strains belonging to phylogroup 3 frequently exhibit an atypical quorum sensing response: the case of MAZ105, a tomato rhizosphere isolate.

is a widespread γ-proteobacterium and an important opportunistic pathogen. The genetically diverse phylogroup 3 strains are characterized by producing the pore-forming ExlA toxin and by their lack of a type III secretion system. However, like all strains of this species, they produce several virulence-associated traits, such as elastase, rhamnolipids and pyocyanin, which are regulated by quorum sensing (QS). The QS response comprises three systems (Las, Rhl and Pqs, respectively) that hierarchically regulate these virulence factors. The Pqs QS system is composed of the PqsR transcriptional factor, which, coupled with the alkyl-quinolones HHQ or PQS, activates the transcription of the operon. The products of the first four genes of this operon produce HHQ, which is then converted to PQS by PqsH, while PqsE forms a complex with RhlR and stabilizes it. In this study we report that mutations affecting the Pqs system are particularly common in phylogroup 3 strains. To better understand QS in phylogroup 3 strains we studied strain MAZ105 isolated from tomato rhizosphere and showed that it contains mutations in the central QS transcriptional regulator, LasR, and in the gene encoding the PqsA enzyme involved in the synthesis of PQS. However, it can still produce QS-regulated virulence factors and is virulent in and mildly pathogenic in the mouse abscess/necrosis model; our results show that this may be due to the expression of from a different PqsR-independent promoter than the promoter. Our results indicate that using anti-virulence therapy based on targeting the PQS system will not be effective against infections by phylogroup 3 strains.