Selection of the N-Acylhomoserine Lactone-Degrading Bacterium Alteromonas stellipolaris PQQ-42 and of Its Potential for Biocontrol in Aquaculture

Interference of AHL signal production in the phytophatogen Pantoea agglomerans as a sustainable biological strategy to reduce its virulence

Pantoea agglomerans is considered one of the most ubiquitous and versatile organisms that include strains that induce diseases in various crops and occasionally cause opportunistic infections in humans. To develop effective strategies to mitigate its impact on plant health and agricultural productivity, a comprehensive investigation is crucial for better understanding its pathogenicity. One proposed eco-friendly approach involves the enzymatic degradation of quorum sensing (QS) signal molecules like N-acylhomoserine lactones (AHLs), known as quorum quenching (QQ), offering potential treatment for such bacterial diseases. In this study the production of C4 and 3-oxo-C6HSL was identified in the plant pathogenic P. agglomerans CFBP 11141 and correlated to enzymatic activities such as amylase and acid phosphatase. Moreover, the heterologous expression of a QQ enzyme in the pathogen resulted in lack of AHLs production and the attenuation of the virulence by mean of drastically reduction of soft rot disease in carrots and cherry tomatoes. Additionally, the interference with the QS systems of P. agglomerans CFBP 11141 by two the plant growth-promoting and AHL-degrading bacteria (PGP-QQ) Pseudomonas segetis P6 and Bacillus toyonensis AA1EC1 was evaluated as a potential biocontrol approach for the first time. P. segetis P6 and B. toyonensis AA1EC1 demonstrated effectiveness in diminishing soft rot symptoms induced by P. agglomerans CFBP 11141 in both carrots and cherry tomatoes. Furthermore, the virulence of pathogen notably decreased when co-cultured with strain AA1EC1 on tomato plants.

Alleviating Coral Thermal Stress via Inoculation with Quorum Quenching Bacteria

In the background of global warming, coral bleaching induced by elevated seawater temperature is the primary cause of coral reef degradation. Coral microbiome engineering using the beneficial microorganisms for corals (BMCs) has become a hot spot in the field of coral reef conservation and restoration. Investigating the potential of alleviating thermal stress by quorum quenching (QQ) bacteria may provide more tools for coral microbial engineering remediation. In this study, QQ bacteria strain Pseudoalteromonas piscicida SCSIO 43740 was screened among 75 coral-derived bacterial strains, and its quorum sensing inhibitor (QSI) compound was isolated and identified as 2,4-di-tert-butylphenol (2,4-DTBP). Then, the thermal stress alleviating potential of QQ bacteria on coral Pocillopora damicornis was tested by a 30-day controlled experiment with three different treatments: control group (Con: 29 °C), high temperature group (HT: 31 °C), and the group of high temperature with QQ bacteria inoculation (HTQQ: 31 °C + QQ bacteria). The results showed that QQ bacteria SCSIO 43740 inoculation can significantly mitigate the loss of symbiotic algae and impairment of photosynthesis efficiency of coral P. damicornis under thermal stress. Significant difference in superoxide dismutase (SOD) and catalase (CAT) enzyme activities between HT and HTQQ was not observed. In addition, QQ bacteria inoculation suppressed the coral microbial community beta-dispersion and improved the stability of microbial co-occurrence network under thermal stress. It was suggested that QQ bacteria inoculation can alleviate coral thermal stress via reshaping microbial interaction and maintain community stability of coral microbiome. This study provided new evidence for the probiotic function of QQ bacteria in corals, which shedding light on the development of new microbiological tools for coral reef conservation.

Review of quorum-quenching probiotics: A promising non-antibiotic-based strategy for sustainable aquaculture

The emergence of antibiotic-resistant bacteria (ARBs) and genes (ARGs) in aquaculture underscores the urgent need for alternative veterinary strategies to combat antimicrobial resistance (AMR). These measures are vital to reduce the likelihood of entering a post-antibiotic era. Identifying environmentally friendly biotechnological solutions to prevent and treat bacterial diseases is crucial for the sustainability of aquaculture and for minimizing the use of antimicrobials, especially antibiotics. The development of probiotics with quorum-quenching (QQ) capabilities presents a promising non-antibiotic strategy for sustainable aquaculture. Recent research has demonstrated the effectiveness of QQ probiotics (QQPs) against a range of significant fish pathogens in aquaculture. QQ disrupts microbial communication (quorum sensing, QS) by inhibiting the production, replication, and detection of signalling molecules, thereby reducing bacterial virulence factors. With their targeted anti-virulence approach, QQPs have substantial promise as a potential alternative to antibiotics. The application of QQPs in aquaculture, however, is still in its early stages and requires additional research. Key challenges include determining the optimal dosage and treatment regimens, understanding the long-term effects, and integrating QQPs with other disease control methods in diverse aquaculture systems. This review scrutinizes the current literature on antibiotic usage, AMR prevalence in aquaculture, QQ mechanisms and the application of QQPs as a sustainable alternative to antibiotics.

Feeding Strategies with Multi-Strain Probiotics Affect Growth, Health Condition, and Disease Resistance in Asian Seabass (Lates calcarifer)

A 16-week feeding trial was done to examine the impacts of continuous feeding (CF) or pulse-feeding (PF) of multi-strain probiotics on Asian seabass (Lates calcarifer, 30.0 ± 0.1 g) juveniles. In this study, three different multi-strain probiotic mixtures were added to a basal diet, including (I) a mixture of different strains of Lactobacillus plantarum, (II) a mixture of the first probiotic (I) + L. delbrueckii sub bulgaricus, L. rhamnosus and L. acidophilus, and (III) a mixture of the second probiotic (II) + two quorum quenching (QQ) bacteria (Bacillus thuringiensis QQ1 and B. cereus QQ2). CF (every day) or PF (every two weeks) strategies were applied for using the abovementioned probiotics to design seven experimental groups including C (control, without probiotics), CF-I (continuous feeding of fish with the probiotic mixture I), CF-II (continuous feeding of fish with the probiotic mixture II), CF-III (continuous feeding of fish with the probiotic mixture III), PF-I (pulse-feeding of fish with the probiotic mixture I), PF-II (pulse-feeding of fish with the probiotic mixture II), and PF-III (pulse-feeding of fish with the probiotic mixture III). Four hundred and twenty fish were stocked into 21 circular polyethylene tanks with 220 L volume (20 fish/tank). Each dietary treatment had three replicates. Tanks were supplied with seawater (temperature = 30.5 °C, salinity = 45 g L-1) in a flow-throw system. Fish in CF-I, CF-II, and CF-III had higher growth rate (ca. 113-145%) and better feed conversion ratio than fish fed C and PF-I (P < 0.05). Fish in the CF-III group had the highest protease activity. Continuous feeding strategy resulted in a higher amount of glutathione and catalase activities in both the liver and plasma as well as higher superoxide dismutase activity in the liver of fish. Pulse-feeding strategy resulted in lower plasma lactate dehydrogenase and aspartate aminotransferase levels than the CF strategy. Regardless of feeding strategy, different probiotic mixtures significantly enhanced blood hemoglobin and hematocrit levels compared to the control. Continuous feeding with the multi-strain probiotics resulted in a higher survival rate against Vibrio harveyi than the PF method. Continuous feeding induced higher mRNA transcription levels of granulocyte-macrophage colony-forming cells and interleukin 10 genes in the gut of fish than PF strategy. In conclusion, continuous feeding with multi-strain probiotics is better than pulse-feeding on growth, feed utilization, antioxidant capacity, and the gut's immune-related genes and led to higher resistance of L. calcarifer in challenge with V. harveyi.

