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We tested the application of 5-bromo-2’-deoxyuridine (BrdU), a thymidine analogue that becomes incorporated into DNA during growth, to measure growth rates of individual marine bacteria cells. Immunocytochemical detection of BrdU incorporation into bacterial DNA has the potential for single-cell-based growth measurement. Optimized procedure for immunocytochemistry was applicable to 14 marine heterotrophic bacterial isolates belonging to g-proteobacteria, α-proteobacteria, Cytophaga-Flavobacterium-Bacteroides (CFB) group and Gram-positive bacteria. The relationship between cell-specific fluorescence intensity and specific growth rate was linearly correlated among CFB group isolates, which indicated a potential of the method for quantitative measurement. Analysis of the detection limit indicated that bacteria with <1 d doubling time could be detected in 5 h incubations using bacterial assemblages in seawater. The method was also applied to visualize actively growing bacteria on phytoplankton detritus in seawater and was sensitive enough to test the variation in growth rate of natural bacterial assemblages in coastal waters using incubations of a few hours in duration. The method has the potential to yield insights into microspatial variability in bacterial growth rates in>seawater.

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