4.4.4 Acquired resistance
Acquired,non-plasmid-encodedresistance occurs whenbacteria are exposedto
gradually increasingconcentrationsof a certainbiocide. Acquired,plasmid-
encoded resistance is, in most cases, someformof resistance againstmetal-
basedbiocides (silver, copperor mercury) (Chapman, 1998;Russell,1999b;
McDonnelland Russell, 1999). However,a recentstudyinvestigatedthe resist-
anceofSalmonellaagainsthypochlorous acid (concentrationsup to 28 mg lˇ^1 )
and indicatedan emergingproblemfor the foodindustry (Mokgatlaet al., 2002).
Normally,a chlorine concentrationof 10 mg lˇ^1 is sufficient to inactivate
vegetative, non-spore-formingmicroorganisms (Krop, 1990). This type of
resistancemightbe causedby remaining organicsubstances(partly)inactivating
the chlorinesolution. It doesshowthat,when applyingcertaindisinfectants,it is
importantto apply to correct concentrationsof disinfectantin combinationwith
a cleansurfacein orderto achieveefficientinactivationof microorganisms.
Therefore,this kind of resistance appears to be unstableand could alsobe
consideredas pseudo-resistance(Heinzel, 1998).Pseudo-resistance occurs when
bacteria appear to be resistantto a certain kindof biocide, but when placed in a
biocide-freeenvironmentthe resistancedisappears.A few reasons are knownto
causethis apparentresistance:
∑ use of an inefficientproduct(i.e. disinfectantwithlimitedspectrumof activity);
∑ incorrect use of the disinfectant (not according to the conditions
recommended by the supplier);
∑ insufficientcontact(time) withthe surfaceto be treated;
∑ insufficientavailabilityof the reactiveagent.
It is obvious that thesereasonsmayleadto survivalof bacteria.Althoughit is
not consideredto be microbial resistance, it is probably the most widespread
formof perceived resistance (Heinzel,1998).In addition,it is eventhought
possible that somemicroorganismsare able to use the intended disinfectantas a
source of energy:instead of beinginactivated, theystartto grow.
4.5 Predictingmicrobial resistance
Predictingpathogenresistance againstcurrentdisinfectants wouldbe veryuseful
for applicationin foodfactoriesand hospitals.Compared withantibiotics,the
modeof actionof preservatives/disinfectants is less well understood.Antibiotics
normallyhaveone specific groupor subgroupof bacteriaas target micro-
organisms whereasdisinfectants attackbacteria in general(Russell, 1991).
Therefore,it is ratherdifficult to determine the exacteffecton microorganisms
beforehand.However, the mechanisms of action of disinfectants become more
and moreclearallowingthe effect they haveon microorganismsto be predicted.
Whether microorganismswill survivedisinfectionin practice depends on more
thanone factor. At least15 factors appear to influencethe possible resistance of
a microbial strain(Baqueroet al., 1998).A modelto predictthe effectof a
84 Handbookof hygiene controlin the foodindustry