Interactions between quorum sensing/quorum quenching and virulence genes may affect coral health by regulating symbiotic bacterial community

Quorum sensing (QS) and quorum quenching (QQ) are two antagonistic processes that may regulate the composition, function and structure of bacterial community. In coral holobiont, autoinducers signaling mediate the communication pathways between interspecies and intraspecies bacteria, which regulate the expression of the virulence factors that can damage host health. However, under environmental stressors, the interaction between the QS/QQ gene and virulence factors and their role in the bacterial communities and coral bleaching is still not fully clear. To address this question, here, metagenomics method was used to examine the profile of QS/QQ and virulence genes from a deeply sequenced microbial database, obtained from three bleached and non-bleached corals species. The prediction of bacterial genes of bleached samples involved in functional metabolic pathways were remarkably decreased, and the bacterial community structure on bleached samples was significantly different compared to non-bleached samples. The distribution and significant difference in QS/QQ and virulence genes were also carried out. We found that Proteobacteria was dominant bacteria among all samples, and AI-1 system is widespread within this group of bacteria. The identified specific genes consistently exhibited a trend of increased pathogenicity in bleached corals relative to non-bleached corals. The abundance of pathogenicity-associated QS genes, including bapA, pfoA and dgcB genes, were significantly increased in bleached corals and can encode the protein of biofilm formation and the membrane damaging toxins promoting pathogenic adhesion and infection. Similarly, the virulence genes, such as superoxide dismutase (Mn-SOD gene), metalloproteinase (yme1, yydH and zmpB), glycosidases (malE, malF, malG, and malK) and LodAB (lodB) genes significantly increased. Conversely, QQ genes that inhibit QS activity and virulence factors to defense the pathogens, including blpA, lsrK, amiE, aprE and gmuG showed a significant decrease in bleached groups. Furthermore, the significant correlations were found among virulence, QS/QQ genes, and coral associated bacterial community, and the virulence genes interact with key QS/QQ genes, directly or indirectly influence symbiotic bacterial communities homeostasis, thereby impacting coral health. It suggested that the functional and structural divergence in the symbiont bacteria may be partially attribute to the interplay, involving interactions among the host, bacterial communication signal systems, and bacterial virulence factors. In conclusion, these data helped to reveal the characteristic behavior of coral symbiotic bacteria, and facilitated a better understanding of bleaching mechanism from a chemical ecological perspective.

Efficacy of ceftiofur N-acyl homoserine lactonase niosome in the treatment of multi-resistant Klebsiella pneumoniae in broilers

In this study, the efficiency of the ceftiofur N-acyl homoserine lactonase niosome against multi-resistant Klebsiella pneumoniae in broilers was evaluated. Fifty-six K. pneumoniae isolates previously recovered from different poultry and environmental samples were screened for the ahlK gene. The lactonase enzyme was extracted from eight quorum-quenching isolates. The niosome was formulated, characterized, and tested for minimal inhibitory concentration (MIC) and cytotoxicity. Fourteen-day-old chicks were assigned to six groups: groups Ӏ and П served as negative and positive controls, receiving saline and K. pneumoniae solutions, respectively. In groups Ш and IV, ceftiofur and niosome were administrated intramuscularly at a dose of 10 mg/kg body weight for five consecutive days, while groups V and VI received the injections following the K. pneumoniae challenge. Signs, mortality, and gross lesions were recorded. Tracheal swabs were collected from groups П, V, and VI for counting K. pneumoniae. Pharmacokinetic parameters were evaluated in four treated groups at nine-time points. The niosome was spherical and 56.5 ± 4.41 nm in size. The viability of Vero cells was unaffected up to 5 × MIC (2.4 gml-1). The niosome-treated challenged group showed mild signs and lesions with lower mortality and colony count than the positive control group. The maximum ceftiofur serum concentrations in treated groups were observed 2 h following administration. The elimination half-life in niosome-treated groups was longer than that reported in ceftiofur-treated groups. This is the first report of the administration of N-acyl homoserine lactonase for the control of multi-resistant K. pneumoniae infections in poultry.

Probiotic Supplementation as an Emerging Alternative to Chemical Therapeutics in Finfish Aquaculture: a Review

Aquaculture is a promising food sector to fulfil nutritional requirements of growing human population. Live weight aquaculture production reached up to 114.5 million tonnes in 2018 and it is further expected to grow by 32% by year 2030. Among total aquaculture production, major product harvested is finfish and its contribution has reached 46% in recent years. Frequent outbreaks of infectious diseases create obstacle in finfish production, result in economic losses to the farmers and threaten the sustainability of aquaculture industry itself. In spite of following the best management practices, the use of antibiotics, chemotherapeutics and phytochemicals often become the method of choice in finfish culture. Among these, phytochemicals have shown lesser effect in animal welfare while antibiotics and other chemotherapeutics have led to negative consequences like emergence of drug-resistant bacteria, and accumulation of residues in host and culture system, resulting in quality degradation of aqua products. Making use of probiotics as viable alternative has paved a way for sustainable aquaculture and minimise the use of antibiotics and other chemotherapeutics that pose adverse effect on host and culture system. This review paper elucidates the knowledge about antibiotics and other chemicals, compilation of probiotics and their effects on health status of finfish as well as overall culture environment. Besides, concoction of probiotics and prebiotics for simultaneous application has also been discussed briefly.

Quorum sensing signals: Aquaculture risk factor

Bacteria produce several virulence factors and cause massive mortality in fish and crustaceans. Abundant quorum sensing (QS) signals and high cell density are essentially required for the production of such virulence factors. Although several strategies have been developed to control aquatic pathogens through antibiotics and QS inhibition, the impact of pre‐existing QS signals in the aquatic environment has been overlooked. QS signals cause detrimental effects on mammalian cells and induce cell death by interfering with multiple cellular pathways. Moreover, QS signals not only function as a messenger, but also annihilate the functions of the host immune system which implies that QS signals should be designated as a major virulence factor. Despite QS signals' role has been well documented in mammalian cells, their impact on aquatic organisms is still at the budding stage. However, many aquatic organisms produce enzymes that degrade and detoxify such QS signals. In addition, physical and chemical factors also determine the stability of the QS signals in the aqueous environment. The balance between QS signals and existing QS signals degrading factors essentially determines the disease progression in aquatic organisms. In this review, we highlight the impact of QS signals on aquatic organisms and further discussed potential alternative strategies to control disease progression.

Diversity of Bacteria with Quorum Sensing and Quenching Activities from Hydrothermal Vents in the Okinawa Trough

Quorum sensing (QS) is a chemical communication system by which bacteria coordinate gene expression and social behaviors. Quorum quenching (QQ) refers to processes of inhibiting the QS pathway. Deep-sea hydrothermal vents are extreme marine environments, where abundant and diverse microbial communities live. However, the nature of chemical communication in bacteria inhabiting the hydrothermal vent is poorly understood. In this study, the QS and QQ activities with N-acyl homoserine lactones (AHLs) as the autoinducer were detected in bacteria isolated from hydrothermal vents in the Okinawa Trough. A total of 18 and 108 isolates possessed AHL-producing and AHL-degrading abilities, respectively. Bacteria mainly affiliated with Rhodobacterales, Hyphomicrobiales, Enterobacterales and Sphingomonadales showed QS activities; QQ was mainly associated with Bacillales, Rhodospirillales and Sphingomonadales. The results showed that the bacterial QS and QQ processes are prevalent in hydrothermal environments in the Okinawa Trough. Furthermore, QS significantly affected the activities of extracellular enzymes represented by β-glucosidase, aminopeptidase and phosphatase in the four isolates with higher QS activities. Our results increase the current knowledge of the diversity of QS and QQ bacteria in extreme marine environments and shed light on the interspecific relationships to better investigate their dynamics and ecological roles in biogeochemical cycling.

The implications of quorum sensing inhibition in bacterial antibiotic resistance- with a special focus on aquaculture

The aquaculture industry is an expanding and demanding industry and due to an increase in urbanization, with rise in income of developing countries population, it offers to provide a sustainable food supply. However, the industry is facing a number of challenges, out of which few needs to be tackled immediately to maximise the productivity. An upcoming problem is the emergence of antibiotic resistant pathogens due to the unchecked use of antibiotics in aquaculture and human clinical practices. A wide variety of aquatic pathogens such as Edwardsiella, Vibrio, and Aeromonas spp. use quorum sensing (QS) systems, a regulatory process involving cell communication via signalling molecules for the collective function of pathogens which regulates the genes expression including virulent genes. Quorum sensing results in bacterial biofilms formation, which leads to their reduced susceptibility towards antimicrobial agents. The usage of quorum sensing inhibitors (QSIs) has been proposed as an attractive strategy to tackle this problem. Due to the modulation of virulence genes expression, QSIs can be used as novel and viable approach to overcome antibiotic resistance in aquaculture. In this review, we direct our attention to the quorum sensing phenomenon and its viability as a target pathway for tackling the ever-growing problem of antimicrobial resistance in aquaculture. This review also provides a concise compilation of the currently available QSIs and investigates possible natural sources for quorum quenching.

