Epidemiologic studies require identification or typing of microbial strains. Macrorestriction DNA mapping analyzed by pulsed-field gel electrophoresis (PFGE) is considered the current gold standard of genomic typing. This technique, however, is difficult to implement because it is labor-intensive and difficult to automate, it requires a long time to obtain results, and results often vary between laboratories. We used direct linear analysis (DLA), which uses a single reagent set and long fragments of microbial genomic DNA to identify various microbes. In this technique, an automated system extracts fragments exceeding 100 kb from restriction enzyme digests of genomic DNA from microbial isolates and hybridizes them with a sequence-dependent fluorescent tag. These fragments are then stretched in a microfluidics chip, and the patterns of the distribution of the tags are discerned with fluorescence confocal microscopy. The tag pattern on each DNA fragment is compared with a database of known microbial DNA sequences or with measured patterns of other microbial DNAs. We used DLA to type 71 Staphylococcus aureus strains. Of these, 9 had been sequenced, 10 were representative of the major pulsed-field types present in the US, and 52 were isolated recently in a hospital in Cambridge, MA. Matching DNA fragments were identified in different samples by a clustering algorithm and were used to quantify the similarities of the strains. DLA-based strain typing is a powerful technique with a resolution comparable to macrorestriction mapping with PFGE, but DLA is faster, more automated, and more reproducible.
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