Isolation and characterization of low nucleic acid (LNA)-content bacteria

Most planktonic bacteria are 'uncultivable' with conventional methods. Flow cytometry (FCM) is one approach that has been taken to study these bacteria. In natural aquatic environments, bacteria with high nucleic acid (HNA) and low nucleic acid (LNA) content are commonly observed with FCM after staining with fluorescent dyes. Although several studies have focused on the relative abundance and in situ activities of these two groups, knowledge on the growth of particularly LNA bacteria is largely limited. In this study, typical LNA bacteria were enriched from three different freshwater sources using extinction dilution (ED) and fluorescence-activated cell sorting (FACS). We have shown for the first time that LNA bacteria can be isolated and cultivated by using sterile freshwater as a growth medium. During growth, the typical LNA characteristics (that is, low-fluorescence intensity and sideward scatter (SSC)) remained distinct from those of typical HNA bacteria. Three LNA pure cultures that are closely affiliated to the Polynucleobacter cluster according to 16S rRNA sequencing results were isolated. Owing to their small size, cells of the isolates remained intact during cryo-transmission electronic microscopy examination and showed a Gram-negative cell-wall structure. The extremely small cell volume (0.05 microm3) observed for all three isolates indicates that they are among the smallest free-living heterotrophic organisms known in culture. Their isolation and cultivation allow further detailed investigation of this group of organisms under defined laboratory conditions.