37.6.5 Molecular methods
Molecularbiologydetectionmethodsincludepolymerasechain reaction(PCR)
and geneprobes (Griffiths& DeCosemo, 1994) as wellas immunological
methods. The PCRanalysismethodpermits the detection of DNAregardless of
the metabolic state of the cells. Themethod may therefore be orders of
magnitudemore sensitive thanculturetechniques. Applicationsof this method
for the quantitationof airborne organismsare still under development. PCR-
basedtechniquesallowthe detectionand identification of microbesat a groupor
species level within a backgroundof othermicrobes. The specificity,sensitivity
and reducedprocessingtimeof this technique are suitablefor the detectionof
small amountsof targetmicrobial cells in a sample(Alvarezet al., 1995).
Specialized equipment and skilled personnel are, however, required for
successfulapplications(Griffiths & DeCosemo,1994).The air samples may
also contain compounds inhibitory to the amplification assay, e.g. high
concentrationof non-target DNA(Alvarezet al., 1995).In many applications,
pre-enrichmentof the sample is needed.PCRis at its best a fast and powerful
methodfor the identificationof biological air contaminants. It is especially
suitable for situations wherea particular contaminant or at the mosta few
contaminant strains in air are searched.The methodhas to be developedand
optimised separately for eachmicrobe and sometimes also for differentsampling
environments, owing to different inhibiting compounds,in order to have
sufficientsensitivityand specificity.
Nucleic acid hybridization has been applied for the detection and
identification of microbes in bioaerosols.Eachhybridizationformatis suitable
for different aerosolconcentrations.It is possible to identifya fast-growing
airborne organismwithin24 h usingthe colony-hybridizationtechnique.Colony
hybridizationcan be usedto detectamountsas low as even1 cfu, whereaswhole-
cell hybridizationrequires a substantially higher aerosol concentration,e.g.
5 104 cfu, fromfiltered air samples(Neefet al., 1995).A two-step detection
strategy, first throughselectivePCRto amplifya groupof the templateDNAs in
a sampleis followedby more specificexaminationvia probehybridizationto a
specific targetin the PCRproducts. Thiscan provide betterspecificity and
sensitivityfor environmental samplescreening (Zenget al., 2003).
Immunological methodsuse specificantibodies whichdetectuniqueepitopes
(antibody-binding sites) expressed by the target organism. Thesample is
exposed to antibodies that bindto the antigenin the sampleand the amountof
antibodybindingdetected.Immunological applicationsare rare at the momentin
bioaerosolmonitoringbut theyhavepotentialin certaincasesprovided that
thereis a functional antibody available for the contamination in question
(McCartneyet al., 1997).
37.6.6 Conclusions regardingassaymethods
Traditionaland relatively simplebioaerosolassay methodssuchas culturing and
microscopyare mostsuitablefor following general hygienic situationsin food634 Handbookof hygiene controlin the foodindustry