Effect of temperature and glucose concentration on the N-butanoyl-l-homoserine lactone production by Aeromonas hydrophila

Carbon metabolism characteristics of quorum quenching bacteria Rhodococcus sp. BH4 determine the bioaugmentation efficiency under different carbon source conditions

Carbon sources are critical factors influencing bacterial bioaugmentation, however, the underlying mechanisms, particularly the metabolic characteristics of bioaugmented bacteria remain poorly understood. The bioaugmented bacterium Rhodococcus sp. BH4 secretes the quorum quenching (QQ) enzyme QsdA to disrupt the quorum sensing (QS) in the activated sludge (AS) process, reducing AS yield in-situ. This study investigated the carbon metabolic characteristics of BH4 and explored the effects on bioaugmentation with different influent carbon sources. Because of the absence of glucose-specific phosphoenol phosphotransferase system (PTS), BH4 prefers sodium acetate to glucose. However, the lactones produced during extracellular glucose metabolism enhance BH4 qsdA expression. Moreover, BH4 possess carbon catabolite repression (CCR), acetate inhibits glucose utilization. BH4 microbeads were added to reactors with different carbon sources (R1: sodium acetate; R2: glucose; R3: a mixture of sodium acetate and glucose) for in-situ AS yield reduction. During operation, AS reduction efficiency decreased in the following order: R1 > R3 > R2. R2 and R3 microbeads exhibited similar QQ activity to R1, with less BH4 biomass at 5 d. 13C labeling and Michaelis-Menten equation showed that, due to differences in the competitiveness of carbon sources, R1 BH4 obtained the most carbon, whereas R2 BH4 obtained the least carbon. Moreover, acetate inhibited glucose utilization of R3 BH4. Transcriptome analysis showed that R1 BH4 qsdA expression was the lowest, R2 BH4 was the most serious form of programmed cell death, and the R3 BH4 PTS pathway was inhibited. At 10 d, R1 BH4 biomass and microbead QQ activity were higher than that in R3, and the R2 BH4 lost viability and QQ activity. This study provides new insights into bioaugmentation from the perspectives of carbon source competitiveness, carbon metabolism pathways, and CCR.

Quorum Sensing Regulates the Hydrolytic Enzyme Production and Community Composition of Heterotrophic Bacteria in Coastal Waters

Heterotrophic microbial communities play a central role in biogeochemical cycles in the ocean by degrading organic matter through the synthesis of extracellular hydrolytic enzymes. Their hydrolysis rates result from the community’s genomic potential and the differential expression of this genomic potential. Cell-cell communication pathways such as quorum sensing (QS) could impact both aspects and, consequently, structure marine ecosystem functioning. However, the role of QS communications in complex natural assemblages remains largely unknown. In this study, we investigated whether N-acylhomoserine lactones (AHLs), a type of QS signal, could regulate both hydrolytic activities and the bacterial community composition (BCC) of marine planktonic assemblages. To this extent, we carried out two microcosm experiments, adding five different AHLs to bacterial communities sampled in coastal waters (during early and peak bloom) and monitoring their impact on enzymatic activities and diversity over 48 h. Several specific enzymatic activities were impacted during both experiments, as early as 6 h after the AHL amendments. The BCC was also significantly impacted by the treatments after 48 h, and correlated with the expression of the hydrolytic activities, suggesting that changes in hydrolytic intensities may drive changes in BCC. Overall, our results suggest that QS communication could participate in structuring both the function and diversity of marine bacterial communities.

Quorum sensing: an emerging link between temperature and membrane biofouling in membrane bioreactors

Lab-scale membrane bioreactors (MBRs) were investigated at 12, 18, and 25 °C to identify the correlation between quorum sensing (QS) and biofouling at different temperatures. The lower the reactor temperature, the more severe the membrane biofouling measured in terms of the transmembrane pressure (TMP) during filtration. More extracellular polymeric substances (EPSs) that cause biofouling were produced at 18 °C than at 25 °C, particularly polysaccharides, closely associated with QS via the production of N-acyl homoserine lactone (AHL). However, at 12 °C, AHL production decreased, but the release of EPSs due to deflocculation increased the soluble EPS concentration. To confirm the temperature effect related to QS, bacteria producing AHL were isolated from MBR sludge and identified as Aeromonas sp., Leclercia sp., and Enterobacter sp. through a 16S rDNA sequencing analysis. Batch assays at 18 and 25 °C showed that there was a positive correlation between QS through AHL and biofilm formation in that temperature range.

