singledisinfectant in an unspecifiedenvironmentmighttherefore be difficult to
realise.Thus,testing underpracticalconditionsremains necessary to determine
the effect of a certain disinfectant. Although not every detailis known,it is
possible to determinewhether a disinfectantwillbe effectivebasedon the
following information:
∑ typeof bacteria± metabolicstate;
∑ revivalof injured cells/biodiversityof microorganisms;
∑ influence of remainingorganicmatter/biofilms;
∑ processing conditions:temperature, pH.
4.5.1 Typeof microorganisms± metabolic state
The metabolic stateof microorganismsis importantin determining the possible
effectof the disinfectant. Withregard to vegetativecells,Gram-negative micro-
organismsappearmore resistantthanGram-positivemicroorganismsowingto
the compositionof the cell wall.The cell wallof Gram-positives contains fewer
lipidscompared withGram-negatives(Russell,1999a). Bacterialspores are
highlyresistantto chemicaland physicalagents, whichis mainly due to the
sporecoat and sporecortex(Bloomfield and Arthur,1994; Setlowet al., 2002).
For chemical agents,sporicidalconcentrationsare in mostcases 10 times (or
more)higherthanbactericidalconcentrations(Russell, 1990).In the caseof
phenols,organicacids, QACs,biguanides, organomercurials (e.g.methyl-Hg,
ethyl-Hg) and alcohols usedat highconcentrationsthe agentshaveno sporicidal
effect(Russell, 1990).
4.5.2 Revival of injuredcells
Anotheraspect is the differencein cell damageafter treatmentwithdisinfectants
or physicalagents. This impliesthata certainamount of cellscan revive.
However, this revivaldoesnot strictly indicatea resistance mechanismbut is
due to the statistical variance of the protection systemsof the microorganism.It
is importantto realisethat sensitivityalso varieswithinthe definedspecies and
resistance is definedas the tolerance of a disinfectantthat exceedsthe natural
variance (Heinzel, 1998;Russell, 1991).
4.5.3 Processingconditions:pH, temperature,concentration
pH is an important factoras it can modify the practicalapplicationof the
disinfectantused(Russell, 1991).For example for chlorine the pH needsto be in
the rangebetween 5 and 8 in orderto be effectiveas hypochlorousacid.Below
pH 5 chlorine gas is producedand abovepH 8 ClO-ions are produced whichare,
apartfromthe acutetoxicityof chlorine gas, not activeas a disinfectant (Krop,
1990).Similareffects are known for otherdisinfectants.Therefore, knowingthe
pH of the environment makes it possible to predict whether a disinfectantwill be
Pathogen resistanceto sanitisers 85