Kraken 2 is the newest version of Kraken, a taxonomic classification
system using exact k-mer matches to achieve high accuracy and fast
classification speeds. This classifier matches each k-mer within a query
sequence to the lowest common ancestor (LCA) of all genomes containing
the given k-mer. The k-mer assignments inform the classification
algorithm. [see: Kraken 1's Webpage for more details].
Kraken 2 provides significant improvements to Kraken 1, with faster
database build times, smaller database sizes, and faster classification
speeds. These improvements were achieved by the following updates to the
Kraken classification program:
1. Storage of Minimizers: Instead of storing/querying entire k-mers,
Kraken 2 stores minimizers (l-mers) of each k-mer. The length of
each l-mer must be ≤ the k-mer length. Each k-mer is treated by
Kraken 2 as if its LCA is the same as its minimizer's LCA.
2. Introduction of Spaced Seeds: Kraken 2 also uses spaced seeds to
store and query minimizers to improve classification accuracy.
3. Database Structure: While Kraken 1 saved an indexed and sorted list
of k-mer/LCA pairs, Kraken 2 uses a compact hash table. This hash
table is a probabilistic data structure that allows for faster
queries and lower memory requirements. However, this data structure
does have a <1% chance of returning the incorrect LCA or returning
an LCA for a non-inserted minimizer. Users can compensate for this
possibility by using Kraken's confidence scoring thresholds.
4. Protein Databases: Kraken 2 allows for databases built from amino
acid sequences. When queried, Kraken 2 performs a six-frame
translated search of the query sequences against the database.
5. 16S Databases: Kraken 2 also provides support for databases not
based on NCBI's taxonomy. Currently, these include the 16S
databases: Greengenes, SILVA, and RDP.
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