Effect of fermentation pH on the structure, rheological properties, and antioxidant activities of exopolysaccharides produced by Alteromonas australica QD

The pH value was essential for the growth and metabolism of microorganisms. Acidic pH exopolysaccharide (AC-EPS) and alkaline pH exopolysaccharide (AL-EPS) secreted by A. australica QD mediated by pH were studied in this paper. The total carbohydrate content and molecular weight of AC-EPS (79.59% ± 2.24% (w/w), 8.374 × 10⁵ Da) and AL-EPS (82.48% ± 1.46% (w/w), 6.182 × 10⁵ Da) were estimated and compared. In AC-EPS, mannose (3.78%) and galactose (3.24%) content was more, while the proportion of glucuronic acid was less in comparison to AL-EPS. The scanning electron microscopy revealed the structural differences among the AC-EPS and AL-EPS. Thermogravimetric analysis showed degradation temperatures of 272.8 °C and 244.9 °C for AC-EPS and AL-EPS, respectively. AC-EPS was found to exhibit better rheological properties and emulsifying capabilities, while AL-EPS had superior antioxidant activities. Overall, both AC-EPS and AL-EPS have the potential to be used as emulsifiers and biological antioxidants.

Peribacillus castrilensis sp. nov.: A Plant-Growth-Promoting and Biocontrol Species Isolated From a River Otter in Castril, Granada, Southern Spain

A strictly aerobic, chemoheterotrophic, endospore-forming, Gram-positive, rod-shaped bacterial strain N3^T was isolated from the feces of a river otter in Castril (Granada, southern Spain). It is halotolerant, motile, and catalase-, oxidase-, ACC deaminase-, and C4- and C8-lipase-positive. It promotes tomato plant growth and can reduce virulence in Erwinia amylovora CECT 222^T and Dickeya solani LMG 25993^T through interference in their quorum-sensing systems, although other antagonistic mechanisms could also occur. A phylogenetic analysis of the 16S rRNA gene sequence as well as the phenotypic and phylogenomic analyses indicated that the strain N3^T is a novel species of the genus Peribacillus, with the highest 16S rRNA sequence similar to that of Bacillus frigoritolerans DSM 8801^T (99.93%) and Peribacillus simplex DSM 1321^T (99.80%). Genomic digital DNA-DNA hybridization (dDDH) between the strain N3^T and Bacillus frigoritolerans DSM 8801^T and Peribacillus simplex was 12.8 and 69.1%, respectively, and the average nucleotide identity (ANIb) of strain N3^T and Bacillus frigoritolerans DSM 8801^T and Peribacillus simplex was 67.84 and 93.21%, respectively. The genomic G + C content was 40.3 mol%. Its main cellular fatty acids were anteiso-C_15:0 and iso-C_15:0. Using 16S rRNA phylogenetic and in silico phylogenomic analyses, together with the chemotaxonomic and phenotypic data, we demonstrated that the type strain N3^T (=CECT 30509^T = LMG 32505^T) is a novel species of the genus Peribacillus and the name Peribacillus castrilensis sp. nov. is proposed.

Isolation of Bacteria Capable of Degrading Various AHLs for Biofouling Control in Membrane Bioreactors

Membrane bioreactors (MBRs) are widely used to treat wastewater, mainly due to the production of high-quality effluent. However, biofilm forming on the surface of membranes can cause many problems, which remains one of the major limitations of this technique. Bacterial quorum quenching (QQ) has been proven to be a successful strategy to control biofouling in MBRs. However, for many QQ bacterial isolates, the detailed degradation rates of acyl homoserine lactones (AHLs) have rarely been reported. Therefore, this study aimed to isolate potential QQ bacteria and investigate their degradation rates against eight different AHLs. Results showed that four isolates (A9, A12, B11, and D3) exhibited consistent C8-HSL–(N-octanoyl-L-homoserine lactone) removal capabilities. These four isolates removed at least 70% of all AHLs tested within 180 min. They might have different QQ enzymes, based on our observation that the locations of enzyme activities differed. The bacteria most closely related to A9, A12, and B11 were Brucella anthropic, Bacillus cereus, and Bacillus toyonensis, respectively. Bacillus species have shown QQ activity in many studies, but AHL-reducing Brucella species have not been previously reported. Overall, this study extends our current knowledge of QQ bacteria that could be used to mitigate biofilm formation on MBR membranes.

Quorum Quenching Strains Isolated from the Microbiota of Sea Anemones and Holothurians Attenuate Vibriocorallilyticus Virulence Factors and Reduce Mortality in Artemiasalina

Interference with quorum-sensing (QS) intercellular communication systems by the enzymatic disruption of N-acylhomoserine lactones (AHLs) in Gram-negative bacteria has become a promising strategy to fight bacterial infections. In this study, seven strains previously isolated from marine invertebrates and selected for their ability to degrade C6 and C10-HSL, were identified as Acinetobacter junii, Ruegeria atlantica, Microbulbifer echini, Reinheimera aquimaris, and Pseudomonas sihuiensis. AHL-degrading activity against a wide range of synthetic AHLs were identified by using an agar well diffusion assay and Agrobacterium tumefaciens NTL4 and Chromobacterium violaceum CV026 and VIR07 as biosensors. High-performance liquid chromatography-mass spectrometry (HPLC-MS) analysis indicated that this activity was not due to an AHL lactonase. All the strains degraded Vibrio coralliilyticus AHLs in coculture experiments, while some strains reduced or abolished the production of virulence factors. In vivo assays showed that strains M3-111 and M3-127 reduced this pathogen’s virulence and increased the survival rate of Artemia salina up to 3-fold, indicating its potential use for biotechnological purposes. To our knowledge, this is the first study to describe AHL-degrading activities in some of these marine species. These findings highlight that the microbiota associated with marine invertebrates constitute an important underexplored source of biological valuable compounds.

Targeting Acyl Homoserine Lactones (AHLs) by the quorum quenching bacterial strains to control biofilm formation in Pseudomonas aeruginosa

Navigating novel biological strategies to mitigate bacterial biofilms have great worth to combat bacterial infections. Bacterial infections caused by the biofilm forming bacteria are 1000 times more resistant to antibiotics than the planktonic bacteria. Among the known bacterial infections, more than 70% involve biofilms which severely complicates treatment options. Biofilm formation is mainly regulated by the Quorum sensing (QS) mechanism. Interference with the QS system by the quorum quenching (QQ) enzyme is a potent strategy to mitigate biofilm. In this study, bacterial strains with QQ activity were identified and their anti-biofilm potential was investigated against the Multidrug Resistant (MDR) Pseudomonas aeruginosa. A Chromobacterium violaceum CV026 and Agrobacterium tumefaciens A136-based bioassays were used to confirm the degradation of different Acyl Homoserine Lactones (AHLs) by QQ isolates. The 16S rRNA gene sequencing of the isolated strains identified them as Bacillus cereus strain QSP03, B. subtilis strain QSP10, Pseudomonas putida strain QQ3 and P. aeruginosa strain QSP01. Biofilm mitigation potential of QQ isolates was tested against MDR P. aeruginosa and the results suggested that 50% biofilm reduction was observed by QQ3 and QSP01 strains, and around 60% reduction by QSP10 and QSP03 bacterial isolates. The presence of AHL degrading enzymes, lactonases and acylases, was confirmed by PCR based screening and sequencing of the already annotated genes aiiA, pvdQ and quiP. Altogether, these results exhibit that QQ bacterial strains or their products could be useful to control biofilm formation in P.aeruginosa.

Bacterial controlled mitigation of dysbiosis in a seaweed disease

Disease in the marine environment is predicted to increase with anthropogenic stressors and already affects major habitat-formers, such as corals and seaweeds. Solutions to address this issue are urgently needed. The seaweed Delisea pulchra is prone to a bleaching disease, which is caused by opportunistic pathogens and involves bacterial dysbiosis. Bacteria that can inhibit these pathogens and/or counteract dysbiosis are therefore hypothesised to reduce disease. This study aimed to identify such disease-protective bacteria and investigate their protective action. One strain, Phaeobacter sp. BS52, isolated from healthy D. pulchra, was antagonistic towards bleaching pathogens and significantly increased the proportion of healthy individuals when applied before the pathogen challenge (pathogen-only vs. BS52 + pathogen: 41-80%), and to a level similar to the control. However, no significant negative correlations between the relative abundances of pathogens and BS52 on D. pulchra were detected. Instead, inoculation of BS52 mitigated pathogen-induced changes in the epibacterial community. These observations suggest that the protective activity of BS52 was due to its ability to prevent dysbiosis, rather than direct pathogen inhibition. This study demonstrates the feasibility of manipulating bacterial communities in seaweeds to reduce disease and that mitigation of dysbiosis can have positive health outcomes.

