Viable but Nonculturable Cells in Plant-Associated Bacterial Populations

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Plants in terrestrial ecosystems are colonized by complex microbial communities composed of bacteria, yeasts, and filamentous fungi. These microbial communities have received much attention because of their effects on plant productivity. While these communities contain some deleterious organisms, such as phytopathogenic bacteria and fungi, they also contain beneficial organisms, such as nitrogen-fixing bacteria, bacteria capable of suppressing plant disease, and bacteria capable of promoting plant growth. Quantification of bacterial populations in these plantassociated microbial communities in epidemiological, pathological, and ecological studies has to date relied almost exclusively upon either plate counts using selective media (4, 48, 87) or upon quantitative immunofluorescence microscopy (13, 28). Only recently have techniques for the quantification of viable or metabolically active bacterial cells become available. Relatively few attempts have been made with plant-associated bacterial species to relate the population size of culturable cells, determined by plating on selective media, with the population size of viable or metabolically active cells (9, 10, 74, 107). The occurrence in these plantassociated microbial communities of a substantial proportion of viable or metabolically active cells which are not culturable would have serious implications for the disciplines of plant pathology, microbial ecology, and phytoremediation.