TdaA regulates Tropodithietic acid synthesis by binding to the tdaC promoter region

Tropodithietic Acid, a Multifunctional Antimicrobial, Facilitates Adaption and Colonization of the Producer, Phaeobacter piscinae

In the marine environment, surface-associated bacteria often produce an array of antimicrobial secondary metabolites, which have predominantly been perceived as competition molecules. However, they may also affect other hallmarks of surface-associated living, such as motility and biofilm formation. Here, we investigate the ecological significance of an antibiotic secondary metabolite, tropodithietic acid (TDA), in the producing bacterium, Phaeobacter piscinae S26. We constructed a markerless in-frame deletion mutant deficient in TDA biosynthesis, S26ΔtdaB. Molecular networking demonstrated that other chemical sulfur-containing features, likely related to TDA, were also altered in the secondary metabolome. We found several changes in the physiology of the TDA-deficient mutant, ΔtdaB, compared to the wild type. Growth of the two strains was similar; however, ΔtdaB cells were shorter and more motile. Transcriptome and proteome profiling revealed an increase in gene expression and protein abundance related to a type IV secretion system, and to a prophage, and a gene transfer agent in ΔtdaB. All these systems may contribute to horizontal gene transfer (HGT), which may facilitate adaptation to novel niches. We speculate that once a TDA-producing population has been established in a new niche, the accumulation of TDA acts as a signal of successful colonization, prompting a switch to a sessile lifestyle. This would lead to a decrease in motility and the rate of HGT, while filamentous cells could form the base of a biofilm. In addition, the antibiotic properties of TDA may inhibit invading competing microorganisms. This points to a role of TDA in coordinating colonization and adaptation. IMPORTANCE Despite the broad clinical usage of microbial secondary metabolites with antibiotic activity, little is known about their role in natural microbiomes. Here, we studied the effect of production of the antibiotic tropodithietic acid (TDA) on the producing strain, Phaeobacter piscinae S26, a member of the Roseobacter group. We show that TDA affects several phenotypes of the producing strain, including motility, cell morphology, metal metabolism, and three horizontal gene transfer systems: a prophage, a type IV secretion system, and a gene transfer agent. Together, this indicates that TDA participates in coordinating the colonization process of the producer. TDA is thus an example of a multifunctional secondary metabolite that can mediate complex interactions in microbial communities. This work broadens our understanding of the ecological role that secondary metabolites have in microbial community dynamics.

Enhanced wastewater treatment performance by understanding the interaction between algae and bacteria based on quorum sensing

In order to further obtain sustainable wastewater treatment technology, in-depth analysis based on algal-bacterial symbiosis, quorum sensing signal molecules and algal-bacterial relationship will lay the foundation for the synergistic algal-bacterial wastewater treatment process. The methods of enhancing algae and bacteria wastewater treatment technology were systematically explored, including promoting symbiosis, reducing algicidal behavior, eliminating the interference of quorum sensing inhibitor, and developing algae and bacteria granular sludge. These findings can provide guidance for sustainable economic and environmental development, and facilitate carbon emissions reduction by using algae and bacteria synergistic wastewater treatment technology in further attempts. The future work should be carried out in the following four aspects: (1) Screening of dominant microalgae and bacteria; (2) Coordination of stable (emerging) contaminants removal; (3) Utilization of algae to produce fertilizers and feed (additives), and (4) Constructing recombinant algae and bacteria for reducing carbon emissions and obtaining high value-added products.

Bioactive Compounds for Quorum Sensing Signal-Response Systems in Marine Phycosphere

Quorum sensing in the phycosphere refers to a sensor system in which bacteria secrete bioactive compounds to coordinate group behavior relying on cell density. It is an important way for algae and bacteria to communicate with each other and achieve interactions. It has been determined that quorum sensing is widely presented in the marine phycosphere, which involves a variety of bioactive compounds. Focused on these compounds in marine phycosphere, this review summarizes the types and structures of the compounds, describes the methods in detection and functional evaluation, discusses the ecological functions regulated by the compounds, such as modulating microbial colonization, achieving algae–bacteria mutualism or competition and contributing to marine biogeochemical cycles. Meanwhile, the application prospects of the compounds are also proposed, including controlling harmful algal blooms and producing biofuel. Future research should focus on improving detection techniques, developing more model systems and investigating the effects of climate change on the quorum-sensing pathway to further understand the mechanism and application potential of quorum sensing compounds. This review aims to present an overview of current research carried out in order to provide the reader with perspective on bioactive compounds involved in quorum sensing.