Quorum quenching Bacillus spp.: an alternative biocontrol agent for Vibrio harveyi infection in aquaculture

Quorum sensing (QS) is a type of cell to cell communication in bacteria that can also regulate the virulence potential in pathogenic strains. Hence, QS disruption, i.e. the quorum quenching (QQ) mechanism, is presently being explored as a novel bio-control strategy to counter bacterial infections. In the present study, we characterized the QQ ability of Bacillus spp. strains to reduce the expression of some virulence factors of a shrimp pathogen, Vibrio harveyi. We screened a total of 118 spore-forming bacterial isolates from aquaculture ponds and mangrove soil for their ability to degrade the synthetic N-acyl-homoserine lactones (AHLs) C4-HSL, C6-HSL, C8-HSL, and C10-HSL. We then selected the top 17 isolates with high AHL-degradation ability for further study. Among them, B. subtilis MFB10, B. lentus MFB2, and B. firmus MFB7 had the highest ability for degradation. These 3 isolates suppressed the expression of virulence genes encoding protease, lipase, phospholipase, caseinase, chitinase, and gelatinase, and potentially inhibited the biofilm formation of V. harveyi MFB32. The reduction in expression of virulence genes like those coding for metalloprotease, serine protease, and haemolysin were confirmed by real-time PCR analysis. Moreover, in an in vivo challenge experiment, these Bacillus spp. protected Penaeus monodon post-larvae against V. harveyi MFB3 infection. Our results demonstrate the potential application of AHL-degrading Bacillus spp. as an alternative to antibiotics in shrimp hatcheries to control luminescent vibriosis. This novel bio-therapeutic method is a promising approach towards disease control in shrimp aquaculture.

Exploration of the Quorum-Quenching Mechanism in Pseudomonas nitroreducens W-7 and Its Potential to Attenuate the Virulence of Dickeya zeae EC1

Quorum quenching (QQ) is a novel, promising strategy that opens up a new perspective for controlling quorum-sensing (QS)-mediated bacterial pathogens. QQ is performed by interfering with population-sensing systems, such as by the inhibition of signal synthesis, catalysis of degrading enzymes, and modification of signals. In many Gram-negative pathogenic bacteria, a class of chemically conserved signaling molecules named N-acyl homoserine lactones (AHLs) have been widely studied. AHLs are involved in the modulation of virulence factors in various bacterial pathogens including Dickeya zeae. Dickeya zeae is the causal agent of plant-rot disease of bananas, rice, maize, potatoes, etc., causing enormous economic losses of crops. In this study, a highly efficient AHL-degrading bacterial strain W-7 was isolated from activated-sludge samples and identified as Pseudomonas nitroreducens. Strain W-7 revealed a superior ability to degrade N-(3-oxododecanoyl)-l-homoserine lactone (OdDHL) and completely degraded 0.2 mmol/L of OdDHL within 48 h. Gas chromatography-mass spectrometry (GC-MS) identified N-cyclohexyl-propanamide as the main intermediate metabolite during AHL biodegradation. A metabolic pathway for AHL in strain W-7 was proposed based on the chemical structure of AHL and intermediate products. In addition to the degradation of OdDHL, this strain was also found to be capable of degrading a wide range of AHLs including N-(3-oxohexanoyl)-l-homoserine lactone (OHHL), N-(3-oxooctanoyl)-l-homoserine lactone (OOHL), and N-hexanoyl-l-homoserine lactone (HHL). Moreover, the application of strain W-7 as a biocontrol agent could substantially attenuate the soft rot caused by D. zeae EC1 to suppress tissue maceration in various host plants. Similarly, the application of crude enzymes of strain W-7 significantly reduced the disease incidence and severity in host plants. These original findings unveil the biochemical aspects of a highly efficient AHL-degrading bacterial isolate and provide useful agents that exhibit great potential for the control of infectious diseases caused by AHL-dependent bacterial pathogens.

AHL-Lactonase Producing Psychrobacter sp. From Palk Bay Sediment Mitigates Quorum Sensing-Mediated Virulence Production in Gram Negative Bacterial Pathogens

Quorum sensing (QS) is a signaling mechanism governed by bacteria used to converse at inter- and intra-species levels through small self-produced chemicals called N-acylhomoserine lactones (AHLs). Through QS, bacteria regulate and organize the virulence factors’ production, including biofilm formation. AHLs can be degraded by an action called quorum quenching (QQ) and hence QQ strategy can effectively be employed to combat biofilm-associated bacterial pathogenesis. The present study aimed to identify novel bacterial species with QQ potential. Screening of Palk Bay marine sediment bacteria for QQ activity ended up with the identification of marine bacterial isolate 28 (MSB-28), which exhibited a profound QQ activity against QS biomarker strain Chromobacterium violaceum ATCC 12472. The isolate MSB-28 was identified as Psychrobacter sp. through 16S-rRNA sequencing. Psychrobacter sp. also demonstrated a pronounced activity in controlling the biofilm formation in different bacteria and biofilm-associated virulence factors’ production in P. aeruginosa PAO1. Solvent extraction, heat inactivation, and proteinase K treatment assays clearly evidence the enzymatic nature of the bioactive lead. Furthermore, AHL’s lactone ring cleavage was confirmed with experiments including ring closure assay and chromatographic analysis, and thus the AHL-lactonase enzyme production in Psychrobacter sp. To conclude, this is the first report stating the AHL-lactonase mediated QQ activity from marine sediment bacteria Psychrobacter sp. Future work deals with the characterization, purification, and mass cultivation of the purified protein and should pave the way to assessing the feasibility of the identified protein in controlling QS and biofilm-mediated multidrug resistant bacterial infections in mono or multi-species conditions.

Community Structure of Protease-Producing Bacteria Cultivated From Aquaculture Systems: Potential Impact of a Tropical Environment

Protease-producing bacteria play vital roles in degrading organic matter of aquaculture system, while the knowledge of diversity and bacterial community structure of protease-producing bacteria is limited in this system, especially in the tropical region. Herein, 1,179 cultivable protease-producing bacterial strains that belonged to Actinobacteria, Firmicutes, and Proteobacteria were isolated from tropical aquaculture systems, of which the most abundant genus was Bacillus, followed by Vibrio. The diversity and relative abundance of protease-producing bacteria in sediment were generally higher than those in water. Twenty-one genera from sediment and 16 genera from water were identified, of which Bacillus dominated by Bacillus hwajinpoensis in both and Vibrio dominated by Vibrio owensii in water were the dominant genera. The unique genera in sediment or water accounted for tiny percentage may play important roles in the stability of community structure. Eighty V. owensii isolates were clustered into four clusters (ET-1-ET-4) at 58% of similarity by ERIC-PCR (enterobacterial repetitive intergenic consensus-polymerase chain reaction), which was identified as a novel branch of V. owensii. Additionally, V. owensii strains belonged to ET-3 and ET-4 were detected in most aquaculture ponds without outbreak of epidemics, indicating that these protease-producing bacteria may be used as potential beneficial bacteria for wastewater purification. Environmental variables played important roles in shaping protease-producing bacterial diversity and community structure in aquaculture systems. In sediment, dissolved oxygen (DO), chemical oxygen demand (COD), and salinity as the main factors positively affected the distributions of dominant genus (Vibrio) and unique genera (Planococcus and Psychrobacter), whereas temperature negatively affected that of Bacillus (except B. hwajinpoensis). In water, Alteromonas as unique genus and Photobacterium were negatively affected by NO₃ ^--N and NO₂ ^--N, respectively, whereas pH as the main factor positively affected the distribution of Photobacterium. These findings will lay a foundation for the development of protease-producing bacterial agents for wastewater purification and the construction of an environment-friendly tropical aquaculture model.

