, 2000). Not surprisingly, the genome Bcl-2 inhibitor contained a high number of genes involved in catabolism, transport, efflux, motility, and signal response regulation. In fact, over 8% of genes in the P. aeruginosa (PAO1) genome were thought to be involved in regulation, which well exceeded the percentage observed in any other bacterial genome. It was immediately clear that the key to Pseudomonas’s success

was the plasticity with which it could express its genes, which was afforded by layers of regulatory complexity. Since 2000, the vast majority of the 1000+ Pseudomonas genomes sequenced have been clinical strains of P. aeruginosa. Collectively, we have learned that the major part of the P. aeruginosa genome (about 4000 genes) is conserved in all strains and represents the ‘core genome’. Up to another 20% of genes reside on genomic islands that collectively represent the ‘accessory genome’. It is this accessory genome that imparts P. aeruginosa’s plasticity and includes many of the genes involved in metabolism, virulence, and antibiotic resistance. As approximately 10 000 unique genes have already been identified in the accessory regions of sequenced isolates, it is estimated that the P. aeruginosa pan-genome could approach, or even exceed, 100 000 genes, meaning that the genetic repertoire of this one species

of Pseudomonas would far click here exceed that of humans (Tummler et al., 2014). In this thematic issue, Sarah Pohl et al. (Pohl et al., 2014) analyzed the expression of the accessory genome of 150 P. aeruginosa clinical isolates. Despite the 10 000 unique genes that have already been sequenced from the accessory regions of P. aeruginosa clinical isolates, the investigators found that almost all of their 150 isolates possessed genes not present in any previously sequenced. Their findings further demonstrate the exceptionally broad P. aeruginosa gene pool. Considering the vast genomic variation in the genus, it is not surprising that there is still much we do

not understand about the relationship between genetic composition and the behavior of pseudomonads. Many of the contributions in this thematic from issue focus on topics in this area. In his MiniReview, Valentin Rybenkov (Rybenkov, 2014) discusses how the replication, organization, and segregation of the P. aeruginosa chromosome add further complexity to the regulation of the transcriptome. The genetic and phenotypic consequences of plasmids on P. aeruginosa, P. putida, and P. stutzeri are investigated in three different reports by Deraspe et al., (2014) Silva-Rocha and de Lorenzo (2014) and Coleman et al., (2014) respectively, while contributions from Song et al. (2014) and González-Valdez et al. (2014) report new findings that influence the regulation of lipopeptide biosynthesis in P. fluorescens and quorum sensing in P. aeruginosa. In all, 12 original reports and MiniReviews are included in this thematic Pseudomonas issue of FEMS Microbiology Letters.