Quorum Sensing and Spoilage Potential of Psychrotrophic Enterobacteriaceae Isolated from Milk

The 16S rDNA of six psychrotrophic Enterobacteriaceae isolated from cold raw milk were sequenced and the isolate 039 was identified as Pantoea sp., isolates 059, 068, and 071 were identified as Hafnia alvei, 067 was identified as Enterobacter sp., and 099 was identified as Aeromonas hydrophila. They presented different spoilage potentials in milk with A. hydrophila 099 being the most deteriorative. Only Pantoea sp. 039 was not able to induce the quorum sensing monitor strains of acyl homoserine lactones (AHLs). The halI gene, which encodes the AHL synthase in H. alvei, was identified in the isolates 059, 067, 068, and 071. After initial sequencing characterization and cloning, this gene showed its function by the heterologous synthesis of N-hexanoyl-DL-homoserine lactone and N-3-oxohexanoyl-L-homoserine lactone in Escherichia coli. In addition to producing AHLs, A. hydrophila 099 produced AI-2 in higher level than the assay's positive control Vibrio harveyi BB120. Therefore, Enterobacteriaceae strains isolated from cooled raw milk produce a rich array of signaling molecules that may influence bacterial traits in the milk environment.

Detection of New Quorum Sensing N-Acyl Homoserine Lactones From Aeromonas veronii

Sturgeon is an important fresh water-culture fish in China. A problem with sturgeon is its high susceptibility to spoilage. Food spoilage is reported to be regulated by quorum sensing (QS). To identify the QS signals acetylated homoserine lactones (AHLs) in sturgeon and test whether QS plays a role in the spoilage of sturgeon, we investigated the specific spoilage organisms (SSOs) in vacuum packaged sturgeon stored at 4°C and the production of AHLs by sturgeon SSOs. 16S rDNA sequencing and spoilage capabilities analysis revealed that Aeromonas veronii LP-11, Citrobacter freundii LPJ-2, and Raoultella ornithinolytica LPC-3 were the SSOs in sturgeon. Among the three SSOs, only A. veronii LP-11 induced the QS biosensors Agrobacterium tumefaciens KYC55 and Chromobacterium violaceum CV026, suggesting that it produced AHLs. Analysis by thin layer chromatography, high-performance liquid chromatography-triple quadrupole tandem mass spectrometry, and high-performance liquid chromatography quadrupole time-of-flight mass spectrometry (HPLC/qTOF-MS) identified that the AHLs produced by A. veronii were C6-SHL, C8-HSL, 3-oxo-C8-HSL, and 3-OH-C8-HSL. Our study revealed that QS system was probably involved in the regulation of sturgeon spoilage and for the first time reported the production of C8-HSL and 3-OH-C8-HSL by genus Aeromonas. As only HPLC/qTOF-MS effectively and accurately identified all the four AHLs produced by A. veronii LP-11, this study also showed that HPLC/qTOF-MS was the most efficient method for rapid analysis of AHLs in complex microbial sample. The study provides new insight into the microbiology of sturgeon spoilage which may be helpful for better sturgeon preservation.

Production of N-acyl Homoserine Lactones and Virulence Factors of Waterborne Aeromonas hydrophila

Aeromonads are inhabitants of aquatic ecosystems and are described as being involved in intestinal disturbances and other infections. The purpose of this study was to investigate the production of N-acyl-homoserine lactone (AHL) signal molecules and some virulence factors, including hemolysins, proteases, extracellular nucleases production and cytotoxicity by waterborne Aeromonas hydrophila. A total of 24 strains isolated from fresh-water or diseased fish were used in the study. The majority A.hydrophila strains produce two AHL molecules (21/24), one is N-butanoyl homoserine lactone (BHL), and the other is N-hexanoyl homoserine lactone (HHL) according to thin-layer chromatography analysis. Among the virulence factors tested, more than 83 % of the isolates produced β haemolysin when inoculated on sheep blood agar, only 50 % of the isolates displayed DNase activity, 75 % of the isolates shown proteolytic activity on skimmed milk plate, and cytotoxic activity was detected in 20 of 24 of the isolates. The strains producing AHLs possessed one or more virulence factors. In conclusion, the production of quorum sensing signal molecules is common among the strains that we examined, and there seems to some relationships between quorum sensing signal production and virulence factors in A. hydrophila.

