Microbial ecology has long been hampered by the fact that most microorganisms cannot be identified in situ because of the lack of morphological diversity. The immunofluorescence approach has yielded important insights into the spatial distribution of microorganisms but has its severe limitations. The recently introduced fluorescently labelled, ribosomal RNA‐targeted oligonucleotide probes have successfully been applied for the detection and identification in situ of individual microbial cells and evade some of the principal problems of the fluorescent antibodies. The design and synthesis of these phylogenetically nested probes does not require cultivation and isolation of the target organism and can therefore be used to monitor the population distribution and dynamics of hitherto uncultured microorganisms.

Publicly available sequence databases of the small subunit ribosomal RNA gene, also known as 16S rRNA in bacteria and archaea, are growing rapidly, and the number of entries currently exceeds 4 million. However, a unified classification and nomenclature framework for all bacteria and archaea does not yet exist. In this Analysis article, we propose rational taxonomic boundaries for high taxa of bacteria and archaea on the basis of 16S rRNA gene sequence identities and suggest a rationale for the circumscription of uncultured taxa that is compatible with the taxonomy of cultured bacteria and archaea. Our analyses show that only nearly complete 16S rRNA sequences give accurate measures of taxonomic diversity. In addition, our analyses suggest that most of the 16S rRNA sequences of the high taxa will be discovered in environmental surveys by the end of the current decade.

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