AhaP, A Quorum Quenching Acylase from Psychrobacter sp. M9-54-1 That Attenuates Pseudomonas aeruginosa and Vibrio coralliilyticus Virulence

Although Psychrobacter strain M9-54-1 had been previously isolated from the microbiota of holothurians and shown to degrade quorum sensing (QS) signal molecules C6 and C10-homoserine lactone (HSL), little was known about the gene responsible for this activity. In this study, we determined the whole genome sequence of this strain and found that the full 16S rRNA sequence shares 99.78-99.66% identity with Psychrobacter pulmonis CECT 5989^T and P. faecalis ISO-46^T. M9-54-1, evaluated using the agar well diffusion assay method, showed high quorum quenching (QQ) activity against a wide range of synthetic N-acylhomoserine lactone (AHLs) at 4, 15, and 28 °C. High-performance liquid chromatography-mass-spectrometry (HPLC-MS) confirmed that QQ activity was due to an AHL-acylase. The gene encoding for QQ activity in strain M9-54-1 was identified from its genome sequence whose gene product was named AhaP. Purified AhaP degraded substituted and unsubstituted AHLs from C4- to C14-HSL. Furthermore, heterologous expression of ahaP in the opportunistic pathogen Pseudomonas aeruginosa PAO1 reduced the expression of the QS-controlled gene lecA, encoding for a cytotoxic galactophilic lectin and swarming motility protein. Strain M9-54-1 also reduced brine shrimp mortality caused by Vibrio coralliilyticus VibC-Oc-193, showing potential as a biocontrol agent in aquaculture.

Black cardamom essential oil prevents Escherichia coli O157:H7 and Salmonella Typhimurium JSG 1748 biofilm formation through inhibition of quorum sensing

This study aimed to investigate the chemical composition, using GC-MS, and anti-biofilm potential of black cardamom essential oil (BCEO) against biofilms of Escherichia coli O157:H7 and Salmonella Typhimurium JSG 1748 through inhibition of bacterial quorum sensing. GC-MS quantification demonstrated that BCEO contains 1,8-cineole (44.24%), α-terpinyl acetate (12.25%), nerolidol (6.03%), and sabinene (5.96%) as the major bioactive compounds. Antioxidant assays for BCEO revealed the total phenolic and flavonoid mean values were 1325.03 ± 7.69 mg GAE 100/g and 168.25 ± 5.26 mg CE/g, respectively. In regards to antimicrobial potential, Candida albicans was the most sensitive species compared to Streptococcus mutans, Staphylococcus aureus, Listeria monocytogenes, Bacillus cereus, and Salmonella Typhimurium with the following zones of inhibition; 14.4 ± 0.52, 13.2 ± 0.42, 11.2 ± 0.28, 11.0 ± 0.52, 8.2 ± 0.24 and 6.6 ± 0.18 mm in diameter, respectively. Biofilm inhibition by BCEO was concentration-dependent, when various concentrations of 0.03, 0.06, 0.12, 0.25 and 0.5% were applied, 33.67, 34.14, 38.66, 46.65 and 50.17% of Salmonella Typhimurium biofilm was inhibited, while 47.31, 54.15, 76.57, 83.36 and 84.63% of Escherichia coli biofilm formation was prevented. Chromobacterium violaceum ATCC 12,472 and its product violacein, was used as a microbial indicator for enhancement or inhibition of quorum sensing. Our data showed that 0.5% of BCEO inhibited violacein production without influencing the growth of Chromobacterium violaceum, while 1% of BCEO, caused 100% inhibtion of violacein production together with 30% inhibition of growth. This study shows that BCEO possesses promising antioxidant and antimicrobial potential, and found anti-biofilm activities linked to the quenching of the quorum sensing system of E. coli and S. Typhimurium.

Whole-Genome Sequencing Analysis of Quorum Quenching Bacterial Strain Acinetobacter lactucae QL-1 Identifies the FadY Enzyme for Degradation of the Diffusible Signal Factor

The diffusible signal factor (DSF) is a fatty acid signal molecule and is widely conserved in various Gram-negative bacteria. DSF is involved in the regulation of pathogenic virulence in many bacterial pathogens, including Xanthomonas campestris pv. campestris (Xcc). Quorum quenching (QQ) is a potential approach for preventing and controlling DSF-mediated bacterial infections by the degradation of the DSF signal. Acinetobacter lactucae strain QL-1 possesses a superb DSF degradation ability and effectively attenuates Xcc virulence through QQ. However, the QQ mechanisms in strain QL-1 are still unknown. In the present study, whole-genome sequencing and comparative genomics analysis were conducted to identify the molecular mechanisms of QQ in strain QL-1. We found that the fadY gene of QL-1 is an ortholog of XccrpfB, a known DSF degradation gene, suggesting that strain QL-1 is capable of inactivating DSF by QQ enzymes. The results of site-directed mutagenesis indicated that fadY is required for strain QL-1 to degrade DSF. The determination of FadY activity in vitro revealed that the fatty acyl-CoA synthetase FadY had remarkable catalytic activity. Furthermore, the expression of fadY in transformed Xcc strain XC1 was investigated and shown to significantly attenuate bacterial pathogenicity on host plants, such as Chinese cabbage and radish. This is the first report demonstrating a DSF degradation enzyme from A. lactucae. Taken together, these findings shed light on the QQ mechanisms of A. lactucae strain QL-1, and provide useful enzymes and related genes for the biocontrol of infectious diseases caused by DSF-dependent bacterial pathogens.

Quorum Quenching in a Novel Acinetobacter sp. XN-10 Bacterial Strain against Pectobacterium carotovorum subsp. carotovorum

Quorum sensing (QS) is a cell density-dependent mechanism that regulates the expression of specific genes in microbial cells. Quorum quenching (QQ) is a promising strategy for attenuating pathogenicity by interfering with the QS system of pathogens. N-Acyl-homoserine lactones (AHLs) act as signaling molecules in many Gram-negative bacterial pathogens and have received wide attention. In this study, a novel, efficient AHL-degrading bacterium, Acinetobacter sp. strain XN-10, was isolated from agricultural contaminated soil and evaluated for its degradation efficiency and potential use against QS-mediated pathogens. Strain XN-10 could effectively degrade N-(3-oxohexanoyl)-L-homoserine lactone (OHHL), N-hexanoyl-L-homoserine lactone (C6HSL), N-(3-oxododecanoyl)-L-homoserine lactone (3OC12HSL), and N-(3-oxooctanoyl)-L-homoserine lactone (3OC8HSL), which all belong to the AHL family. Analysis of AHL metabolic products by gas chromatography-mass spectrometry (GC-MS) led to the identification of N-cyclohexyl-propanamide, and pentanoic acid, 4-methyl, methyl ester as the main intermediate metabolites, revealing that AHL could be degraded by hydrolysis and dehydroxylation. All intermediates were transitory and faded away without any non-cleavable metabolites at the end of the experiment. Furthermore, strain XN-10 significantly attenuated the pathogenicity of Pectobacterium carotovorum subsp. carotovorum (Pcc) to suppress tissue maceration in carrots, potatoes, and Chinese cabbage. Taken together, our results shed light on the QQ mechanism of a novel AHL-degrading bacterial isolate, and they provide useful information which show potential for biocontrol of infectious diseases caused by AHL-dependent bacterial pathogens.

Acyl homoserine lactone-mediated quorum sensing in the oral cavity: a paradigm revisited

Acyl homoserine lactones (AHLs), the quorum sensing (QS) signals produced by Gram-negative bacteria, are currently considered to play a minor role in the development of oral biofilm since their production by oral pathogens has not been ascertained thus far. However, we report the presence of AHLs in different oral samples and their production by the oral pathogen Porphyromonas gingivalis. The importance of AHLs is further supported by a very high prevalence of AHL-degradation capability, up to 60%, among bacteria isolated from dental plaque and saliva samples. Furthermore, the wide-spectrum AHL-lactonase Aii20J significantly inhibited oral biofilm formation in different in vitro biofilm models and caused important changes in bacterial composition. Besides, the inhibitory effect of Aii20J on a mixed biofilm of 6 oral pathogens was verified using confocal microscopy. Much more research is needed in order to be able to associate specific AHLs with oral pathologies and to individuate the key actors in AHL-mediated QS processes in dental plaque formation. However, these results indicate a higher relevance of the AHLs in the oral cavity than generally accepted thus far and suggest the potential use of inhibitory strategies against these signals for the prevention and treatment of oral diseases.

