Identification and characterization of a LuxI/R-type quorum sensing system in Pseudoalteromonas

Development of a Quorum Sensing-Mediated Bacterial Autolytic System in Escherichia coli for Automatic Release of Intracellular Products

Escherichia coli, one of the most efficient expression hosts for recombinant proteins, is widely used in chemical, medical, food, and other industries. De novo engineering of gene regulation circuits and cell density-controlled E. coli cell lysis are promising directions for the release of intracellular bioproducts. Here, we developed an E. coli autolytic system, named the quorum sensing-mediated bacterial autolytic (QS-BA) system, by incorporating an acyl-homoserine lactone (AHL)-based YasI/YasR-type quorum sensing circuit from Pseudoalteromonas into E. coli cells. The results showed that the E. coli QS-BA system can release the intracellular bioproducts into the cell culture medium in terms of E. coli cell density, which offers an environmentally-friendly, economical, efficient, and flexible E. coli lysis platform for production of recombinant proteins. The QS-BA system has the potential to serve as an integrated system for the large-scale production of target products in E. coli for medical and industrial applications.

AHL-Based Quorum Sensing Regulates the Biosynthesis of a Variety of Bioactive Molecules in Bacteria

Bacteria are social microorganisms that use communication systems known as quorum sensing (QS) to regulate diverse cellular behaviors including the production of various secreted molecules. Bacterial secondary metabolites are widely studied for their bioactivities including antibiotic, antifungal, antiparasitic, and cytotoxic compounds. Besides playing a crucial role in natural bacterial niches and intermicrobial competition by targeting neighboring organisms and conferring survival advantages to the producer, these bioactive molecules may be of prime interest to develop new antimicrobials or anticancer therapies. This review focuses on bioactive compounds produced under acyl homoserine lactone-based QS regulation by Gram-negative bacteria that are pathogenic to humans and animals, including the Burkholderia, Serratia, Pseudomonas, Chromobacterium, and Pseudoalteromonas genera. The synthesis, regulation, chemical nature, biocidal effects, and potential applications of these identified toxic molecules are presented and discussed in light of their role in microbial interactions.

Biosynthetic mechanism of the yellow pigments in the marine bacterium Pseudoalteromonas sp. strain T1lg65

The Pseudoalteromonas genus marine bacteria have attracted increasing interest because of their abilities to produce bioactive metabolites. The pigmented Pseudoalteromonas group encodes more secondary metabolite biosynthetic gene clusters (BGCs) than the non-pigmented group. Here, we report a yellow pigmented bacterium Pseudoalteromonas sp. strain T1lg65, which was isolated from a mangrove forest sediment. We showed that the yellow pigments of T1lg65 belong to the group of lipopeptide alterochromides. Further genetic analyses of the alterochromide BGC revealed that the yellow pigments are biosynthesized by aryl-polyene synthases and nonribosomal peptide synthases. Within the gene cluster, altA encodes a tyrosine ammonia acid lyase, which catalyzes synthesis of the precursor 4-hydroxycinnamic acid (4-HCA) from tyrosine in the alterochromide biosynthetic pathway. In addition, altN, encoding a putative flavin-dependent halogenase, was proven to be responsible for the bromination of alterochromides based on gene deletion, molecular docking, and site mutagenesis analyses. In summary, the biosynthetic pathway, precursor synthesis, and bromination mechanism of the lipopeptide alterochromides were studied in-depth. Our results expand the knowledge on biosynthesis of Pseudoalteromonas pigments and could promote the development of active pigments in the future.IMPORTANCEThe marine bacteria Pseudoalteromonas spp. are important biological resources because they are producers of bioactive natural products, including antibiotics, pigments, enzymes, and antimicrobial peptides. One group of the microbial pigments, alterochromides, holds a great value for their novel lipopeptide structures and antimicrobial activities. Previous studies were limited to the structural characterization of alterochromides and genome mining for the alterochromide biosynthesis. This work focused on the biosynthetic mechanism for alterochromide production, especially revealing functions of two key genes within the gene cluster for the alterochromide biosynthesis. On the one hand, our study provides a target for metabolic engineering of the alterochromide biosynthesis; on the other hand, the 4-HCA synthase AltA and brominase AltN show potential in the biocatalyst industry.

