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Unformatted text preview: APPUED MICROBILOGY, JUlY1973,p. 9-13 Copyright 0 1973 American SocietyforMicrobiology Vol.26,No. 1 PrintedinU.S.A. DifferentialCounting in Mixed Cultureswith CoulterCounters J. F. DRAKE AND H. M. TSUCHIYA Department ofChemicalEngineeringandMaterialsScienceandDepartment ofMicrobiology, Universityof Minnesota, Minneapolis, Minnesota 55455 Received forpublication 15February 1973 A critical comparison of Coulter, viable, and microscope counts forseveral mixed culturesofmicroorganisms hasbeenmade. Thisinvestigationshowsthat Coultercounting can provide reliableestimatesofmicrobialnumbers inmixed cultures.Precautions and limitationsofCoultercountinginmixed cultures are discussed. Studieson mixedculturesofmicroorganisms require differentialcounting ofthe component organisms. A varietyoftechniquescanbeused dependingupon the particularsystemstudied. Our experience with mixed cultures of more thanonepairofmicrobialformshasshownthat the Coulter counter can be used to count microorganisms in mixtures; however, certain precautionsmustbefollowedanditslimitations must beborneinmind. This paper reports some of the details of preparingsamplesandcountingmixedcultures withtheCoultercounter,orinstrumentssimilar to it.An appreciation ofthe potential ofthe conventionalCoultercountersformixedculture studiesshould prove usefultoworkers who do nothave accesstomoreelaborateinstrumenta- tion. Numerous descriptions of the theory and operation of the Coulter counter have been givenandthecapabilitiesandlimitationsofits use in particlecounting are welldocumented. An excellent discussion pertaining to microor- ganismsisgivenbyKubitschek(3).Countingis based on the simultaneous passage of a con- ducting fluid and current through a small aperture. The aperture serves as a resistance elementinameasuringcircuit;thepassageofa suspended particle momentarily blocks a por- tionoftheaperture andchangesitsresistance. The magnitude of the resistance pulse is a measureofthevolumeoftheparticle.Thepulse height is proportional, or nearly so, to the volume oftheparticle(3)undersuitablecondi- tions. As particles of different volumes pass through the aperture, a series of pulses are generated.The pulseheightscorrespondtothe volume distributionofthesuspended particles. The Coulter counter has threshold dials that may besetforcounting onlythosepulsesofa certain magnitude. By varying the threshold settings, particleswithin limits ofa givenvol- ume rangecan beselectivelycounted. Forcountingpurposes, a mixed culturemay be viewed as a series of overlapping volume distributions.Ifthemixedcultureiscomposed of organisms which differ in size to such an extent that theirvolume distributions do not overlapappreciably, theycan thenbecounted with aconventionalCoultercounter.The aper- tures and threshold settings can be chosen so thatthepulse-heightdistributionsofthecom- ponent organismscan beisolated....
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