Plant growth-promoting activity and quorum quenching-mediated biocontrol of bacterial phytopathogens by Pseudomonas segetis strain P6

Given the major threat of phytopathogenic bacteria to food production and ecosystem stability worldwide, novel alternatives to conventional chemicals-based agricultural practices are needed to combat these bacteria. The objective of this study is to evaluate the ability of Pseudomonas segetis strain P6, which was isolated from the Salicornia europaea rhizosphere, to act as a potential biocontrol agent given its plant growth-promoting (PGP) and quorum quenching (QQ) activities. Seed biopriming and in vivo assays of tomato plants inoculated with strain P6 resulted in an increase in seedling height and weight. We detected QQ activity, involving enzymatic degradation of signal molecules in quorum sensing communication systems, against a broad range of N-acylhomoserine lactones (AHLs). HPLC-MRM data and phylogenetic analysis indicated that the QQ enzyme was an acylase. The QQ activity of strain P6 reduced soft rot symptoms caused by Dickeya solani, Pectobacterium atrosepticum and P. carotovorum on potato and carrot. In vivo assays showed that the PGP and QQ activities of strain P6 protect tomato plants against Pseudomonas syringae pv. tomato, indicating that strain P6 could have biotechnological applications. To our knowledge, this is the first report to show PGP and QQ activities in an indigenous Pseudomonas strain from Salicornia plants.

Degradability and biofouling of oxo-biodegradable polyethylene in the planktonic and benthic zones of the Arabian Gulf

Little is known about the degradability of oxo-biodegradable polyethylene (OXO-PE) and its surface fouling bacterial communities in the marine environment. The degradation of OXO-PE, PE and polyethylene terephthalate (PET) was compared at two depths (2 m and 6 m) in the Arabian Gulf. Scanning electron microcopy (SEM) revealed more fissure formation on OXO-PE and PE than on PET, indicating physical degradation. The formation of hydroxyl groups and carbonyl bonds, by Fourier-transform infrared spectroscopy (FTIR), suggests chemical degradation of OXO-PE. Plastisphere bacterial communities on OXO-PE and PE were different than on PET. Proteobacteria, Bacteriodetes and Planctomycetes were detected on all plastics, however, sequences of Alteromonas and Zoogloea were OXO-PE-specific suggesting a possible involvement of these bacterial genera in OXO-PE degradation. We conclude that OXO-PE shows increased signs of degradation with time owing to the combination of abiotic and biotic processes, and its degradation is higher in the benthic than in the planktonic zone.

Quorum Quenching Properties and Probiotic Potentials of Intestinal Associated Bacteria in Asian Sea Bass Lates calcarifer

Quorum quenching (QQ), the enzymatic degradation of N-acyl homoserine lactones (AHLs), has been suggested as a promising strategy to control bacterial diseases. In this study, 10 AHL-degrading bacteria isolated from the intestine of barramundi were identified by 16S rDNA sequencing. They were able to degrade both short and long-chain AHLs associated with several pathogenic Vibrio species (spp.) in fish, including N-[(RS)-3-Hydroxybutyryl]-l-homoserine lactone (3-oh-C₄-HSL), N-Hexanoyl-l-homoserine lactone (C₆-HSL), N-(β-Ketocaproyl)-l-homoserine lactone (3-oxo-C₆-HSL), N-(3-Oxodecanoyl)-l-homoserine lactone (3-oxo-C₁₀-HSL), N-(3-Oxotetradecanoyl)-l-homoserine lactone (3-oxo-C₁₄-HSL). Five QQ isolates (QQIs) belonging to the Bacillus and Shewanella genera, showed high capacity to degrade both synthetic AHLs as well as natural AHLs produced by Vibrio harveyi and Vibrio alginolyticus using the well-diffusion method and thin-layer chromatography (TLC). The genes responsible for QQ activity, including aiiA, ytnP, and aaC were also detected. Analysis of the amino acid sequences from the predicted lactonases revealed the presence of the conserved motif HxHxDH. The selected isolates were further characterized in terms of their probiotic potentials in vitro. Based on our scoring system, Bacillus thuringiensis QQ1 and Bacillus cereus QQ2 exhibited suitable probiotic characteristics, including the production of spore and exoenzymes, resistance to bile salts and pH, high potential to adhere on mucus, appropriate growth abilities, safety to barramundi, and sensitivity to antibiotics. These isolates, therefore, constitute new QQ probiotics that could be used to control vibriosis in Lates calcalifer.

Silencing of Phytopathogen Communication by the Halotolerant PGPR Staphylococcus equorum Strain EN21

Increasing world food demand together with soil erosion and indiscriminate use of chemical fertilization highlight the need to adopt sustainable crop production strategies. In this context, a combination of plant growth-promoting rhizobacteria (PGPR) and pathogen management represents a sustainable and efficient alternative. Though little studied, halophilic and halotolerant PGPR could be a beneficial plant growth promotion strategy for saline and non-saline soils. The virulence of many bacterial phytopathogens is regulated by quorum sensing (QS) systems. Quorum quenching (QQ) involves the enzymatic degradation of phytopathogen-generated signal molecules, mainly N-acyl homoserine lactones (AHLs). In this study, we investigate plant growth-promoting (PGP) activity and the capacity of the halotolerant bacterium Staphylococcus equorum strain EN21 to attenuate phytopathogens virulence through QQ. We used biopriming and in vivo tomato plant experiments to analyse the PGP activity of strain EN21. AHL inactivation was observed to reduce Pseudomonas syringae pv. tomato infections in tomato and Arabidopsis plants. Our study of Dickeya solani, Pectobacterium carotovorum subsp. carotovorum and Erwinia amylovora bacteria in potato tubers, carrots and pears, respectively, also demonstrated the effectiveness of QS interruption by EN21. Overall, this study highlights the potential of strain S. equorum EN21 in plant growth promotion and QQ-driven bacterial phytopathogen biocontrol.

A Quorum-Sensing Inhibitor Strain of Vibrio alginolyticus Blocks Qs-Controlled Phenotypes in Chromobacterium violaceum and Pseudomonas aeruginosa

The cell density-dependent mechanism, quorum sensing (QS), regulates the expression of virulence factors. Its inhibition has been proposed as a promising new strategy to prevent bacterial pathogenicity. In this study, 827 strains from the microbiota of sea anemones and holothurians were screened for their ability to produce quorum-sensing inhibitor (QSI) compounds. The strain M3-10, identified as Vibrio alginolyticus by 16S rRNA gene sequencing, as well as ANIb and dDDH analyses, was selected for its high QSI activity. Bioassay-guided fractionation of the cell pellet extract from a fermentation broth of strain M3-10, followed by LC–MS and NMR analyses, revealed tyramine and N-acetyltyramine as the active compounds. The QS inhibitory activity of these molecules, which was confirmed using pure commercially available standards, was found to significantly inhibit Chromobacterium violaceum ATCC 12472 violacein production and virulence factors, such as pyoverdine production, as well as swarming and twitching motilities, produced by Pseudomonas aeruginosa PAO1. This constitutes the first study to screen QSI-producing strains in the microbiota of anemones and holothurians and provides an insight into the use of naturally produced QSI as a possible strategy to combat bacterial infections.

Activity Improvement and Vital Amino Acid Identification on the Marine-Derived Quorum Quenching Enzyme MomL by Protein Engineering

MomL is a marine-derived quorum-quenching (QQ) lactonase which can degrade various N-acyl homoserine lactones (AHLs). Intentional modification of MomL may lead to a highly efficient QQ enzyme with broad application potential. In this study, we used a rapid and efficient method combining error-prone polymerase chain reaction (epPCR), high-throughput screening and site-directed mutagenesis to identify highly active MomL mutants. In this way, we obtained two candidate mutants, MomL_I144V and MomL_V149A. These two mutants exhibited enhanced activities and blocked the production of pathogenic factors of Pectobacterium carotovorum subsp. carotovorum (Pcc). Besides, seven amino acids which are vital for MomL enzyme activity were identified. Substitutions of these amino acids (E238G/K205E/L254R) in MomL led to almost complete loss of its QQ activity. We then tested the effect of MomL and its mutants on Pcc-infected Chinese cabbage. The results indicated that MomL and its mutants (MomL_L254R, MomL_I144V, MomL_V149A) significantly decreased the pathogenicity of Pcc. This study provides an efficient method for QQ enzyme modification and gives us new clues for further investigation on the catalytic mechanism of QQ lactonase.

