microorganisms Perspective The Primary Physiological Roles of Autoinducer 2 in Escherichia coli Are Chemotaxis and Biofilm Formation Sooyeon Song 1 and Thomas K. Wood 2,* 1 Department of Animal Science, Jeonbuk National University, Jeonju-si 54896, Jeollabuk-do, Korea; [email protected] 2 Department of Chemical Engineering, Pennsylvania State University, University Park, PA 16802-7000, USA * Correspondence: [email protected] Abstract: Autoinducer 2 (AI-2) is a ubiquitous metabolite but, instead of acting as a “universal signal,” relatively few phenotypes have been associated with it, and many scientists believe AI-2 is often a metabolic byproduct rather than a signal. Here, the aim is to present evidence that AI-2 influences both biofilm formation and motility (swarming and chemotaxis), using Escherichia coli as the model system, to establish AI-2 as a true signal with an important physiological role in this bacterium. In addition, AI-2 signaling is compared to the other primary signal of E. coli, indole, and it is shown that they have opposite effects on biofilm formation and virulence. Keywords: AI-2; chemotaxis; aggregation; biofilm; motility; Escherichia coli 1. Introduction Citation: Song, S.; Wood, T.K. The Quorum sensing (QS) is the process by which bacteria communicate via secreted Primary Physiological Roles of signals (autoinducers); once the concentration of the autoinducers reaches a threshold, Autoinducer 2 in Escherichia coli Are the signal is detected, and gene expression is altered [1]. The roles of QS are diverse and Chemotaxis and Biofilm Formation. include population density detection, virulence, biofilm formation, and the maintenance Microorganisms 2021, 9, 386. of the stress response [2]. Although inhibitors of QS (quorum-quenching compounds) https://doi.org/10.3390/ are still promoted as a means to reduce virulence without promoting resistance [3], these microorganisms9020386 compounds will indubitably and unfortunately fail. The main problem is that the inhibition of QS leads to pleiotropic effects that affect growth; hence, lab strains and clinical isolates Academic Editor: Charles M. Dozois rapidly evolve resistance to these compounds [4–6]. Clearly, it is imperative to have a better understanding of QS in order to be in a position to better control bacteria to prevent Received: 7 January 2021 diseases, such as stomach cancer and ulcers caused by Helicobacter pylori and Lyme disease Accepted: 12 February 2021 by Borrelia burgdorferi [7], and to utilize them for synthetic biology applications. Therefore, Published: 14 February 2021 in this opinion piece, we probe the physiological role of AI-2 by focusing on the best-studied bacterium, Escherichia coli. Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in 2. Autoinducer-2 published maps and institutional affil- iations. Commensal E. coli has several QS pathways, including one system based on in- dole (Figure1)[ 8–10], which is produced by TnaA from tryptophan, and another sys- tem based on autoinducer 2 (AI-2) (Figure1)[ 11], which is produced by LuxS from S- ribosylhomocysteine [12]. It appears AI-2 is used primarily for communication inside the gastrointestinal tract at 37 ◦C, while indole is used primarily at lower temperatures (30 ◦C Copyright: © 2021 by the authors. and lower) when the bacterium is outside of its eucaryotic host [9]. Although E. coli can Licensee MDPI, Basel, Switzerland. detect homoserine lactones through the autoinducer-1 sensor SdiA (a LuxR homolog), it This article is an open access article E. coli distributed under the terms and lacks a homoserine lactone synthase to produce the homoserine lactone signal, so conditions of the Creative Commons uses SdiA to eavesdrop on signals of other bacteria [13]. Moreover, there is an interaction Attribution (CC BY) license (https:// between these systems in that SdiA has been shown to be important for indole signaling in creativecommons.org/licenses/by/ E. coli [8]. 4.0/). Microorganisms 2021, 9, 386. https://doi.org/10.3390/microorganisms9020386 https://www.mdpi.com/journal/microorganisms Microorganisms 2021, 9, 386 2 of 5 Once produced by LuxS, the AI-2 precursor 4,5-dihydroxy-2,3-pentanedione is con- verted spontaneously into R-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran (R-THMF) in E. coli (Figure 1), and R-THMF is the active form of AI-2 [7]. Hydrophilic AI-2 is trans- ported from the cell by the membrane protein TqsA [14]. Once a threshold concentration is reached in the late exponential phase, AI-2 is imported into E. coli through its recogni- tion by the AI-2 receptor LsrB [15]. In addition to LsrB in E. coli, LuxP (e.g., Vibrio harveyi) and the dCACHE-domain proteins PctA/TlpQ (Pseudomonas aeruginosa) are receptors for AI-2 [15], so there are at least three forms of AI-2 receptors in different bacteria. Further- more, upon import, AI-2 is phosphorylated by LsrK in E. coli, and phosphorylated AI-2 Microorganisms 2021, 9, 386 2 of 5 binds and inhibits the repressor LsrR, which leads to changes in gene expression primarily at 37 °C [9]. Figure 