a reservoir for continuous contamination.

Scrupulous cleaning can help to remove biofilms on endoscopes and thus eliminate this problem.

For more information on cleaning see the sections about pre-cleaning and cleaning in the reprocessing module.Click on 'Next >' at the bottom of this page for a general discussion of cleaning and cleaning agents in the next section.


For discussion of the problems associated with biofilms, see the automatic flexible endoscope reprocessors section.

Last Updated: 29 August 2011
Last Reviewed: 29 August 2011