Role is in the eye of the beholder-the multiple functions of the antibacterial compound tropodithietic acid produced by marine Rhodobacteraceae

Many microbial secondary metabolites have been studied for decades primarily because of their antimicrobial properties. However, several of these metabolites also possess nonantimicrobial functions, both influencing the physiology of the producer and their ecological neighbors. An example of a versatile bacterial secondary metabolite with multiple functions is the tropone derivative tropodithietic acid (TDA). TDA is a broad-spectrum antimicrobial compound produced by several members of the Rhodobacteraceae family, a major marine bacterial lineage, within the genera Phaeobacter, Tritonibacter, and Pseudovibrio. The production of TDA is governed by the mode of growth and influenced by the availability of nutrient sources. The antibacterial effect of TDA is caused by disruption of the proton motive force of target microorganisms and, potentially, by its iron-chelating properties. TDA also acts as a signaling molecule, affecting gene expression in other bacteria, and altering phenotypic traits such as motility, biofilm formation, and antibiotic production in the producer. In microbial communities, TDA-producing bacteria cause a reduction of the relative abundance of closely related species and some fast-growing heterotrophic bacteria. Here, we summarize the current understanding of the chemical ecology of TDA, including the environmental niches of TDA-producing bacteria, and the molecular mechanisms governing the function and regulation of TDA.

Bacterial Tropone Natural Products and Derivatives: Overview of their Biosynthesis, Bioactivities, Ecological Role and Biotechnological Potential

Tropone natural products are non-benzene aromatic compounds of significant ecological and pharmaceutical interest. Herein, we highlight current knowledge on bacterial tropones and their derivatives such as tropolones, tropodithietic acid, and roseobacticides. Their unusual biosynthesis depends on a universal CoA-bound precursor featuring a seven-membered carbon ring as backbone, which is generated by a side reaction of the phenylacetic acid catabolic pathway. Enzymes encoded by separate gene clusters then further modify this key intermediate by oxidation, CoA-release, or incorporation of sulfur among other reactions. Tropones play important roles in the terrestrial and marine environment where they act as antibiotics, algaecides, or quorum sensing signals, while their bacterial producers are often involved in symbiotic interactions with plants and marine invertebrates (e. g., algae, corals, sponges, or mollusks). Because of their potent bioactivities and of slowly developing bacterial resistance, tropones and their derivatives hold great promise for biomedical or biotechnological applications, for instance as antibiotics in (shell)fish aquaculture.

Causes and Consequences of a Variant Strain of Phaeobacter inhibens With Reduced Competition

Phaeobacter inhibens 2.10 is an effective biofilm former and colonizer of marine surfaces and has the ability to outcompete other microbiota. During biofilm dispersal P. inhibens 2.10 produces heritable phenotypic variants, including those that have a reduced ability to inhibit the co-occurring bacterium Pseudoalteromonas tunicata. However, the genetic changes that underpin the phenotypic variation and what the ecological consequences are for variants within the population are unclear. To answer these questions we sequenced the genomes of strain NCV12a1, a biofilm variant of P. inhibens 2.10 with reduced inhibitory activity and the P. inhibens 2.10 WT parental strain. Genome wide analysis revealed point mutations in genes involved in synthesis of the antibacterial compound tropodithietic acid (TDA) and indirectly in extracellular polymeric substances (EPS) production. However, confocal laser scanning microscopy analyses found little differences in biofilm growth between P. inhibens 2.10 WT (parental) and NCV12a1. P. inhibens NCV12a1 was also not outcompeted in co-cultured biofilms with P. tunicata, despite its reduced inhibitory activity, rather these biofilms were thicker than those produced when the WT strain was co-cultured with P. tunicata. Notably, dispersal populations from biofilms of P. inhibens NCV12a1 had a higher proportion of WT-like morphotypes when co-cultured with P. tunicata. These observations may explain why the otherwise non-inhibiting variant persists in the presence of a natural competitor, adding to our understanding of the relative importance of genetic diversification in microbial biofilms.