Comprehensive computational investigation for ligand recognition and binding dynamics of SdiA: a degenerate LuxR -type receptor in Klebsiella pneumoniae

SdiA is a LuxR-type receptor that controls the virulence of Klebsiella pneumoniae, a Gram-negative bacterium that causes various infections in humans. SdiA senses exogenous acyl-homoserine lactones (AHLs) and autoinducer-2 (AI-2), two types of quorum sensing signals produced by other bacterial species. However, the molecular details of how SdiA recognizes and binds to different ligands and how this affects its function and regulation in K. pneumoniae still need to be better understood. This study uses computational methods to explore the protein-ligand binding dynamics of SdiA with 11 AHLs and 2 AI-2 ligands. The 3D structure of SdiA was predicted through homology modeling, followed by molecular docking with AHLs and AI-2 ligands. Binding affinities were quantified using MM-GBSA, and complex stability was assessed via Molecular Dynamics (MD) simulations. Results demonstrated that SdiA in Klebsiella pneumoniae exhibits a degenerate binding nature, capable of interacting with multiple AHLs and AI-2. Specific ligands, namely C10-HSL, C8-HSL, 3-oxo-C8-HSL, and 3-oxo-C10-HSL, were found to have high binding affinities and formed critical hydrogen bonds with key amino acid residues of SdiA. This finding aligns with the observed preference of SdiA for AHLs having 8 to 10 carbon-length acyl chains and lacking hydroxyl groups. In contrast, THMF and HMF demonstrated poor binding properties. Furthermore, AI-2 exhibited a low affinity, corroborating the inference that SdiA is not the primary receptor for AI-2 in K. pneumoniae. These findings provide insights into the protein-ligand binding dynamics of SdiA and its role in quorum sensing and virulence of K. pneumoniae.

A growing battlefield in the war against biofilm-induced antimicrobial resistance: insights from reviews on antibiotic resistance

Biofilms are a common survival strategy employed by bacteria in healthcare settings, which enhances their resistance to antimicrobial and biocidal agents making infections difficult to treat. Mechanisms of biofilm-induced antimicrobial resistance involve reduced penetration of antimicrobial agents, increased expression of efflux pumps, altered microbial physiology, and genetic changes in the bacterial population. Factors contributing to the formation of biofilms include nutrient availability, temperature, pH, surface properties, and microbial interactions. Biofilm-associated infections can have serious consequences for patient outcomes, and standard antimicrobial therapies are often ineffective against biofilm-associated bacteria, making diagnosis and treatment challenging. Novel strategies, including antibiotics combination therapies (such as daptomycin and vancomycin, colistin and azithromycin), biofilm-targeted agents (such as small molecules (LP3134, LP3145, LP4010, LP1062) target c-di-GMP), and immunomodulatory therapies (such as the anti-PcrV IgY antibodies which target Type IIIsecretion system), are being developed to combat biofilm-induced antimicrobial resistance. A multifaceted approach to diagnosis, treatment, and prevention is necessary to address this emerging problem in healthcare settings.

Unraveling the Diverse Profile of N-Acyl Homoserine Lactone Signals and Their Role in the Regulation of Biofilm Formation in Porphyra haitanensis-Associated Pseudoalteromonas galatheae

N-acyl homoserine lactones (AHLs) are small, diffusible chemical signal molecules that serve as social interaction tools for bacteria, enabling them to synchronize their collective actions in a density-dependent manner through quorum sensing (QS). The QS activity from epiphytic bacteria of the red macroalgae Porphyra haitanensis, along with its involvement in biofilm formation and regulation, remains unexplored in prior scientific inquiries. Therefore, this study explores the AHL signal molecules produced by epiphytic bacteria. The bacterium isolated from the surface of P. haitanensis was identified as Pseudoalteromonas galatheae by 16s rRNA gene sequencing and screened for AHLs using two AHL reporter strains, Agrobacterium tumefaciens A136 and Chromobacterium violaceum CV026. The crystal violet assay was used for the biofilm-forming phenotype. The inferences revealed that P. galatheae produces four different types of AHL molecules, i.e., C4-HSL, C8-HSL, C18-HSL, and 3-oxo-C16-HSL, and it was observed that its biofilm formation phenotype is regulated by QS molecules. This is the first study providing insights into the QS activity, diverse AHL profile, and regulatory mechanisms that govern the biofilm formation phenotype of P. galatheae. These findings offer valuable insights for future investigations exploring the role of AHL producing epiphytes and biofilms in the life cycle of P. haitanensis.