Discovering, Characterizing, and Applying Acyl Homoserine Lactone-Quenching Enzymes to Mitigate Microbe-Associated Problems Under Saline Conditions

Quorum quenching (QQ) is proposed as a new strategy for mitigating microbe-associated problems (e.g., fouling, biocorrosion). However, most QQ agents reported to date have not been evaluated for their quenching efficacies under conditions representative of seawater desalination plants, cooling towers or marine aquaculture. In this study, bacterial strains were isolated from Saudi Arabian coastal environments and screened for acyl homoserine lactone (AHL)-quenching activities. Five AHL quenching bacterial isolates from the genera Pseudoalteromonas, Pontibacillus, and Altererythrobacter exhibited high AHL-quenching activity at a salinity level of 58 g/L and a pH of 7.8 at 50°C. This result demonstrates the potential use of these QQ bacteria in mitigating microbe-associated problems under saline and alkaline conditions at high (>37°C) temperatures. Further characterizations of the QQ efficacies revealed two bacterial isolates, namely, Pseudoalteromonas sp. L11 and Altererythrobacter sp. S1-5, which could possess enzymatic QQ activity. The genome sequences of L11 and S1-5 with a homologous screening against reported AHL quenching genes suggest the existence of four possible QQ coding genes in each strain. Specifically, two novel AHL enzymes, AiiA_S1-5 and Est_S1-5 from Altererythrobacter sp. S1-5, both contain signal peptides and exhibit QQ activity over a broad range of pH, salinity, and temperature values. In particular, AiiA_S1-5 demonstrated activity against a wide spectrum of AHL molecules. When tested against three bacterial species, namely, Aeromonas hydrophila, Pseudomonas aeruginosa, and Vibrio alginolyticus, AiiA_S1-5 was able to inhibit the motility of all three species under saline conditions. The biofilm formation associated with P. aeruginosa was also significantly inhibited by AiiA_S1-5. AiiA_S1-5 also reduced the quorum sensing-mediated virulence traits of A. hydrophila, P. aeruginosa, and V. alginolyticus during the mid and late exponential phases of cell growth. The enzyme did not impose any detrimental effects on cell growth, suggesting a lower potential for the target bacterium to develop resistance over long-term exposure. Overall, this study suggested that some QQ enzymes obtained from the bacteria that inhabit saline environments under high temperatures have potential applications in the mitigation of microbe-associated problems.

Stenotrophomonas maltophilia AHL-Degrading Strains Isolated from Marine Invertebrate Microbiota Attenuate the Virulence of Pectobacterium carotovorum and Vibrio coralliilyticus

Many Gram-negative aquacultural and agricultural pathogens control virulence factor expression through a quorum-sensing (QS) mechanism involving the production of N-acylhomoserine (AHL) signalling molecules. Thus, the interruption of QS systems by the enzymatic degradation of signalling molecules, known as quorum quenching (QQ), has been proposed as a novel strategy to combat these infections. Given that the symbiotic bacteria of marine invertebrates are considered to be an important source of new bioactive molecules, this study explores the presence of AHL-degrading bacteria among 827 strains previously isolated from the microbiota of anemones and holothurians. Four of these strains (M3-1, M1-14, M3-13 and M9-54-2), belonging to the species Stenotrophomonas maltophilia, were selected on the basis of their ability to degrade a broad range of AHLs, and the enzymes involved in their activity were identified. Strain M9-54-2, which showed the strongest AHL-degrading activity, was selected for further study. High-performance liquid chromatography-mass-spectrometry confirmed that the QQ enzyme is not a lactonase. Strain M9-54-2 degraded AHL accumulation and reduced the production of enzymatic activity in Pectobacterium carotovorum CECT 225^T and Vibrio coralliilyticus VibC-Oc-193 in in vitro co-cultivation experiments. The effect of AHL inactivation was confirmed by a reduction in potato tuber maceration and brine shrimp (Artemia salina) mortality caused by P. carotovorum and Vibrio coralliilyticus, respectively. This study strengthens the evidence of marine organisms as an underexplored and promising source of QQ enzymes, useful to prevent infections in aquaculture and agriculture. To our knowledge, this is the first time that anemones and holothurians have been studied for this purpose.

Saline Environments as a Source of Potential Quorum Sensing Disruptors to Control Bacterial Infections: A Review

Saline environments, such as marine and hypersaline habitats, are widely distributed around the world. They include sea waters, saline lakes, solar salterns, or hypersaline soils. The bacteria that live in these habitats produce and develop unique bioactive molecules and physiological pathways to cope with the stress conditions generated by these environments. They have been described to produce compounds with properties that differ from those found in non-saline habitats. In the last decades, the ability to disrupt quorum-sensing (QS) intercellular communication systems has been identified in many marine organisms, including bacteria. The two main mechanisms of QS interference, i.e., quorum sensing inhibition (QSI) and quorum quenching (QQ), appear to be a more frequent phenomenon in marine aquatic environments than in soils. However, data concerning bacteria from hypersaline habitats is scarce. Salt-tolerant QSI compounds and QQ enzymes may be of interest to interfere with QS-regulated bacterial functions, including virulence, in sectors such as aquaculture or agriculture where salinity is a serious environmental issue. This review provides a global overview of the main works related to QS interruption in saline environments as well as the derived biotechnological applications.

Novel N-Acyl Homoserine Lactone-Degrading Bacteria Isolated From Penicillin-Contaminated Environments and Their Quorum-Quenching Activities

N-Acyl homoserine lactones (AHLs) are signaling molecules used in the quorum sensing (QS) of Gram-negative bacteria. Some bacteria interfere with the QS system using AHL-inactivating enzymes, commonly known as quorum-quenching (QQ) enzymes. We have recently isolated a new QQ bacterium showing high resistance to multiple β-lactam antibiotics, and its QQ enzyme (MacQ) confers β-lactam antibiotic resistance and exhibits QQ activities. This observation suggests the possibility of isolating novel QQ bacteria from β-lactam antibiotic-resistant bacteria. In this direction, we attempted to isolate penicillin G (PENG)-resistant bacteria from penicillin-contaminated river sediments and activated sludge treating penicillin-containing wastewater and characterize their QQ activities. Of 19 PENG-resistant isolates, six isolates showed high QQ activity toward a broad range of AHLs, including AHLs with 3-oxo substituents. Five of the six AHL-degraders showed AHL-acylase activity and hydrolyzed the amide bond of AHLs, whereas the remaining one strain did not show AHL-acylase activity, suggesting that this isolate may likely possess alternative degradation mechanism such as AHL-lactonase activity hydrolyzing the lactone ring of AHLs. The 16S rRNA gene sequence analysis results categorized these six AHL-degrading isolates into at least five genera, namely, Sphingomonas (Alphaproteobacteria), Diaphorobacter (Betaproteobacteria), Acidovorax (Betaproteobacteria), Stenotrophomonas (Gammaproteobacteria), and Mycobacterium (Actinobacteria); of these, Mycobacterium sp. M1 has never been known as QQ bacteria. Moreover, multiple β-lactam antibiotics showed high minimum inhibitory concentrations (MICs) when tested against all of isolates. These results strongly demonstrate that a wide variety of β-lactam antibiotic-resistant bacteria possess QQ activities. Although the genetic and enzymatic elements are yet unclear, this study may infer the functional and evolutionary correlation between β-lactam antibiotic resistance and QQ activities.

Genomic analyses of two Alteromonas stellipolaris strains reveal traits with potential biotechnological applications

The Alteromonas stellipolaris strains PQQ-42 and PQQ-44, previously isolated from a fish hatchery, have been selected on the basis of their strong quorum quenching (QQ) activity, as well as their ability to reduce Vibrio-induced mortality on the coral Oculina patagonica. In this study, the genome sequences of both strains were determined and analyzed in order to identify the mechanism responsible for QQ activity. Both PQQ-42 and PQQ-44 were found to degrade a wide range of N-acylhomoserine lactone (AHL) QS signals, possibly due to the presence of an aac gene which encodes an AHL amidohydrolase. In addition, the different colony morphologies exhibited by the strains could be related to the differences observed in genes encoding cell wall biosynthesis and exopolysaccharide (EPS) production. The PQQ-42 strain produces more EPS (0.36 g l-1) than the PQQ-44 strain (0.15 g l-1), whose chemical compositions also differ. Remarkably, PQQ-44 EPS contains large amounts of fucose, a sugar used in high-value biotechnological applications. Furthermore, the genome of strain PQQ-42 contained a large non-ribosomal peptide synthase (NRPS) cluster with a previously unknown genetic structure. The synthesis of enzymes and other bioactive compounds were also identified, indicating that PQQ-42 and PQQ-44 could have biotechnological applications.

