Barcoded primer structures. Design of barcoded oligonucleotides. "Barcode" indicates the location of sequences unique to each PCR primer. "Template-specificity" indicates sequences required for PCR amplification. In this example, template-specificity regions are broadly specific for bacterial large-subunit rRNA genes (A: LSU559R, B: LSU130F) and are joined to the rest of the primer through a two-nucleotide, randomized linker. Sequences labeled "Sequencing" refer to the primer A and B sequences required by the GS-FLX instrument. Although this example is based on specifications required for operation of the 454 LifeSciences Inc. GS-FLX system, the use of barcrawl can be abstracted to other platforms capable of sequencing PCR amplicons. The "sequencing" segments may not be required by other platforms and are not required for barcrawl to function.