A FadR-Type Regulator Activates the Biodegradation of Polycyclic Aromatic Hydrocarbons by Mediating Quorum Sensing in Croceicoccus naphthovorans Strain PQ-2

Bacteria employ multiple transcriptional regulators to orchestrate cellular responses to adapt to constantly varying environments. The bacterial biodegradation of polycyclic aromatic hydrocarbons (PAHs) has been extensively described, and yet, the PAH-related transcriptional regulators remain elusive. In this report, we identified an FadR-type transcriptional regulator that controls phenanthrene biodegradation in Croceicoccus naphthovorans strain PQ-2. The expression of fadR in C. naphthovorans PQ-2 was induced by phenanthrene, and its deletion significantly impaired both the biodegradation of phenanthrene and the synthesis of acyl-homoserine lactones (AHLs). In the fadR deletion strain, the biodegradation of phenanthrene could be recovered by supplying either AHLs or fatty acids. Notably, FadR simultaneously activated the fatty acid biosynthesis pathway and repressed the fatty acid degradation pathway. As intracellular AHLs are synthesized with fatty acids as substrates, boosting the fatty acid supply could enhance AHL synthesis. Collectively, these findings demonstrate that FadR in C. naphthovorans PQ-2 positively regulates PAH biodegradation by controlling the formation of AHLs, which is mediated by the metabolism of fatty acids. IMPORTANCE Master transcriptional regulation of carbon catabolites is extremely important for the survival of bacteria that face changes in carbon sources. Polycyclic aromatic hydrocarbons (PAHs) can be utilized as carbon sources by some bacteria. FadR is a well-known transcriptional regulator involved in fatty acid metabolism; however, the connection between FadR regulation and PAH utilization in bacteria remains unknown. This study revealed that a FadR-type regulator in Croceicoccus naphthovorans PQ-2 stimulated PAH biodegradation by controlling the biosynthesis of the acyl-homoserine lactone quorum-sensing signals that belong to fatty acid-derived compounds. These results provide a unique perspective for understanding bacterial adaptation to PAH-containing environments.

Inhibitory effects of hexanal on acylated homoserine lactones (AHLs) production to disrupt biofilm formation and enzymes activity in Erwinia carotovora and Pseudomonas fluorescens

Erwinia carotovora and Pseudomonas fluorescens were two bacteria commonly caused the spoilage of vegetables through biofilm formation and secretion of extracellular enzymes. In this study, N-(3-oxohexanoyl)-L-homoserine lactone (3-oxo-C6-HSL) and N-Octanoyl-L-homoserine lactone (C8-HSL) were confirmed as acylated homoserine lactones (AHLs) signal molecule produced by E. carotovora and P. fluorescens, respectively. In addition, quorum sensing inhibitory (QSI) effects of hexanal on AHLs production were evaluated. Hexanal at 1/2 minimum inhibitory concentration (MIC) was achieved 76.27% inhibitory rate of 3-oxo-C6-HSL production in E. carotovora and a inhibitory rate of C8-HSL (60.78%) in P. fluorescens. The amount of biofilm formation and activity of extracellular enzymes treated with 1/2 MIC of hexanal were restored with different concentrations (10 ng/mL, 50 ng/mL, 100 ng/mL) of exogenous AHLs (P < 0.05), which verified QSI effect of hexanal on biofilm and extracellular enzymes were due to its inhibition on AHLs production. Molecular docking analysis showed that hexanal could interact with EcbI and PcoI protein to disrupt AHLs production. Furthermore, results showed that sub-MICs of hexanal could suppress expressions of ecbI and pcoI genes in AHL-mediated QS system of E. carotovora and P. fluorescens. This study provides theoretical support for the application of essential oils as QS inhibitors in the preservation of vegetables.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13197-022-05624-9.