Draft Genome Sequence of the Marine Bacterium Alteromonas sp. Strain KS69

Alteromonas spp. are Gram-negative, aerobic, marine bacteria. Here, we report the draft genome sequence of Alteromonas sp. strain KS69, isolated from Narragansett Bay deep water samples. Unpublished preliminary data suggest that KS69 reduces expression of the 3-oxo-C12-HSL-dependent, virulence-associated gene lasB of Pseudomonas aeruginosa PAO1, suggesting that it produces a quorum sensing inhibitor.

Alteromonas spp. are Gram-negative, aerobic, marine bacteria. Here, we report the draft genome sequence of Alteromonas sp. strain KS69, isolated from Narragansett Bay deep water samples. Unpublished preliminary data suggest that KS69 reduces expression of the 3-oxo-C12-HSL-dependent, virulence-associated gene lasB of Pseudomonas aeruginosa PAO1, suggesting that it produces a quorum sensing inhibitor.

Quorum-Quenching Bacteria Isolated From Red Sea Sediments Reduce Biofilm Formation by Pseudomonas aeruginosa

Quorum sensing (QS) is the process by which bacteria communicate with each other through small signaling molecules such as N-acylhomoserine lactones (AHLs). Certain bacteria can degrade AHL molecules by a process called quorum quenching (QQ); therefore, QQ can be used to control bacterial infections and biofilm formation. In this study, we aimed to identify new species of bacteria with QQ activity. Red Sea sediments were collected either from the close vicinity of seagrass or from areas with no vegetation. We isolated 72 bacterial strains, which were tested for their ability to degrade/inactivate AHL molecules. Chromobacterium violaceum CV026-based bioassay was used for the initial screening of isolates with QQ activity. QQ activity was further quantified using high-performance liquid chromatography-tandem mass spectrometry. We found that these isolates could degrade AHL molecules of different acyl chain lengths as well as modifications. 16S-rRNA sequencing of positive QQ isolates showed that they belonged to three different genera. Specifically, two isolates belonged to the genus Erythrobacter; four, Labrenzia; and one, Bacterioplanes. The genome of one representative isolate from each genus was sequenced, and potential QQ enzymes, namely, lactonases and acylases, were identified. The ability of these isolates to degrade the 3OXOC12-AHLs produced by Pseudomonas aeruginosa PAO1 and hence inhibit biofilm formation was investigated. Our results showed that the isolate VG12 (genus Labrenzia) is better than other isolates at controlling biofilm formation by PAO1 and degradation of different AHL molecules. Time-course experiments to study AHL degradation showed that VG1 (genus Erythrobacter) could degrade AHLs faster than other isolates. Thus, QQ bacteria or enzymes can be used in combination with an antibacterial to overcome antibiotic resistance.

AHL-lactonase expression in three marine emerging pathogenic Vibrio spp. reduces virulence and mortality in brine shrimp (Artemia salina) and Manila clam (Venerupis philippinarum)

Bacterial infectious diseases produced by Vibrio are the main cause of economic losses in aquaculture. During recent years it has been shown that the expression of virulence genes in some Vibrio species is controlled by a population-density dependent gene-expression mechanism known as quorum sensing (QS), which is mediated by the diffusion of signal molecules such as N-acylhomoserine lactones (AHLs). QS disruption, especially the enzymatic degradation of signalling molecules, known as quorum quenching (QQ), is one of the novel therapeutic strategies for the treatment of bacterial infections. In this study, we present the detection of AHLs in 34 marine Vibrionaceae strains. Three aquaculture-related pathogenic Vibrio strains, V. mediterranei VibC-Oc-097, V. owensii VibC-Oc-106 and V. coralliilyticus VibC-Oc-193 were selected for further studies based on their virulence and high production of AHLs. This is the first report where the signal molecules have been characterized in these emerging marine pathogens and correlated to the expression of virulence factors. Moreover, the results of AHL inactivation in the three selected strains have been confirmed in vivo against brine shrimps (Artemia salina) and Manila clams (Venerupis philippinarum). This research contributes to the development of future therapies based on AHL disruption, the most promising alternatives for fighting infectious diseases in aquaculture.

High Prevalence of Quorum-Sensing and Quorum-Quenching Activity among Cultivable Bacteria and Metagenomic Sequences in the Mediterranean Sea

There is increasing evidence being accumulated regarding the importance of N-acyl homoserine lactones (AHL)-mediated quorum-sensing (QS) and quorum-quenching (QQ) processes in the marine environment, but in most cases, data has been obtained from specific microhabitats, and subsequently little is known regarding these activities in free-living marine bacteria. The QS and QQ activities among 605 bacterial isolates obtained at 90 and 2000 m depths in the Mediterranean Sea were analyzed. Additionally, putative QS and QQ sequences were searched in metagenomic data obtained at different depths (15-2000 m) at the same sampling site. The number of AHL producers was higher in the 90 m sample (37.66%) than in the 2000 m sample (4.01%). However, the presence of QQ enzymatic activity was 1.63-fold higher in the 2000 m sample. The analysis of putative QQ enzymes in the metagenomes supports the relevance of QQ processes in the deepest samples, found in cultivable bacteria. Despite the unavoidable biases in the cultivation methods and biosensor assays and the possible promiscuous activity of the QQ enzymes retrieved in the metagenomic analysis, the results indicate that AHL-related QS and QQ processes could be common activity in the marine environment.

Evaluating the antimicrobial potential of green cardamom essential oil focusing on quorum sensing inhibition of Chromobacterium violaceum

Spices are well known for their taste and flavor imparting properties. Green cardamom (Elletaria cardamomum), a herb spice belongs to family Zingiberaceae. In current study, GC-MS analysis of green cardamom essential oil (CEO) resulted in identification of twenty-six compounds with α-terpinyl acetate (38.4%), 1,8-cineole (28.71%), linalool acetate (8.42%), sabinene (5.21%), and linalool (3.97%) as major bioactive components. Present study also described the antimicrobial properties like zone of inhibition, minimum inhibitory concentration against microbial strains with special emphasis on quorum sensing inhibition. Disk diffusion assay showed that C. albicans and S. mutans were the most sensitive microorganisms followed by S. aureus, L. monocytogenes, B. cereus and S. typhimurium sensor strains, respectively. Whilst P. aeruginosa was found most resistant strain as CEO did not inhibited its growth. The minimum inhibitory concentration (MIC) values of CEO against tested strains were 10 ± 0.00 mg/mL against S. typhimurium, S. aureus and 5 ± 0.00 mg/mL against S. mutans, C. albicans strains, respectively. Regarding quorum sensing inhibition the tested concentrations 0.625 and 0.313 mg/mL of CEO inhibited violacein production with very little effect on growth of C. violaceum. Conclusively, study proved that quorum sensing inhibition values of CEO were much lower compared to MIC revealed values. Hence, cardamom bioactive constituents can effectively be used to develop novel antimicrobial drugs against conventional antibiotics.

HqiA, a novel quorum-quenching enzyme which expands the AHL lactonase family

The screening of a metagenomic library of 250,000 clones generated from a hypersaline soil (Spain) allowed us to identify a single positive clone which confers the ability to degrade N-acyl homoserine lactones (AHLs). The sequencing of the fosmid revealed a 42,318 bp environmental insert characterized by 46 ORFs. The subcloning of these ORFs demonstrated that a single gene (hqiA) allowed AHL degradation. Enzymatic analysis using purified HqiA and HPLC/MS revealed that this protein has lactonase activity on a broad range of AHLs. The introduction of hqiA in the plant pathogen Pectobacterium carotovorum efficiently interfered with both the synthesis of AHLs and quorum-sensing regulated functions, such as swarming motility and the production of maceration enzymes. Bioinformatic analyses highlighted that HqiA showed no sequence homology with the known prototypic AHL lactonases or acylases, thus expanding the AHL-degrading enzymes with a new family related to the cysteine hydrolase (CHase) group. The complete sequence analysis of the fosmid showed that 31 ORFs out of the 46 identified were related to Deltaproteobacteria, whilst many intercalated ORFs presented high homology with other taxa. In this sense, hqiA appeared to be assigned to the Hyphomonas genus (Alphaproteobacteria), suggesting that horizontal gene transfer had occurred.