antimicrobialproductshouldbe evaluatedwithstandardprotocolsthat investigate
a rangeof conditions.Manyantimicrobialtests,notablypracticaltests,include
variousparametersin theirdesign,suchas concentration,contacttime,tempera-
ture,soiling,typeand numberof microorganisms.Generally,thesefactorscan be
dividedinto thoseinherentto the biocideand thoseinherentto the microorganisms.Factorsinherent to the product
Concentration is probably the most important factor to consider when
antimicrobial efficacyis concerned(Russelland McDonnell,2000).Therehave
beenseveralpublished reportsof microbial contamination following chemical
disinfection,or microbial survivalwithinbiocidal products/formulations(Poole
2002;Russell, 2002).For example, many reportsconcern the failure of QAC
disinfectants, althoughin manycases inappropriate concentrationswereused
(Princeand Ayliffe, 1972; Ehrenkranzet al., 1980). Holahand colleagues
(2002)pointedout that whenthe concentrationof QACsremainshigh(i.e.
1000 mg Lˇ^1 ), survival of vegetative microorganismsis unlikely. Likewise,
failure of high-level disinfectants such as glutaraldehyde to eliminate all
microorganismsfromendoscopewasher disinfectorshavebeenreported(van
Klingerenand Pullen,1993;Griffithset al., 1997).
Theeffectof changes in concentrationon antimicrobial efficacycan be
estimatedby the concentrationexponent () and is givenby the equation:àlogt 2 ˇlogt 1
logC 1 ˇlogC 2
whereC 1 andC 2 represent two concentrationsandt 1 andt 2 the respectivetimes
to reduce the populationto the samelevel.The concentrationexponent varies
among biocides (Table 38.4). It gives an indication of the effectof dilutingan
in-use concentration;i.e. biocides withhighconcentrationexponent will rapidly
loseactivity upondilution,whereasthosewith a low concentrationexponent
will retain activity upon dilution. This in effect allows the selection of
appropriateconcentrationsto be evaluated withantimicrobial test protocols.
Contact timeis an important factorof all antimicrobial testingprotocolsand
the choice of timeof exposure usually reflectsconditionsin practice.Thereis no
simple relationshipbetween activity and contacttime,althoughlonger exposure
timeis usually associated with betteractivity and mightbe essentialto eliminate
the resistant'clones of a microbialpopulation.Standard antimicrobialtest protocols,for manufacturers' and hygienicguidelinesusuallyspecify a set con- tact timeor the minimumcontacttimerequired.For example, the European Standard for the testingof surfacedisinfectants(CEN1276,1997a)stipulates that5 log 10 reductionin bacterial concentration mustbe attained within 5 minutes of exposure time. Likewise, the hygienic hand-wash procedure (CEN1499, 1997 b) recommendsa minimumof 1 minute contacttime, which reflectsacceptable hand-washingtimein practice. Organicloador soiling (e.g.serum,blood,pus, earth,foodresidues, faecal materials)contributesto decreasingbiocidal activity by eithermoppingup' the
646 Handbookof hygiene controlin the foodindustry