Pulse Generation in the Quorum Machinery of Pseudomonas aeruginosa

Bull Math Biol DOI 10.1007/s11538-017-0288-z ORIGINAL ARTICLE Pulse Generation in the Quorum Machinery of Pseudomonas aeruginosa Cicik Alfiniyah1,2 A. Jamie Wood1,3 · Martin A. Bees1 · Received: 28 September 2016 / Accepted: 3 May 2017 © The Author(s) 2017. This article is an open access publication Abstract Pseudomonas aeruginosa is a Gram-negative bacterium that is responsible for a wide range of infections in humans. Colonies employ quorum sensing (QS) to coordinate gene expression, including for virulence factors, swarming motility and complex social traits. The QS signalling system of P. aeruginosa is known to involve multiple control components, notably the las, rhl and pqs systems. In this paper, we examine the las system and, in particular, the repressive interaction of rsaL, an embedded small regulative protein, employing recent biochemical information to aid model construction. Using analytic methods, we show how this feature can give rise to excitable pulse generation in this subsystem with important downstream consequences for rhamnolipid production. We adopt a symmetric competitive inhibition to capture the binding in the lasI–rsaL intergenic region and show our results are not dependent on the exact choice of this functional form. Furthermore, we examine the coupling of lasR to the rhl system, the impact of the predicted capacity for pulse generation and the biophysical consequences of this behaviour. We hypothesize that the interaction between the las and rhl systems may provide a quorum memory to enable cells to trigger rhamnolipid production only when they are at the edge of an established aggregation. Keywords Pseudomonas aeruginosa · Quorum sensing · Excitable behaviour · Bifurcation analysis B Cicik Alfiniyah 1 Department of Mathematics, University of York, York YO10 5DD, UK 2 Department of Mathematics, Universitas Airlangga, Surabaya 60115, Indonesia 3 Department of Biology, University of York, York YO10 5DD, UK 123 C. Alfiniyah et al. 1 Introduction Pseudomonas aeruginosa is a common Gram-negative bacterium responsible for a wide range of infections, including those of the urinary and gastrointestinal tract, the skin, and, most prominently, the respiratory system in immunocompromised hosts and sufferers of cystic fibrosis (CF). P. aeruginosa is a well-studied opportunistic pathogen in many contexts; it is well known for its ability to form biofilms (O’Loughlin et al. 2013; Singh et al. 2015), its swarming behaviour (Daniels et al. 2004; Shrout et al. 2006), its rapid acquisition of resistance to antibiotics (Shih and Huang 2002) and its quorum sensing (QS) behaviour (Fuqua et al. 2001). QS in P. aeruginosa is of particular interest because the mechanism is more complex than the originally discovered, prototypical Lux homolog positive-feedback loop (e.g. James et al. 2000; Shadel and Baldwin 1991) and the number of genes regulated by QS is large (Sitnikov et al. 1995), especially those associated with virulence (O’Loughlin et al. 2013). Mathematical models of QS in Pseudomonas aeruginosa have received a lot of attention. They provide the formalism to summarize current understanding as well as the means to explore mechanisms and evaluate emergent solution behaviour. Here, we develop a model description, employing recent genomic information and bioinformatic techniques, and explore mechanisms for the generation of pulses and memory effects for downstream rhamnolipid production. In P. aeruginosa quorum sensing is governed by a hierarchical Luxl/LuxR system, which consists of two homolog pairs: LasI/LasR and RhlI/RhlR (Miller and Bassler 2001). Under this process, formation of the HSL autoinducers N-(3-oxododecanoyl)HSL and N-(butyryl)-HSL is synthesised by LasI and RhlI, respectively (see Fig. 1). It should be noted, however, that signalling systems of las and rhl are specific in their activation of autoinducers, i.e. N-(3-oxododecanoyl)-HSL is unable to activate RhlR and, similarly, LasR cannot be activated by N-(butyryl)- HSL (Latifi et al. 1995; Pearson et al. 1997). Although biochemically independent, the las system is able to exert control of the rhl system through the transcriptional promotion of the RhlR gene by LasR/N-(3-oxododecanoyl)-HSL (Pesci et al. 1997; Latifi et al. 1996). As well as gene regulation effects, the rhl system has an important function of modulating rhamnolipid production via rhlAB. Rhamnolipids are particularly important in swarming motility where they are postulated to lower surface tension and allow expansion of the colony through their surfactant and wetting properties, driving the bacteria to swarm on surfaces (Glick et al. 2010; Kohler et al. 2000). In addition, a quinolone system (Dubern and Diggle 2008) may also modulate these interconnecting feedback loops (for simplicity, we do not model this aspect of QS here). The first models of QS in P. aeruginosa were of the Lux (James et al. 2000) and the Las systems (Dockery and Keener 2001). Both descriptions, and subsequent models, highlight the existence of a fold bifurcation structure for the concentration of the response regulator in response to bulk cell concentration. The seminal paper by Dockery and Keener (2001) provides the foundation for the emergence of QS based on formal mass action arguments. However, there have been significant increases in biochemical knowledge of this system in the last 15 years. Fagerlind et al. (2003) constructed a large mass action model of the coupled Las and Rhl systems, including the effects of both RsaL and Vfr. This work also emphasizes the existence of 123 Pulse Generation in the Quorum Machinery of… Extracellular 3O-C12-HSL Hex 3O-C12-HSL HL LasR RL s ms ms LasR/ 3O-C12-HSL RL LasI lasI HL IL min ms-s s ms-s rsaL SL min s rhlR min RhlR C4-HSL HR RR ms RhlR/ C4-HSL RR s RhlI s ms HR IR min ms-s rhlI Fig. 1 The quorum sensing signalling system in Pseudomonas aeruginosa is composed of las and rhl systems. Arrows and barred arrows indicate activating (positive) and inhibiting (negative) regulatory interactions, respectively. Shapes on the diagram depict autoregulation terminology. Letters associated with each arrow reflect the associated time scale (ms = millisecond, s = second, and min = minute). Symbols associated with each shape are detailed in Table 1. Adopted from Van Delden and Iglewski (1998) (Color figure online) the classic twofold bifurcation diagram for the activation level of the system (typically for the levels of liganded LasR) with respect to the external concentration of HSL. Subsequent work Fagerlind et al. (2005) then explored how anti-virulence drugs 123 C. Alfiniyah et al. Table 1 Description of dimensional variables Variable Description Unit RL LasR nM HL 3O-C12-HSL nM RL H LasR/3O-C12-HSL complex nM IL IˆL LasI nM lasI mRNA nM SL RsaL nM Ŝ L rsaL mRNA nM RR RhlR nM HR C4-HSL nM RR H RhlR/C4-HSL complex nM R̂ R rhlR mRNA nM (...truncated)