A Complex Mechanism Involving LysR and TetR/AcrR That Regulates Iron Scavenger Biosynthesis in Pseudomonas donghuensis HYS

7-Hydroxytropolone (7-HT) is a symmetrical seven-membered heteroatomic ring with a carboxyl group and two hydroxyl groups and was recently reported to be an iron scavenger of Pseudomonas donghuensis HYS. Cluster 1 includes 12 genes related to the synthesis of 7-HT; among these genes, those for two regulators, Orf1 and Orf12, were predicted to regulate 7-HT biosynthesis and to be LysR-type transcriptional regulators (LTTRs) and TetR/AcrR family transcriptional regulators, respectively. Data from real-time quantitative PCR and β-galactosidase and classical siderophore assays indicated that the transcription levels of orf1 and orf12, as well as those of crucial genes orf6 to orf9, were repressed under high-iron conditions. The deletion of orf1 and orf12 led to an absence of 7-HT and a decrease in orf6-orf9 expression. Orf1 and Orf12 were essential for the production of 7-HT through orf6-orf9 These two regulators are regulated by the Gac/Rsm system; Orf1 facilitates the expression of Orf12, and Orf12 concomitantly stimulates the expression of orf6-orf9 to synthesize 7-HT. The overexpression of Orf12 decreased 7-HT yields, possibly through decreased orf6-orf9 expression. This work thus outlines a complex mechanism regulating the biosynthesis of the iron scavenger 7-HT in P. donghuensis HYS. The synergy between Orf1 and Orf12 ensures that 7-HT acts as an iron chelator despite being toxic to bacteria and provides new ideas for the novel regulation of dual-functional secondary metabolism and research on 7-HT and its derivates in other bacteria.IMPORTANCE A complex regulation mechanism including two regulators, LysR and TetR/AcrR, in the biosynthesis of the novel iron scavenger 7-hydroxytropolone (7-HT) was verified in Pseudomonas donghuensis HYS. The coaction of LysR Orf1 and TetR/AcrR Orf12 may balance the toxicity and iron chelation of 7-HT in P. donghuensis HYS to overcome iron deficiency, as well as improve the bacterial competitiveness under iron-scarce conditions because of the toxicity of 7-HT toward other bacteria, making the accurate regulation of 7-HT biosynthesis indispensable. This regulation mechanism may be ubiquitous in the Pseudomonas putida group but may better explain the group's strong adaptability.

Phylogenetic structure of bacterial assemblages co-occurring with Ostreopsis cf. ovata bloom

Extensive blooms of the toxic epiphytic/benthic dinoflagellate Ostreopsis cf. ovata are being reported with increasing frequency and spatial distribution in temperate coastal regions including the Mediterranean. These blooms are of human and environmental health concern due to the production of isobaric palytoxin and a wide range of ovatoxins by Ostreopsis cf. ovata. Bacterial-microalgal interactions are important regulators in algal bloom dynamics and potentially toxin dynamics. This study investigated the bacterial assemblages co-occurring with O. cf. ovata (OA) and from ambient seawaters (SW) during the early and peak phases of bloom development in NW Adriatic Sea. Fractions of the bacterial assemblages co-occurring with O. cf. ovata (OA) and more closely associated to the mucilage layer (LA) embedding O. cf. ovata cells were also reported. In total, 14 bacterial phyla were detected by targeted 454 pyrosequencing of the 16S rRNA gene. The dominant bacterial phyla in the OA assemblages were Proteobacteria and Bacteroidetes; while at the class level, Alphaproteobacteria were the most abundant (83 and 66%, relative abundance, early and peak bloom phases), followed by Flavobacteria (7 and 19%, early and peak phases). Actinobacteria and Cyanobacteria were of minor importance (<5% of the relative bacterial abundance each). Gammaproteobacteria showed a notably presence in OA assemblage only at the early phase of the bloom (genus Haliea, 13%). The Alphaproteobacteria were predominately composed by the genera Ruegeria, Jannaschia and Erythrobacter which represented about half of the total phylotypes' contribution of OA at both early and peak phases of the O. cf. ovata bloom, suggesting interactions between this consortium and the microalga. Moreover, the highest contribution of Ruegeria (30% of the total phylotypes) was observed at the early phase of the bloom in LA assemblage. Microbial assemblages associated with the ambient seawaters while being also dominated by Alphaproteobacteria and Flavobacteria were partially distinct from those associated with O. cf. ovata due to the presence of genera almost not retrieved in the latter assemblages.

