38.3.3 Criteriainherent to the test methods
Quenchingantimicrobialactivity
The use of a neutralisingagentto quenchthe activityof an antimicrobialor the
eliminationof the antimicrobial from the recoverymedium is essential to
determinethe lethalactivity of the disinfectant/sanitiser.Unfortunately, this
crucial stephas often been overlooked in many investigations. The true' bactericidalactivity of a compoundthenbecomesdifficultto determineand to distinguish from a residual bacteriostatic effect in the recovery medium. Examples of neutralising agents that quench appropriately the activity of specific biocidesare givenin Table38.6. Severaluniversal neutralisers' are
also available and contain usuallya rangeof chemicals. However, it is unclearas
to whether they inactivate the activity of all biocides.Recommendedneutralisers
in standard protocols may contain several compounds (e.g. Table 38.7).
Table38.6 Examplesof neutralisingagents
Antimicrobialagent Possibleneutraliser Comments
Phenols None(dilution) Phenolicshavehigh-concentration
Tweens(polysorbate) exponent.The activityreadily
dropswithdilution.
Alcohols None(dilution)
Cationiccompounds
Chlorhexidine Lecithin+ tween
QACs Lethicin+ Lubrol
Aldehydes Sodiumsulphite/ Sodiumsulphite/bisulphitemight
Glutaraldehyde bisulphite be toxicto somemicroorganisms.
Ortho-phthalaldehyde Glycine Glycineis inefficientin quenching
OPA.Higherconcentrationof
sodiumbisulphiteneededfor
OPA.
Halogen-releasingagents Sodiumthiosulphate Sodiumthiosulphatemightbe
Chlorine toxicto somebacterialspecies.
Sodiumhypochlorite
Iodine
Hydrogenperoxide Catalaseor peroxidase 1 unit catalysesthe decomposition
Dilution of 1 mol of hydrogenperoxideper
minuteat 25 ÎC and at pH 7.
Parabens None(dilution)
Tweens
Metallicsalts/compounds
Mercurycompounds SH-compounds;e.g. Thioglycollatemightbe toxicto
Silvercompounds thioglycollate somebacterialspecies.
Organicarsenical S- and SS-compoundsare
inefficientto quenchsilver
compounds.
654 Handbookof hygiene controlin the foodindustry