Antimicrobial Activity of Lactones

The development of bacterial resistance to antibiotics and the consequent lack of effective therapy is one of the biggest problems in modern medicine. A consequence of these processes is an urgent need to continuously design and develop novel antimicrobial agents. Among the compounds showing antimicrobial potential, lactones are a group to explore. For centuries, their antimicrobial activities have been used in folk medicine. Currently, novel lactone compounds are continuously described in the literature. Some of those structures exhibit high antimicrobial potential and some are an inspiration for design and synthesis of future drugs. This paper describes recent developments on antimicrobial lactones with smaller ring sizes, up to seven membered ε-lactones. Their isolation from natural sources, chemical synthesis, synergistic activity with antibiotics, and effects on quorum sensing are presented herein.

Molecular Relationships in Biofilm Formation and the Biosynthesis of Exoproducts in Pseudoalteromonas spp

Most members of the Pseudoalteromonas genus have been isolated from living surfaces as members of epiphytic and epizooic microbiomes on marine macroorganisms. Commonly Pseudoalteromonas isolates are reported as a source of bioactive exoproducts, i.e., secondary metabolites, such as exopolymeric substances and extracellular enzymes. The experimental conditions for the production of these agents are commonly associated with sessile metabolic states such as biofilms or liquid cultures in the stationary growth phase. Despite this, the molecular mechanisms that connect biofilm formation and the biosynthesis of exoproducts in Pseudoalteromonas isolates have rarely been mentioned in the literature. This review compiles empirical evidence about exoproduct biosynthesis conditions and molecular mechanisms that regulate sessile metabolic states in Pseudoalteromonas species, to provide a comprehensive perspective on the regulatory convergences that generate the recurrent coexistence of both phenomena in this bacterial genus. This synthesis aims to provide perspectives on the extent of this phenomenon for the optimization of bioprospection studies and biotechnology processes based on these bacteria.

Complete Genome Analysis Reveals the Quorum Sensing-Related Spoilage Potential of Pseudomonas fluorescens PF08, a Specific Spoilage Organism of Turbot (Scophthalmus maximus)

Pseudomonas fluorescens is a common specific spoilage organism (SSO) of aquatic products. The spoilage ability of SSO can be regulated by the quorum sensing (QS) system. However, the QS system in P. fluorescens and their relationship with the spoilage potential have not been systematically analyzed. In the present study, the complete genome of P. fluorescens PF08 isolated from spoilage turbot was sequenced. The identification of key genes that involved in the QS, enzyme synthesis, sulfur, and amino acid metabolism explained the spoilage potential of P. fluorescens PF08. Results of quantitative real-time PCR revealed the key role of the P. fluorescens PF08 QS system in regulating the transcription of spoilage-related genes and its sensitivity to environmental stress. These findings provide insight into the spoilage features of P. fluorescens PF08 from a genomic perspective. The knowledge may be valuable in the development of new strategies for the targeted inhibition of aquatic product spoilage based on QS interference.

Characterization of differentiated autoregulation of LuxI/LuxR-type quorum sensing system in Pseudoalteromonas

Gram-negative bacteria usually use acyl-homoserine lactones (AHLs)-mediated LuxI/LuxR-type quorum sensing (QS) systems for cell-cell cooperation and/or bacteria-environment communication. LuxI and LuxR are AHLs synthase and receptor, respectively. These two parts could form a positive regulatory feedback loop, controlling various types of group behaviors. However, the autoregulation mechanisms between them are fragmented and could be highly differentiated in different bacteria. Here, we clarified the autoregulation mechanism between LuxI and LuxR in Pseudoalteromonas sp. R3. YasI (LuxI in strain R3) synthesizes two types of AHLs, C8-HSL and 3-OH-C8-HSL. It is worth noting that YasR (LuxR in strain R3) only responds to C8-HSL rather than 3-OH-C8-HSL. YasR-C8HSL can activate the yasI transcription by recognizing "lux box" at yasI upstream. Interestingly, YasR can directly promote the yasR expression with AHL-independent manner, but AHL absence caused by the yasI-deficiency led to the significant decrease in the yasR expression. Further study demonstrated that the yasI-deficiency can result in the decrease in the yasR mRNA stability. Notably, both yasI-deficiency and yasR-deficiency led to the significant decrease in the expression of hfq encoding RNA chaperone. Therefore, it was speculated that not only YasR itself can directly regulate the yasR transcription, but YasR-C8HSL complex indirectly affects the yasR mRNA stability by regulating Hfq.