Investigation of the Genetics and Biochemistry of Roseobacticide Production in the Roseobacter Clade Bacterium Phaeobacter inhibens

Roseobacter clade bacteria are abundant in surface waters and are among the most metabolically diverse and ecologically significant species. This group includes opportunistic symbionts that associate with micro- and macroalgae. We have proposed that one representative member, Phaeobacter inhibens, engages in a dynamic symbiosis with the microalga Emiliania huxleyi. In one phase, mutualistically beneficial molecules are exchanged, including the Roseobacter-produced antibiotic tropodithietic acid (TDA), which is thought to protect the symbiotic interaction. In an alternative parasitic phase, triggered by algal senescence, the bacteria produce potent algaecides, the roseobacticides, which kill the algal host. Here, we employed genetic and biochemical screens to identify the roseobacticide biosynthetic gene cluster. By using a transposon mutagenesis approach, we found that genes required for TDA synthesis—the tda operon and paa catabolon—are also necessary for roseobacticide production. Thus, in contrast to the one-cluster–one-compound paradigm, the tda gene cluster can generate two sets of molecules with distinct structures and bioactivities. We further show that roseobacticide production is quorum sensing regulated via an N-acyl homoserine lactone signal (3-OH–C₁₀-HSL). To ensure tight regulation of algaecide production, and thus of a lifestyle switch from mutualism to parasitism, roseobacticide biosynthesis necessitates the presence of both an algal senescence molecule and a quorum sensing signal.

Marine Roseobacter species are abundant in the oceans and engage in symbiotic interactions with microscopic algae. One member, P. inhibens, produces the antibiotic TDA and a growth hormone thought to protect and promote algal growth. However, in the presence of molecules released by senescing algae, the bacteria produce potent algaecides, the roseobacticides, which kill the host. We examined the regulatory networks and biosynthetic genes required for roseobacticide production. We found that P. inhibens uses largely the same set of genes for production of both TDA and roseobacticides, thus providing a rare case in which one gene cluster synthesizes two structurally and functionally distinct molecules. Moreover, we found roseobacticide production to be regulated by quorum sensing. Thus, two small molecules, the algal metabolite and the quorum-sensing signal, ensure tight control in the production of roseobacticides. These results highlight the role of small molecules in regulating microbial symbioses.

Mode of action and resistance studies unveil new roles for tropodithietic acid as an anticancer agent and the γ-glutamyl cycle as a proton sink

While we have come to appreciate the architectural complexity of microbially synthesized secondary metabolites, far less attention has been paid to linking their structural features with possible modes of action. This is certainly the case with tropodithietic acid (TDA), a broad-spectrum antibiotic generated by marine bacteria that engage in dynamic symbioses with microscopic algae. TDA promotes algal health by killing unwanted marine pathogens; however, its mode of action (MoA) and significance for the survival of an algal-bacterial miniecosystem remains unknown. Using cytological profiling, we herein determine the MoA of TDA and surprisingly find that it acts by a mechanism similar to polyether antibiotics, which are structurally highly divergent. We show that like polyether drugs, TDA collapses the proton motive force by a proton antiport mechanism, in which extracellular protons are exchanged for cytoplasmic cations. The α-carboxy-tropone substructure is ideal for this purpose as the proton can be carried on the carboxyl group, whereas the basicity of the tropylium ion facilitates cation export. Based on similarities to polyether anticancer agents we have further examined TDA's cytotoxicity and find it to exhibit potent, broad-spectrum anticancer activities. These results highlight the power of MoA-profiling technologies in repurposing old drugs for new targets. In addition, we identify an operon that confers TDA resistance to the producing marine bacteria. Bioinformatic and biochemical analyses of these genes lead to a previously unknown metabolic link between TDA/acid resistance and the γ-glutamyl cycle. The implications of this resistance mechanism in the context of the algal-bacterial symbiosis are discussed.