1. ComparisonFigure 1. Comparison of the phenotypes of the phenotypes affected affected by by(A ()A autoinducer) autoinducer 2 2 (AI-2) (AI-2) and and (B) ( indole.B) indole. Curved Curved black arrows black indicate arrows indicate cell motility/movement,cell motility/movement, QS is quorum QS is quorum sensing, sensing, EHEC EHEC is isEscherichiaEscherichia colicoliO157:H7, O157:H7, and and flagella flagella are indicated are indicated by two lines by two at lines at one of the cellone ofpoles. the cell Human poles. Human cells are cells indicated are indicated by bypink pink hexa hexagons.gons. GreenGreen lightning lightning indicates indicates the application the application of indole. of The indole. The R R R-2-methyl-2,3,3,4-tetrah-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuranydroxytetrahydrofuran (R- (THMF)-THMF) form form ofof AI-2 AI-2 is shown.is shown. Once produced by LuxS, the AI-2 precursor 4,5-dihydroxy-2,3-pentanedione is con- 3. AI-2verted and spontaneouslyBiofilm Formation into R-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran (R-THMF) in E. coli (Figure1), and R-THMF is the active form of AI-2 [7]. Hydrophilic AI-2 is trans- Although indole reduces both pathogenic [16] and non-pathogenic E. coli biofilm for- ported from the cell by the membrane protein TqsA [14]. Once a threshold concentration is mationreached [17], inAI-2 the increases late exponential E. coli phase, biofilm AI-2 isformation imported into(FigureE. coli 1).through Initially, its recognition QS was linked to biofilmby formation the AI-2 receptor using LsrB non- [15E.]. Incoli addition species to LsrBand inbasedE. coli on, LuxP non-AI-2 (e.g., Vibrio signaling, harveyi) and specifically, for homoserinethe dCACHE-domain lactone increasing proteins PctA/TlpQ Pseudomonas (Pseudomonas aeruginosa aeruginosa [18].) areLater receptors studies, for AI- with Vibrio cholerae2 [ 15[19],], so Serratia there are liquefaciens at least three forms[20], ofand AI-2 Streptococcus receptors in different mutans bacteria. [21], confirmed Furthermore, the link of upon import, AI-2 is phosphorylated by LsrK in E. coli, and phosphorylated AI-2 binds QS to andbiofilm inhibits formation. the repressor LsrR, which leads to changes in gene expression primarily at The37 ◦ C[first9]. report of AI-2 and biofilm formation was indirect and based on masking AI- 2 signaling in E. coli with the QS inhibitor (5Z)-4-bromo-5-(bromomethylene)-3-butyl- 3. AI-2 and Biofilm Formation 2(5H)-furanone (henceforth furanone) from the alga Delisea pulchra; in this report, biofilm Although indole reduces both pathogenic [16] and non-pathogenic E. coli biofilm formation was reduced by 60 µg/mL furanone [22]. Later reports of AI-2 influencing bio- formation [17], AI-2 increases E. coli biofilm formation (Figure1). Initially, QS was linked film formationto biofilm formation were based using on non- luxSE. coli mutantsspecies and rather based than on non-AI-2 purified signaling, AI-2. For specifically, example, a luxS mutationfor homoserine in Streptococcus lactone increasinggordonii influencedPseudomonas aeruginosamixed-species[18]. Later biofilm studies, formation with Vibrio with Por- phyromonascholerae gingivalis[19], Serratia [23], liquefaciens a luxS[20 mutation], and Streptococcus had a mutanssmall [21impact], confirmed on the the architecture link of of KlebsiellaQS to pneumoniae biofilm formation. (although there was no effect for a luxS mutant for intestinal coloni- The first report of AI-2 and biofilm formation was indirect and based on masking zationAI-2 and signaling colonization in E. coli onwith polystyrene) the QS inhibitor [24], (5Z )-4-bromo-5-(bromomethylene)-3-butyl-and a luxS mutant increased biofilm for- mation2( 5Hin )-furanoneHelicobacter (henceforth pylori [25]. furanone) Unfortunately, from the alga theseDelisea earl pulchra;y resultsin this related report, biofilmto AI-2 via luxS formation was reduced by 60 µg/mL furanone [22]. Later reports of AI-2 influencing biofilm formation were based on luxS mutants rather than purified AI-2. For example, a Microorganisms 2021, 9, 386 3 of 5 luxS mutation in Streptococcus gordonii influenced mixed-species biofilm formation with Porphyromonas gingivalis [23], a luxS mutation had a small impact on the architecture of Klebsiella pneumoniae (although there was no effect for a luxS mutant for intestinal colonization and colonization on polystyrene) [24], and a luxS mutant increased biofilm formation in Helicobacter pylori [25]. Unfortunately, these early results related to AI-2 via luxS mutations do not provide compelling evidence due to pleiotropic changes resulting from the luxS mutations. The first direct demonstration that AI-2 was responsible for influencing biofilm forma- tion was the 4- to 24-fold increase in biofilm formation in microtiter plates for three E. coli strains upon the addition of 11 µM of purified AI-2 [11]. Moreover, AI-2 failed to stimulate biofilm formation for an lsrK AI-2 regulation mutant, and AI-2 stimulated biofilm forma- tion five-fold in flow cells [11].