Journal of Microbiological Methods 9 (1989) 257-266 257 Elsevier MIMET 00306 Enumeration and sizing of aquatic bacteria by use of a silicon-intensified target camera linked-image analysis system Andrew W. David and John H. Paul Department of Marine Science, University of South Florida, St Petersburg, Florida, USA (Received 29 July 1988; revision received 22 December 1988; accepted 3 January 1989) Summary Modern image analysis systems incorporating video cameras and epifluorescence microscopy provide ac- curate cell counts and size measurements of natural populations of aquatic bacteria. This paper presents comparisons of counts and size measurements made manually and with an Image Technology Corporation Model 2000 image analysis system of bacteria from diverse aquatic environments of southwestern Florida, including the Alafia and Crystal Rivers, Tampa Bay, and the southeastern Gulf of Mexico. The modification of automated counting and sizing software and addition of a silicon-intensifiedtarget camera to the image- analysis system enabled collection of data statistically equivalent to manually derived data in all cases. Bac- terial cells from 0.18 to >7.00 #m were measured at cell densities up to 1.14 x 10 7 cells .ml -l. Estimation of biomass based upon published values of g C.#m -3 and image analysis-derived cell abundances and volumes ranged between 2.79 × 10 -8 g C.m1-1 for the oligotrophic Crystal River and 1.27 x 10 -6 g C.m1-1 for the eutrophic Alafia River. Key words: Image analysis; SIT camera Introduction Bacterial cell counts and measurements in aquatic systems can be used to monitor water quality as well as assess growth and survival, contamination, and seasonal and diel periodicity in bacterial abundance and biomass. Epifluorescence microscopy is the most widely used technique to enumerate aquatic bacteria [1]. DNA-staining fluorochromes used in epifluorescence microscopy include acridine orange, Hoechst 33258 and 33342, and DAPI [2- 4]. Counts have traditionally been performed manual- ly while SEM and photomicroscopy have been often used in bacterial size measure- ment [5- 7]. Considerable time can be saved through employment of video-enhanced Correspondence to: J.H. Paul, Department of Marine Science, University of South Florida, St Petersburg, FL 33701, USA. 0167-7012/89/$ 3.50 © 1989 Elsevier Science Publishers B.V. (Biomedical Division)