The P. aeruginosa core genome size based on variable definition of “core” sequences. The nucleotide core genome size is plotted as a function of the minimum number of the twelve reference genomes in which a particular genomic element must be present to be considered “core”. As flexibility is introduced into the definition of the core genome (i.e. an element is considered “core” if it is present in eleven of the twelve genomes, or ten of the twelve genomes, etc.), the “core” genome size increases. A “core” genome requirement of presence in only one of the twelve genomes therefore yields the pangenome of these twelve strains. Each symbol represents the average core genome size of all possible permutations of genome orders for twelve (12 permutations), eleven (132 permutations), and ten (1320 permutations) genome minimums, and the average core genome size for 10,000 randomly generated permutations at all other minimum genome numbers. Standard errors of the means at each value are too small to be visible at the scale of the figure.