The selective pressure of quorum quenching on microbial communities in membrane bioreactors

Highlights

• QQ changed sludge morphology via controlling TB-EPS and PS/PN in EPS.

• The endless ‘battle’ between QQ and QS resulted in huge varations in sludge floc size and EPS content.

• AHL based QQ altered microbial composition, thus facilitated Gram-positive community.

• Survival of QQ strains within beads need to be maintained and monitored.

Abstract

In conventional membrane bioreactor (MBR) treatment systems, Gram-negative bacterial population appears to be always outnumbered Gram-positive community. Thereby, acyl homoserine lactones (AHLs), major signaling molecules utilized by Gram-negative bacteria, have been targeted for biofouling control in quorum quenching (QQ) based studies. This study investigated the impact of AHL and autoinducer-2 (AI-2)-degrading QQ consortium on the selective accumulation of microbial communities in a QQ MBR (MBR-QQb). The results show that addition of the QQ consortium (in the form of beads) increased the filtration time of MBR-QQb by 3.5 times. The distribution of mixed liquor extracellular polymeric substances (EPS), especially the tightly bound (TB) proteinous EPS and the floc size were strongly affected by the QQ activity, and the endless ‘battle’ between QQ and quorum sensing (QS). More importantly, QQ induced the significant suppression of Gram-negative bacterial community. The average abundance of Gram-positive bacteria at the genus level in the biocake of MBR-QQb (51%) was significantly higher than that of the control MBR (11%) and the MBR with vacant beads (28%). These findings suggest that an unintended condition is created to favor the growth of Gram-positive bacteria in QQ MBRs, resulting in a distinct microbial social network in both bulk sludge and biocake.

Introduction

Membrane biofouling has been recognized as the most challenging issue in membrane bioreactor (MBR) technology, due to its strong undesirable effects such as membrane clogging, fast trans-membrane pressure (TMP) build-up, low treatment efficiency and high energy demands (Lee et al., 2014; Vrouwenvelder et al., 2008). Various measures such as changing hydro-dynamical parameters (aeration rate and air diffuser position, relaxation/filtration mode and backwashing), pre-treatment of feed by coagulation/flocculation, chemical cleaning, membrane surface conditioning, and modifications in reactor design and filtration modes, have been attempted to mitigate membrane biofouling (Kobayashi et al., 2003; Liu et al., 2010; Yang et al., 2013). However, no significant progress had been made until the quorum quenching (QQ)-based biofouling control technology emerged a decade ago. This biological approach retards biocake formation via disrupting bacteria social communication, i.e. quorum sensing, and thus reducing the production of extracellular polymeric substances (EPS) – the backbone providing structural properties and mechanical strength of biofilms – in biocake. Since the first demonstration of its feasibility by Yeon et al. (2009), this technology has attracted extensive attention over the last decade (Oh and Lee, 2018).

Application of a single QQ bacterial strain such as Rhodococcus BH4, or a QQ consortium in disrupting the quorum sensing (QS) mechanism at lab-scale MBRs has been extensively investigated and proved to be effective in MBR systems (Kim et al., 2013; Maqbool et al., 2015; Oh et al., 2012; Waheed et al., 2017; Xiao et al., 2018). Moreover, the composition of bacteria-entrapping media and their geometric structure have been continuously modified to enhance their durability at large scale applications (Iqbal et al., 2018; Kim et al., 2015; Lee et al., 2016; Nahm et al., 2017). Most of these studies have been limited to target the acyl-homoserine lactone (AHL) -based QS, which is known to regulate the Gram-negative bacterial populations only. This regulation may affect the relative distribution of Gram-positive and -negative bacteria in QQ-MBR. However, this has rarely been reported in the literature, except that Jo et al. (2016) observed a lower abundance of Gram-negative phylum/genus e.g. Proteobacteria and Thiothrix sp. in the biofilm of a QQ MBR than a conventional MBR. Hence, it is expected that in QQ-MBRs, a selective pressure is created to favor the growth of Gram-positive bacteria, resulting in distinct microbial community structures in both bulk sludge and biocake from those in conventional MBRs. Therefore, one of the objectives of this study was to investigate the selective accumulation of Gram-positive bacteria in QQ-MBRs.

Most of the QQ related studies monitored the changes in EPS contents and QS signal molecule levels in the sludge, but overlooked the impacts of several other factors such as sludge microbial ecology, sludge physicochemical properties and soluble microbial products (SMPs). In addition to the similar role of EPS in determining sludge hydrophobicity, surface charge, adhesion and flocculation ability, and thus affecting biofouling indirectly, SMPs also contribute to the membrane fouling directly and have been considered as a major (26–52%) organic foulant (Bouhabila et al., 2001; Liang et al., 2007). However, the role of these factors and their possible variations under QQ conditions are far from completely understood. It was, therefore, our second objective to understand the changes of key sludge components, sludge properties and microbial composition due to the quorum quenching phenomenon.

Overall, the objectives of the present study were to i) check the biofouling control potential of an AHL- and AI-2 degrading QQ consortium, ii) investigate the role of EPS in sludge filterability and membrane fouling, and iii) examine microbial responses to the QQ. To this end, a consortium of QQ bacteria with overlapping potential to mineralize various signal molecules, immobilized in a modified alginate-polysulfone mixture, was applied in a lab-scale MBR system. The QQ effects on biocake and mixed liquor were explored in terms of sludge dewaterability, settleability, microbial activity, floc size, production of SMPs and EPS. The microbial abundance and composition in mixed liquor, biocake and the immobilizing media were also investigated. This study is the first of this type to comprehensively examine the impacts of QQ on microbial ecology and highlights the overlooked microbial response.