37.6 Bioaerosolassaymethods
37.6.1 Culturingtechniques
Culturingof microbesdirectlyor through a brothon solidagarmediawith
incubationfor a certainperiodand temperatureis the traditional methodfor
enumeratingthe microbial countin an air sample. The culturetechniqueis easyto
use and requires no specializedequipmentfor sampling. The totalpopulationof
microbes in a samplecan,however,be severely underestimated if assessmentis
carried out withthis type of method (Griffiths& DeCosemo, 1994). Microbesin the
air may lose theirabilityto form colonies and still be viable,i.e. be non-culturable.
In cases where the airbornemicrobesare non-culturable,dataon cfusdoes not
describe the actual microbialpopulation(Heidelberget al., 1997).Epifluorescent
microscopicmethods for countingthe totalamountof microbes have shownthat
thereis widevariation in the culturabilityof microbialcells;the culturableamount
of totalairbornebacteria variedfrom0.02%to 10.6%(Lighthart,1997).
In the developmentof methodsthat utilizeculture techniquesas part of the
assessmentcriteria,growthconditions, diluentand culture plate media must be
standardized to be ableto estimate microbial counts in the air (Griffiths&
DeCosemo, 1994). The culture techniqueis useful for directidentificationof
certain pathogensor spoilage microbes in the food-processingair, e.g. spoilage
moulds on specificagars (Lund,1996).
Collectionof microbesin the fluidis usedin the impingers. The selection of a
liquidcollectionmediumis dependent upon the particular organismsto be
isolated. In quantitativestudiesa medium that will minimize bothmultiplication
and deathof the organism mustbe employed(Kang & Frank,1989a). The
usefulnessof organic compounds, e.g. di-, tri- and polysaccharides, sugar
alcohols,proteins,polypeptides,organic acid salts,antibiotics, chelating agents
and culturesupernatantfluidin enhancing cell survival has beeninvestigated.
The majority of the most effectiveorganicadditivesare sugars or polyhydric
alcohols(Griffiths & DeCosemo, 1994).Marthi and Lighthart (1990)showed
thatadditionof an organiccompound,e.g. betaine, to the impingerbuffer
significantlyincreased the colony-forming abilitiesof the airborne microbes.
The compoundactedin somecases as a protective agent,maintaining the
culturabilityof somebacteria.Heidelberget al.(1997),however, showedthat
the effectcould be the oppositein othercases, owingto strainvariability against
betaine. Thus,protectiveagents in routinemonitoring mayintroduce a bias,
protectingthe culturabilityof onlysomepartsof the microbialpopulation.
37.6.2 Microscopy and fluorescencetechniques
Microscopyis a methodwithwhichthe totalcountof microbes as wellas
morphological data on the microbescan be obtained.The microscopicalmethod
is relativelysimple and rapid.Automatic countingand size evaluationin the
assessmentof airborne microbes by meansof imageprocessing of fluorescence
microscopy data reduce analysis time (Griffiths & DeCosemo, 1994).
Fluorescencemicroscopycan be appliedin the evaluationof airborne microbes
632 Handbookof hygiene controlin the foodindustry