active. In addition to pH (as discussedearlier), processingparameterssuchas
temperature, concentration and application time are important factors
concerning (pseudo)resistance of microorganisms. For example, when the
temperature of a solution of formaldehyde is increased by 10 ÎC the
effectivenessis increasedbetween threeand five times(Krop,1990;Russell,
1999a). Concerning concentration, when the concentration of the applied
disinfectantis too low,the disinfectant onlyworksbacteriostatically instead of
bactericidally. This impliesthatas soonas the disinfectantis usedup the
bacteriastart growingagain. For example chlorhexidine is bacteriostaticat
0.0001 mg lˇ^1 and bactericidal at 0.002 mg lˇ^1 (Russell, 1990). When
concentrationsare too highthe disinfectantwill act fasterbut the question is
whether that is strictly necessary. Whenthis is not the caseit will cost money
and maybe dangerous for the environment(personnel, equipment).
4.5.4 Residualorganic matter/biofilms
The guideline for cleaning and disinfectionis that disinfectioncan be effective
onlywhen the equipment or surface is properly cleanedpriorto the disinfection
(Krop, 1990).Thiscan be explainedby the fact that remainingorganicmatter
will inactivate the disinfectant and microorganisms will not be affected
(Kraemer,1998).A second reasonis that organic compoundsact as a protective
layer for the microorganisms.Thisis also the casewhenmicroorganismshave
formeda biofilm where,as a resultof nutrient limitation,a reducedgrowthrate
makes the specificmicroorganismsless susceptible to disinfectants(Brownand
Gilbert, 1993;Luppens, 2002).The fact that microorganismscan formbiofilms,
implying a changein theirgrowthcharacteristics,can also result in resistance
against disinfectants for the followingreasons(Brownand Gilbert, 1993):
∑ exclusion/influencingof the disinfectant by the glycocalyx(a slimylayer
surroundingthe cell);
∑ chemical reactionof the glycocalyxwithdisinfecting agents;
∑ limited availabilityof key nutrientsresults in decreasedgrowthrate;
∑ the attachmentto surfaces,causing depressing of genesassociated with
sessile (directlyto the substrate) existence,which coincidentally affects
antimicrobialsusceptibility.
These effectscan be regardedas pseudo-resistance as the effect will end as soon
as the biofilmno longerexists.
4.6 Future trends
As microorganismsevolve and adaptto disinfecting strategies,the development
of more effectivecleaning and disinfectingstrategies and new toolsto monitor
the efficiency of thesestrategies willcontinue. Thefollowing trendscan be
distinguished.
86 Handbookof hygiene controlin the foodindustry