P. aeruginosais an opportunistic pathogenic bacterium responsible for both acute and chronic infections. Beyond its natural resistance to many drugs, its ability to form biofilm, a complex biological system, renders ineffective the clearance by immune defense systems and antibiotherapy. The objective of this report is to provide an overview (i) onP. aeruginosabiofilm lifestyle cycle, (ii) on the main key actors relevant in the regulation of biofilm formation byP. aeruginosaincluding QS systems,GacS/GacAandRetS/LadStwo-component systems and C-di-GMP-dependent polysaccharides biosynthesis, and (iii) finally on reported natural and synthetic products that interfere with control mechanisms of biofilm formation byP. aeruginosawithout affecting directly bacterial viability. Concluding remarks focus on perspectives to consider biofilm lifestyle as a target for eradication of resistant infections caused byP. aeruginosa.

Quorum-sensing (QS) regulates the production of key virulence factors in Pseudomonas aeruginosa and other important pathogenic bacteria. In this report, extracts of leaves and bark of Combretum albiflorum (Tul.) Jongkind ( Combretaceae ) were found to quench the production of QS-dependent factors in P. aeruginosa PAO1. Chromatographic fractionation of the crude active extract generated several active fractions containing flavonoids, as shown by their typical spectral features. Purification and structural characterization of one of the active compounds led to the identification of the flavan-3-ol catechin [(2 R ,3 S )-2-(3,4-dihydroxyphenyl)-3,4-dihydro-1(2 H )-benzopyran-3,5,7-triol]. The identity of catechin as one of the active molecules was confirmed by comparing the high-pressure liquid chromatography profiles and the mass spectrometry spectra obtained for a catechin standard and for the active C. albiflorum fraction. Moreover, standard catechin had a significant negative effect on pyocyanin and elastase productions and biofilm formation, as well as on the expression of the QS-regulated genes lasB and rhlA and of the key QS regulatory genes lasI , lasR , rhlI , and rhlR . The use of RhlR- and LasR-based biosensors indicated that catechin might interfere with the perception of the QS signal N -butanoyl- l -homoserine lactone by RhlR, thereby leading to a reduction of the production of QS factors. Hence, catechin, along with other flavonoids produced by higher plants, might constitute a first line of defense against pathogenic attacks by affecting QS mechanisms and thereby virulence factor production.

Preliminary screening of the Malagasy plantCombretum albiflorumfor compounds attenuating the production of quorum sensing (QS)-controlled virulence factors in bacteria led to the identification of active fractions containing flavonoids. In the present study, several flavonoids belonging to the flavone, flavanone, flavonol and chalcone structural groups were screened for their capacity to reduce the production of QS-controlled factors in the opportunistic pathogenPseudomonas aeruginosa(strain PAO1). Flavanones (i.e. naringenin, eriodictyol and taxifolin) significantly reduced the production of pyocyanin and elastase inP. aeruginosawithout affecting bacterial growth. Consistently, naringenin and taxifolin reduced the expression of several QS-controlled genes (i.e.lasI,lasR,rhlI,rhlR,lasA,lasB,phzA1andrhlA) inP. aeruginosaPAO1. Naringenin also dramatically reduced the production of the acylhomoserine lactonesN-(3-oxododecanoyl)-l-homoserine lactone (3-oxo-C12-HSL) andN-butanoyl-l-homoserine lactone (C4-HSL), which is driven by thelasIandrhlIgene products, respectively. In addition, using mutant strains deficient for autoinduction (ΔlasIand ΔrhlI) and LasR- and RhlR-based biosensors, it was shown that QS inhibition by naringenin not only is the consequence of a reduced production of autoinduction compounds but also results from a defect in the proper functioning of the RlhR–C4-HSL complex. Widely distributed in the plant kingdom, flavonoids are known for their numerous and determinant roles in plant physiology, plant development and in the success of plant–rhizobia interactions, but, as shown here, some of them also have a role as inhibitors of the virulence of pathogenic bacteria by interfering with QS mechanisms.

The role and metabolism of indole-3-acetic acid in gram-negative bacteria is well documented, but little is known about indole-3-acetic acid biosynthesis and regulation in gram-positive bacteria. The phytopathogenRhodococcus fascians, a gram-positive organism, incites diverse developmental alterations, such as leafy galls, on a wide range of plants. Phenotypic analysis of a leafy gall suggests that auxin may play an important role in the development of the symptoms. We show here for the first time thatR. fasciansproduces and secretes the auxin indole-3-acetic acid. Interestingly, whereas noninfected-tobacco extracts have no effect, indole-3-acetic acid synthesis is highly induced in the presence of infected-tobacco extracts when tryptophan is not limiting. Indole-3-acetic acid production by a plasmid-free strain shows that the biosynthetic genes are located on the bacterial chromosome, although plasmid-encoded genes contribute to the kinetics and regulation of indole-3-acetic acid biosynthesis. The indole-3-acetic acid intermediates present in bacterial cells and secreted into the growth media show that the main biosynthetic route used byR. fasciansis the indole-3-pyruvic acid pathway with a possible rate-limiting role for indole-3-ethanol. The relationship between indole-3-acetic acid production and the symptoms induced byR. fasciansis discussed.

The gram-positive plant pathogenRhodococcus fasciansprovokes leafy gall formation on a wide range of plants through secretion of signal molecules that interfere with the hormone balance of the host. Crucial virulence genes are located on a linear plasmid, and their expression is tightly controlled. A mutant with a mutation in a chromosomal locus that affected virulence was isolated. The mutation was located in genevicA, which encodes a malate synthase and is functional in the glyoxylate shunt of the Krebs cycle. VicA is required for efficient in planta growth in symptomatic, but not in normal, plant tissue, indicating that the metabolic requirement of the bacteria or the nutritional environment in plants or both change during the interaction. We propose that induced hyperplasia on plants represents specific niches for the causative organisms as a result of physiological alterations in the symptomatic tissue. Hence, such interaction could be referred to as metabolic habitat modification.

▪ Abstract Rhodococcus fascians infects a wide range of plants, initiating the formation of leafy galls that consist of centers of shoot amplification and shoot growth inhibition. R. fascians is an epiphyte but it also can establish endophytic populations. Bacterial signals involved in symptom development initiate de novo cell division and shoot meristem formation in differentiated tissues. The R. fascians signals exert activities that are distinct from mere cytokinin effects, and the evidence points to a process that adopted cytokinin biosynthetic enzymes to form derivatives with unique activity. Genes implicated in leafy gall formation are located on a linear plasmid and are subject to a highly controlling, complex regulatory network, integrating autoregulatory compounds and environmental signals. Leafy galls are considered as centers with specific metabolic features, a niche where populations of R. fascians experience a selective advantage. Such “metabolic habitat modification” might be universal for gall-inducing bacteria.