Quorum sensing in the context of food microbiology

Food spoilage may be defined as a process that renders a product undesirable or unacceptable for consumption and is the outcome of the biochemical activity of a microbial community that eventually dominates according to the prevailing ecological determinants. Although limited information are reported, this activity has been attributed to quorum sensing (QS). Consequently, the potential role of cell-to-cell communication in food spoilage and food safety should be more extensively elucidated. Such information would be helpful in designing approaches for manipulating these communication systems, thereby reducing or preventing, for instance, spoilage reactions or even controlling the expression of virulence factors. Due to the many reports in the literature on the fundamental features of QS, e.g., chemistry and definitions of QS compounds, in this minireview, we only allude to the types and chemistry of QS signaling molecules per se and to the (bioassay-based) methods of their detection and quantification, avoiding extensive documentation. Conversely, we attempt to provide insights into (i) the role of QS in food spoilage, (ii) the factors that may quench the activity of QS in foods and review the potential QS inhibitors that might "mislead" the bacterial coordination of spoilage activities and thus may be used as biopreservatives, and (iii) the future experimental approaches that need to be undertaken in order to explore the "gray" or "black" areas of QS, increase our understanding of how QS affects microbial behavior in foods, and assist in finding answers as to how we can exploit QS for the benefit of food preservation and food safety.

Cell-cell signalling in bacteria: not simply a matter of quorum

Bacterial signalling known as quorum sensing (QS) relies on the synthesis of autoinducing signals throughout growth; when a threshold concentration is reached, these signals interact with a transcriptional regulator, allowing the expression of specific genes at a high cell density. One of the most studied intraspecies signalling is based on the use of N-acyl-homoserine lactones (AHL). Many factors other than cell density were shown to affect AHL accumulation and interfere with the QS signalling process. At the cellular level, the genetic determinants of QS are integrated in a complex regulatory network, including QS cascades and various transcriptional and post-transcriptional regulators that affect the synthesis of the AHL signal. In complex environments where bacteria exist, AHL do not accumulate at a constant rate; the diffusion and perception of the AHL signal outside bacterial cells can be compromised by abiotic environmental factors, by members of the bacterial community such as AHL-degrading bacteria and also by compounds produced by eukaryotes acting as an AHL mimic or inhibitor. This review aims to present all factors interfering with the AHL-mediated signalling process, at the levels of signal production, diffusion and perception.

Insights into the role of quorum sensing in food spoilage

Food spoilage is a consequence of the degrading enzymatic activity of some food-associated bacteria. Several proteolytic, lipolytic, chitinolytic, and pectinolytic activities associated with the deterioration of goods are regulated by quorum sensing, suggesting a potential role of such cell-to-cell communication in food spoilage. Here we review quorum sensing signaling molecules and methods of their detection and quantification, and we provide insights into the role of quorum sensing in food spoilage and address potential quorum sensing inhibitors that might be used as biopreservatives.

Quorum-sensing-dependent switch to butanediol fermentation prevents lethal medium acidification in Aeromonas hydrophila AH-1N

Aeromonas hydrophila AH-1N shows biphasic growth in glucose containing broth at 30 degrees C. The first phase is characterized by strong acidification, while the second phase is accompanied by an increase in medium pH. Disruption of AHL production by knockout of ahyI did not affect the first growth phase, but resulted in further acidification which completely blocked the second growth phase and even caused cell death. The block could be relieved by addition of 5 microM (or less) synthetic N-butanoyl-L-homoserine lactone (C4-HSL), the major AHL produced by A. hydrophila AH-1N. Further analysis revealed that, as shown previously for Serratia plymuthica, butanediol fermentation is regulated by AHL-mediated quorum sensing in A. hydrophila AH-1N. These results indicate that A. hydrophila switches to butanediol fermentation to reduce medium acidification under certain conditions and to allow further growth if nutrient resources are not yet exhausted.