Potential microbial functions and quorum sensing systems in partial nitritation and anammox processes

Diverse microbial communities coexist in the partial nitritation-anaerobic ammonium oxidation (PNA) process, in which nitrogen metabolism and information exchange are two important microbial interactions. In the PNA process, the existence of diverse microorganisms including nitrifiers, anammox bacteria, and heterotrophs makes it challenging to achieve a balanced relationship between anaerobic ammonium oxidation bacteria and ammonia oxidizing bacteria. In this study, potential microbial functions in nitrogen conversion and acyl-homoserine lactones (AHLs)-based quorum sensing (QS) in PNA processes were examined. Candidatus_Kuenenia and Nitrosomonas were the key functional bacteria responsible for PNA, while Nitrospira was detected as the dominant nitrite oxidizing bacteria (NOB). Heterotrophs containing nxr might play a similar function to NOB. The AHLs-QS system was an important microbial communication pathway in PNA systems. N-octanoyl-L-homoserine lactone, N-decanoyl homoserine lactone, and N-dodecanoyl homoserine lactone were the main AHLs, which might be synthesized by nitrogen converting microorganisms and heterotrophs. However, only heterotrophs had the potential to sense and degrade AHLs, such as Saccharophagus (sensing) and Leptospira (degradation). These results provide comprehensive information about the possible microbial functions and interactions in the PNA system and clues for system optimization from a microbial perspective. PRACTITIONER POINTS: ●Potential functions of anammox bacteria, nitrifiers, and heterotrophs were revealed. ●Diverse nitrogen conversion and AHLs-quorum sensing related genes were detected. ●Anammox bacteria and AOB played important roles in the AHLs synthesis process. ●Heterotrophs could sense and degrade AHLs during information exchange.

Hfq and sRNA00002 positively regulate the LuxI/LuxR-type quorum sensing system in Pseudoalteromonas

Pseudoalteromonas spp. are Gram-negative bacteria which are ubiquitous in marine environments. Our previous work found that there is a classic LuxI/LuxR-type quorum sensing (QS) system which was named YasI/YasR in Pseudoalteromonas sp. R3, but the factors that control QS in strain R3 are unclear yet. Here, we found that the deficiency of hfq encoding RNA chaperon Hfq down-regulated the transcription levels of yasI encoding acyl-homoserine lactones (AHLs) synthase and yasR encoding AHLs receptor in strain R3. The assay based on fusion reporter of yasI-lacZ showed that Hfq regulates the expression of yasR at both transcriptional and translational levels. In addition, Hfq affects the expression of yasI via yasR. Further analysis indicated that the 5'UTR region of yasR is necessary for Hfq to control QS. In addition, the deletion of hfq increases the unstability of the target yasR mRNA. Based on transcriptome sequencing and bioinformatic analysis together with molecular experiments, Hfq-dependent sRNA00002 was identified to be involved in positively regulating QS in Pseudoalternas sp. R3. It was found that sRNA00002 deficiency causes the decrease in expression of yasI and yasR, and thus abolishes the production of AHLs in strain R3. It was concluded that Hfq-dependent sRNA00002 regulates yasR expression by base-pairing with target yasR mRNA at 5'UTR region and altering the stability of yasR mRNA. Our work paves the way for understanding the regulation mechanism of Hfq-dependent sRNAs on QS in Pseudoalteromonas.

Stringent Starvation Protein Regulates Prodiginine Biosynthesis via Affecting Siderophore Production in Pseudoalteromonas sp. Strain R3

