harvested, e.g. on filters(Kildes ̆& Nielsen,1997)and in impinger liquids
(Terzievaet al., 1996).However, manualfocusing of the microscopeis needed
due to the impurities, e.g. largerparticles, present in the sample(Kildes ̆&
Nielsen,1997).Phase-contrast microscopyis particularlyuseful for counting
bacterialendosporesas theyappear phase-brightagainstthe darkervegetative
cells(Griffiths& DeCosemo,1994).The addition of UV fluorescencecapability
into aerosolcounters offersa way to distinguish biological particles frommost
organic and inorganic particles (Seaveret al., 1999).Viablestainingmethods
havebeenapplied for the detectionof viable microbes (Terzievaet al., 1996,
Hernandezet al., 1999).The actualviablecountmaybe underestimatedwhen
assessedby this techniquebecausemicrobes injured in the samplingstage may
recoverlateron (Terzievaet al., 1996).Understandingthe fluorescence effect on
the cell viability,the presenceof non-biological particles and the interference
frommixtures has not yet beenachieved(Brosseauet al., 2000).Technical
problems related to fluorescencemicroscopyincludethe low-contrastand low-
lightintensity, renderingdifficulties in automaticimageprocessing (Kildes ̆&
Nielsen,1997).
37.6.3 Flowcytometry
Flowcytometry(FCM)has developedto a rapid screeningand enumeration
methodfor airborne particles demanding, however,samplingand samplepre-
treatment beforeanalysis.FCMis basedon multiple parameters suchas forward
light scatter (FSC), side light scatter (SSC) and fluorescence emission at
wavelengthsof interest. It can determine individuallya largenumber of cellsin
a shorttime(Prigioneet al., 2004).Airbornefungi(Prigioneet al., 2004)and
bacteria(Langeet al., 1997)havebeenquantitatively analysedfromnatural
environments. However, the background contaminants cause problems in
detection and the sensitivityof the methodand it still needsto be improved
beforebeingapplicablein the foodindustry.
37.6.4 ATPbioluminescence
Adenosine triphosphate(ATP) is present bothin microbial cellsand food
ingredientsand can be measuredusingthe luciferaseenzymecomplexfoundin
fireflies.The lightoutputof a sampleis directlyproportionalto the amountof
ATPpresent.The detectionlimitof the methodis about 104 cells(Wirtanen,
1995).Thismethodis non-specific,i.e. it measuresthe ATPcontentof the
microbialpopulationin the sampleas a whole(Griffiths& DeCosemo,1994).The
ATPcontentin airbornecells,whichare stressed throughassessment,can be
alteredby the effect of aerosolizationand this can also affectthe detectionlevelof
the method. The totalconcentrationof adenylatewithinthe cellsmay give a better
estimateof cell concentration (Griffiths& DeCosemo,1994).Applicationof a
fullyautomatedATPmonitoringsystem,AutoTrac,for continuouscheckingfor
microbialcontaminationin air has beendeveloped(Brady, 1999).
Improving air sampling 633