PopPUNK documentation

PopPUNK (Population Partitioning Using Nucleotide K-mers)

In straightforward cases, usage can be as simple as:

poppunk --easy-run --r-files references.txt --output poppunk_db

Where references.txt is a list of assembly fasta files, one per line. See Quickstart and the Tutorial for full details.

Details

A full description of the method can be found in the pre-print.

PopPUNK uses the fast k-mer distance estimation enabled by mash to calculate core and accessory distances between all pairs of isolates of bacteria in a collection. By clustering these distances into ‘within-strain’ and ‘between-strain’ distances a network of within-strain comparisons can be constructed. The use of a network has a number of convenient properties, the first being that the connected components represent a cluster of strains.

As well as identifying strains, the pairwise distance distribution also helps with assembly quality control (particularly in the case of contaminated contigs) and may be informative of the level of recombination in the population. The network representation also allows definition of representative isolates by sampling one example from each clique, and calculation of various statistics which can show how good the clustering is.

The advantages of this approach are broadly that:

  • It is fast, and scalable to \(10^{4}\) genomes in a single run.
  • Assigning new query sequences to a cluster using an existing database is scalable even beyond this.
  • Databases can be updated online (as sequences arrive).
  • Online updating is equivalent to building databases from scratch.
  • Databases can be kept small and managable by only keeping representative isolates.
  • There is no bin cluster. Outlier isolates will be in their own cluster.
  • Pre-processing, such as generation of an alignment, is not required.
  • The definition of clusters is biologically relevant to how bacteria evolve.
  • There is a lot of quantitative and graphical output to assist with clustering.
  • A direct import into microreact is available, as well as cytoscape, grapetree and phandango.
  • Everything is available within a single python executable.