Quorum sensing is a language of chemical signals and plays an ecological role in algal-bacterial interactions

Algae are ubiquitous in the marine environment, and the ways in which they interact with bacteria are of particular interest in marine ecology field. The interactions between primary producers and bacteria impact the physiology of both partners, alter the chemistry of their environment, and shape microbial diversity. Although algal-bacterial interactions are well known and studied, information regarding the chemical-ecological role of this relationship remains limited, particularly with respect to quorum sensing (QS), which is a system of stimuli and response correlated to population density. In the microbial biosphere, QS is pivotal in driving community structure and regulating behavioral ecology, including biofilm formation, virulence, antibiotic resistance, swarming motility, and secondary metabolite production. Many marine habitats, such as the phycosphere, harbour diverse populations of microorganisms and various signal languages (such as QS-based autoinducers). QS-mediated interactions widely influence algal-bacterial symbiotic relationships, which in turn determine community organization, population structure, and ecosystem functioning. Understanding infochemicals-mediated ecological processes may shed light on the symbiotic interactions between algae host and associated microbes. In this review, we summarize current achievements about how QS modulates microbial behavior, affects symbiotic relationships, and regulates phytoplankton chemical ecological processes. Additionally, we present an overview of QS-modulated co-evolutionary relationships between algae and bacterioplankton, and consider the potential applications and future perspectives of QS.

Biosynthesis of the antibiotic tropodithietic acid by the marine bacterium Phaeobacter inhibens

The biosynthesis of tropodithietic acid was investigated using a combinatorial approach of feeding experiments, gene knockouts and bioinformatic analyses. The mechanism of sulfur introduction is distinct from known mechanisms in holomycin, thiomarinol A and gliotoxin biosynthesis.

A dual-species co-cultivation system to study the interactions between Roseobacters and dinoflagellates

Some microalgae in nature live in symbiosis with microorganisms that can enhance or inhibit growth, thus influencing the dynamics of phytoplankton blooms. In spite of the great ecological importance of these interactions, very few defined laboratory systems are available to study them in detail. Here we present a co-cultivation system consisting of the toxic phototrophic dinoflagellate Prorocentrum minimum and the photoheterotrophic alphaproteobacterium Dinoroseobacter shibae. In a mineral medium lacking a carbon source, vitamins for the bacterium and the essential vitamin B₁₂ for the dinoflagellate, growth dynamics reproducibly went from a mutualistic phase, where both algae and bacteria grow, to a pathogenic phase, where the algae are killed by the bacteria. The data show a “Jekyll and Hyde” lifestyle that had been proposed but not previously demonstrated. We used RNAseq and microarray analysis to determine which genes of D. shibae are transcribed and differentially expressed in a light dependent way at an early time-point of the co-culture when the bacterium grows very slowly. Enrichment of bacterial mRNA for transcriptome analysis was optimized, but none of the available methods proved capable of removing dinoflagellate ribosomal RNA completely. RNAseq showed that a phasin encoding gene (phaP₁) which is part of the polyhydroxyalkanoate (PHA) metabolism operon represented approximately 10% of all transcripts. Five genes for aerobic anoxygenic photosynthesis were down-regulated in the light, indicating that the photosynthesis apparatus was functional. A betaine-choline-carnitine-transporter (BCCT) that may be used for dimethylsulfoniopropionate (DMSP) uptake was the highest up-regulated gene in the light. The data suggest that at this early mutualistic phase of the symbiosis, PHA degradation might be the main carbon and energy source of D. shibae, supplemented in the light by degradation of DMSP and aerobic anoxygenic photosynthesis.

Acyl-homoserine lactone quorum sensing in the Roseobacter clade

Members of the Roseobacter clade are ecologically important and numerically abundant in coastal environments and can associate with marine invertebrates and nutrient-rich marine snow or organic particles, on which quorum sensing (QS) may play an important role. In this review, we summarize current research progress on roseobacterial acyl-homoserine lactone-based QS, particularly focusing on three relatively well-studied representatives, Phaeobacter inhibens DSM17395, the marine sponge symbiont Ruegeria sp. KLH11 and the dinoflagellate symbiont Dinoroseobacter shibae. Bioinformatic survey of luxI homologues revealed that over 80% of available roseobacterial genomes encode at least one luxI homologue, reflecting the significance of QS controlled regulatory pathways in adapting to the relevant marine environments. We also discuss several areas that warrant further investigation, including studies on the ecological role of these diverse QS pathways in natural environments.

