SummaryN -acylhomoserine lactones (AHLs) are used as signal molecules by many quorum-sensing Proteobacteria. Diverse plant and animal pathogens use AHLs to regulate infection and virulence functions. These signals are subject to biological inactivation by AHLlactonases and AHL-acylases. Previously, little was known about the molecular details underlying the latter mechanism. An AHL signal-inactivating bacterium, identified as a Ralstonia sp., was isolated from a mixed-species biofilm. The signal inactivation encoding gene from this organism, which we call aiiD , was cloned and successfully expressed in Escherichia coli and inactivated three AHLs tested. The predicted 794-amino-acid polypeptide was most similar to the aculeacin A acylase (AAC) from Actinoplanes utahensis and also shared significant similarities with cephalosporin acylases and other N-terminal (Ntn) hydrolases. However, the most similar homologues of AiiD are deduced proteins of undemonstrated function from available Ralstonia , Deinococcus and Pseudomonas genomes. LC-MS analyses demonstrated that AiiD hydrolyses the AHL amide, releasing homoserine lactone and the corresponding fatty acid. Expression of AiiD in Pseudomonas aeruginosa PAO1 quenched quorum sensing by this bacterium, decreasing its ability to swarm, produce elastase and pyocyanin and to paralyse nematodes. Thus, AHLacylases have fundamental implications and hold biotechnological promise in quenching quorum sensing.

No abstract

No abstract

A novel coccobacilli group found previously in enhanced biological phosphorus removal (EBPR) systems was further revealed to have a high degree of diversity and distribution in various activated sludge systems. Phylogenetic analysis based on 14 existing and 18 newly retrieved 16S rRNA sequences revealed that these sequences formed a novel cohesive cluster with seven subgroups in the gamma-Proteobacteria. Fluorescence in situ hybridization with a set of probes designed specifically targeting the novel group at different hierarchical levels showed that the novel group with a coccoid (2-4 micro m) to occasionally long-rod (up to 20 micro m) shape widely distributed and in some cases predominated in sludge samples taken from nine lab- and full-scale EBPR systems (10-50% of total cells) and four conventional activated sludge systems (1-10%). Variation of predominance was also observed among those subgroups in systems showing deteriorated or effective EBPR activity.

In this study, four bench-scale pre-anoxic submerged membrane bioreactors (MBR) were operated simultaneously at different mean cell residence times (MCRTs) (3, 5, 10, and 20 days) to systematically elucidate the contributing factors of membrane fouling. Severe membrane fouling was first observed in the 3-day followed by the 5-day MCRT MBRs. Minimal membrane fouling was detected in the 10 and 20-day MCRT MBRs. The fouling of microfiltration membrane was not controlled by mixed liquor suspended solids concentration or zeta potential of biomass. Instead, membrane fouling rate increased with increasing soluble microbial products and extracellular polymeric substances concentrations, which both increased with decreasing MCRT. Total organic carbon, protein, carbohydrate, and UV254 absorbance in the mixed liquor supernatant increased with decreasing MCRT and were consistently higher than those of the effluent. Accumulation of carbohydrates rather than proteins in the mixed liquor supernatant was found to decrease with increasing MCRT. Normalized capilliary suction time value rather than the capilliary suction time value would indicate membrane fouling potential of a mixed liquor. Image analysis of the fouled membrane using scanning electron microscope and confocal laser scanning microscope showed that biofilm formation was the cause of membrane fouling.