Prodiginines are a family of red-pigmented secondary metabolites with multiple biological activities. The biosynthesis of prodiginines is affected by various physiological and environmental factors. Thus, prodiginine biosynthesis regulation is highly complex and multifaceted. Although the regulatory mechanism for prodiginine biosynthesis has been extensively studied in Serratia and Streptomyces species, little is known about that in the marine betaproteobacterium Pseudoalteromonas In this study, we report that stringent starvation protein A (SspA), an RNA polymerase-associated regulatory protein, is required for the biosynthesis of prodiginine in Pseudoalteromonas sp. strain R3. The strain lacking sspA (ΔsspA) fails to produce prodiginine, which resulted from the downregulation of the prodiginine biosynthetic gene (pig) cluster. The effect of SspA on prodiginine biosynthesis is independent of histone-like nucleoid structuring protein (H-NS) and RpoS (σ^S). Further analysis demonstrates that the ΔsspA strain has a significant decrease in the transcription of the siderophore biosynthesis gene (pvd) cluster, leading to the inhibition of siderophore production and iron uptake. The ΔsspA strain regains the ability to synthesize prodiginine by cocultivation with siderophore producers or the addition of iron. Therefore, we conclude that SspA-regulated prodiginine biosynthesis is due to decreased siderophore levels and iron deficiency. We further show that the iron homeostasis master regulator Fur is also essential for pig transcription and prodiginine biosynthesis. Overall, our results suggest that SspA indirectly regulates the biosynthesis of prodiginine, which is mediated by the siderophore-dependent iron uptake pathway.IMPORTANCE The red-pigmented prodiginines are attracting increasing interest due to their broad biological activities. As with many secondary metabolites, the biosynthesis of prodiginines is regulated by both environmental and physiological factors. At present, studies on the regulation of prodiginine biosynthesis are mainly restricted to Serratia and Streptomyces species. This work focused on the regulatory mechanism of prodiginine biosynthesis in Pseudoalteromonas sp. R3. We found that stringent starvation protein A (SspA) positively regulates prodiginine biosynthesis via affecting the siderophore-dependent iron uptake pathway. The connections among SspA, iron homeostasis, and prodiginine biosynthesis were investigated. These findings uncover a novel regulatory mechanism for prodigiosin biosynthesis.

Use of Quorum Sensing Inhibition Strategies to Control Microfouling

Interfering with the quorum sensing bacterial communication systems has been proposed as a promising strategy to control bacterial biofilm formation, a key process in biofouling development. Appropriate in vitro biofilm-forming bacteria models are needed to establish screening methods for innovative anti-biofilm and anti-microfouling compounds. Four marine strains, two Pseudoalteromonas spp. and two Vibrio spp., were selected and studied with regard to their biofilm-forming capacity and sensitivity to quorum sensing (QS) inhibitors. Biofilm experiments were performed using two biofilm cultivation and quantification methods: the xCELLigence^® system, which allows online monitoring of biofilm formation, and the active attachment model, which allows refreshment of the culture medium to obtain a strong biofilm that can be quantified with standard staining methods. Although all selected strains produced acyl-homoserine-lactone (AHL) QS signals, only the P. flavipulchra biofilm, measured with both quantification systems, was significantly reduced with the addition of the AHL-lactonase Aii20J without a significant effect on planktonic growth. Two-species biofilms containing P. flavipulchra were also affected by the addition of Aii20J, indicating an influence on the target bacterial strain as well as an indirect effect on the co-cultured bacterium. The use of xCELLigence^® is proposed as a time-saving method to quantify biofilm formation and search for eco-friendly anti-microfouling compounds based on quorum sensing inhibition (QSI) strategies. The results obtained from these two in vitro biofilm formation methods revealed important differences in the response of biosensor bacteria to culture medium and conditions, indicating that several strains should be used simultaneously for screening purposes and the cultivation conditions should be carefully optimized for each specific purpose.

Stringent starvation protein A and LuxI/LuxR-type quorum sensing system constitute a mutual positive regulation loop in Pseudoalteromonas

Bacteria commonly exhibit social activities through acyl-homoserine lactones (AHLs)-based quorum sensing (QS) systems to form their unique social network. The sigma factor RpoS is an important regulator that controls QS system in different bacteria. However, the upstream of RpoS involving regulation on QS system remains unclear. In Escherichia coli RpoS is regulated by stringent starvation protein A (SspA), which is dependent of histone-like nucleoid structuring protein (H-NS). To date, the connection between SspA and QS system is essentially unknown. Here, we characterized a typical LuxI/LuxR-type QS system in marine bacterium Pseudoalteromonas sp. T1lg65 which can produce four types of AHLs. The luxI encoding AHLs synthase and luxR encoding AHLs-responsive receptor are co-transcribed, providing advantages in rapidly amplifying QS signaling. Notably, SspA positively regulated luxI/luxR transcription by activating RpoS expression, which is mediated by H-NS. Interestingly, LuxR in turn positively regulated SspA expression. Therefore, SspA and QS system constitute a mutual positive regulation loop in T1lg65. In view of the crucial roles of SspA and QS system in environmental adaption, we believe that the improvement of bacterial tolerance to marine environments could be related to rapidly tuning SspA-involved QS programming.