Biofilm formation and antibiotic production in Ruegeria mobilis are influenced by intracellular concentrations of cyclic dimeric guanosinmonophosphate

In many species of the marine Roseobacter clade, periods of attached life, in association with phytoplankton or particles, are interspersed with planktonic phases. The purpose of this study was to determine whether shifts between motile and sessile life in the globally abundant Roseobacter clade species Ruegeria mobilis are associated with intracellular concentrations of the signal compound cyclic dimeric guanosinmonophosphate (c-di-GMP), which in bacteria regulates transitions between motile and sessile life stages. Genes for diguanylate cyclases and phosphodiesterases, which are involved in c-di-GMP signalling, were found in the genome of R. mobilis strain F1926. Ion pair chromatography-tandem mass spectrometry revealed 20-fold higher c-di-GMP concentrations per cell in biofilm-containing cultures than in planktonic cells. An introduced diguanylate cyclase gene increased c-di-GMP and enhanced biofilm formation and production of the potent antibiotic tropodithietic acid (TDA). An introduced phosphodiesterase gene decreased c-di-GMP and reduced biofilm formation and TDA production. tdaC, a key gene for TDA biosynthesis, was expressed only in attached or biofilm-forming cells, and expression was induced immediately after initial attachment. In conclusion, c-di-GMP signalling controls biofilm formation and biofilm-associated traits in R. mobilis and, as suggested by presence of GGDEF and EAL domain protein genes, also in other Roseobacter clade species.

A novel inducer of Roseobacter motility is also a disruptor of algal symbiosis

Silicibacter sp. strain TM1040, a member of the Roseobacter clade, forms a symbiosis with unicellular phytoplankton, which is inextricably linked to the biphasic "swim or stick" lifestyle of the bacteria. Mutations in flaC bias the population toward the motile phase. Renewed examination of the FlaC(-) strain (HG1016) uncovered that it is composed of two different cells: a pigmented type, PS01, and a nonpigmented cell, PS02, each of which has an identical mutation in flaC. While monocultures of PS01 and PS02 had few motile cells (0.6 and 6%, respectively), coculturing the two strains resulted in a 10-fold increase in the number of motile cells. Cell-free supernatants from coculture or wild-type cells were fully capable of restoring motility to PS01 and PS02, which was due to increased fliC3 (flagellin) transcription, FliC3 protein levels per cell, and flagella synthesis. The motility-inducing compound has an estimated mass of 226 Da, as determined by mass spectrometry, and is referred to as Roseobacter Motility Inducer (RMI). Mutations affecting genes involved in phenyl acetic acid synthesis significantly reduced RMI, while defects in tropodithietic acid (TDA) synthesis had marginal or no effect on RMI. RMI biosynthesis is induced by p-coumaric acid, a product of algal lignin degradation. When added to algal cultures, RMI caused loss of motility, cell enlargement, and vacuolization in the algal cells. RMI is a new member of the roseobacticide family of troponoid compounds whose activities affect roseobacters, by shifting their population toward motility, as well as their phytoplankton hosts, through an algicidal effect.

Tropodithietic acid production in Phaeobacter gallaeciensis is regulated by N-acyl homoserine lactone-mediated quorum sensing

The production of N-acyl homoserine lactones (AHLs) is widely distributed within the marine Roseobacter clade, and it was proposed that AHL-mediated quorum sensing (QS) is one of the most common cell-to-cell communication mechanisms in roseobacters. The traits regulated by AHL-mediated QS are yet not known for members of the Roseobacter clade, but production of the antibiotic tropodithietic acid (TDA) was supposed to be controlled by AHL-mediated QS in Phaeobacter spp. We describe here for the first time the functional role of luxR and luxI homologous genes of an organism of the Roseobacter clade, i.e., pgaR and pgaI in Phaeobacter gallaeciensis. Our results demonstrate that the AHL synthase gene pgaI is responsible for production of N-3-hydroxydecanoylhomoserine lactone (3OHC(10)-HSL). Insertion mutants of pgaI and pgaR are both deficient in TDA biosynthesis and the formation of a yellow-brown pigment when grown in liquid marine broth medium. This indicates that in P. gallaeciensis the production of both secondary metabolites is controlled by AHL-mediated QS. Quantitative real-time PCR showed that the transcription level of tdaA, which encodes an essential transcriptional regulator for TDA biosynthesis, decreased 28- and 51-fold in pgaI and pgaR genetic backgrounds, respectively. These results suggest that both the response regulator PgaR and the 3OHC(10)-HSL produced by PgaI induce expression of tdaA, which in turn positively regulates expression of the tda genes. Moreover, we confirmed that TDA can also act as autoinducer in P. gallaeciensis, as previously described for Silicibacter sp. strain TM1040, but only in the presence of the response regulator PgaR.