Alterocin, an Antibiofilm Protein Secreted by Pseudoalteromonas sp. Strain 3J6

We sought to identify and study the antibiofilm protein secreted by the marine bacterium Pseudoalteromonas sp. strain 3J6. The latter is active against marine and terrestrial bacteria, including Pseudomonas aeruginosa clinical strains forming different biofilm types. Several amino acid sequences were obtained from the partially purified antibiofilm protein, named alterocin. The Pseudoalteromonas sp. 3J6 genome was sequenced, and a candidate alt gene was identified by comparing the genome-encoded proteins to the sequences from purified alterocin. Expressing the alt gene in another nonactive Pseudoalteromonas sp. strain, 3J3, demonstrated that it is responsible for the antibiofilm activity. Alterocin is a 139-residue protein that includes a predicted 20-residue signal sequence, which would be cleaved off upon export by the general secretion system. No sequence homology was found between alterocin and proteins of known functions. The alt gene is not part of an operon and adjacent genes do not seem related to alterocin production, immunity, or regulation, suggesting that these functions are not fulfilled by devoted proteins. During growth in liquid medium, the alt mRNA level peaked during the stationary phase. A single promoter was experimentally identified, and several inverted repeats could be binding sites for regulators. alt genes were found in about 30% of the Pseudoalteromonas genomes and in only a few instances of other marine bacteria of the Hahella and Paraglaciecola genera. Comparative genomics yielded the hypothesis that alt gene losses occurred within the Pseudoalteromonas genus. Overall, alterocin is a novel kind of antibiofilm protein of ecological and biotechnological interest.IMPORTANCE Biofilms are microbial communities that develop on solid surfaces or interfaces and are detrimental in a number of fields, including for example food industry, aquaculture, and medicine. In the latter, antibiotics are insufficient to clear biofilm infections, leading to chronic infections such as in the case of infection by Pseudomonas aeruginosa of the lungs of cystic fibrosis patients. Antibiofilm molecules are thus urgently needed to be used in conjunction with conventional antibiotics, as well as in other fields of application, especially if they are environmentally friendly molecules. Here, we describe alterocin, a novel antibiofilm protein secreted by a marine bacterium belonging to the Pseudoalteromonas genus, and its gene. Alterocin homologs were found in about 30% of Pseudoalteromonas strains, indicating that this new family of antibiofilm proteins likely plays an important albeit nonessential function in the biology of these bacteria. This study opens up the possibility of a variety of applications.

The LuxI/LuxR-Type Quorum Sensing System Regulates Degradation of Polycyclic Aromatic Hydrocarbons via Two Mechanisms

Members of the Sphingomonadales are renowned for their ability to degrade polycyclic aromatic hydrocarbons (PAHs). However, little is known about the regulatory mechanisms of the degradative pathway. Using cross-feeding bioassay, a functional LuxI/LuxR-type acyl-homoserine lactone (AHL)-mediated quorum sensing (QS) system was identified from Croceicoccus naphthovorans PQ-2, a member of the order Sphingomonadales. Inactivation of the QS system resulted in a significant decrease in PAHs degradation. The QS system positively controlled the expression of three PAH-degrading genes (ahdA1e, xylE and xylG) and a regulatory gene ardR, which are located on the large plasmid. Interestingly, the transcription levels of these three PAH-degrading genes were significantly down-regulated in the ardR mutant. In addition, bacterial cell surface hydrophobicity and cell morphology were altered in the QS-deficient mutant. Therefore, the QS system in strain PQ-2 positively regulates PAH degradation via two mechanisms: (i) by induction of PAH-degrading genes directly and/or indirectly; and (ii) by an increase of bacterial cell surface hydrophobicity. The findings of this study improve our understanding of how the QS system influences the degradation of PAHs, therefore facilitating the development of new strategies for the bioremediation of PAHs.