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Neisseria gonorrhoeae responds to host-derived antimicrobials by inducing the expression of the mtrCDE-encoded multidrug efflux pump, which expels microbicides, such as bile salts, fatty acids, and multiple extrinsically administered drugs, from the cell. In the absence of these cytotoxins, the TetR family member MtrR represses the mtrCDE genes. Although antimicrobial-dependent derepression of mtrCDE is clear, the physiological inducers of MtrR are unknown. Here, we report the crystal structure of an induced form of MtrR. In the binding pocket of MtrR, we observed electron density that we hypothesized was N-cyclohexyl-3-aminopropanesulfonic acid (CAPS), a component of the crystallization reagent. Using the MtrR-CAPS structure as an inducer-bound template, we hypothesized that bile salts, which bear significant chemical resemblance to CAPS, are physiologically relevant inducers. Indeed, characterization of MtrR-chenodeoxycholate and MtrR-taurodeoxycholate interactions, both in vitro and in vivo, revealed that these bile salts, but not glyocholate or taurocholate, bind MtrR tightly and can act as bona fide inducers. Furthermore, two residues, W136 and R176, were shown to be important in binding chenodeoxycholate but not taurodeoxycholate, suggesting different binding modes of the bile salts. These data provide insight into a crucial mechanism utilized by the pathogen to overcome innate human defenses. IMPORTANCE Neisseria gonorrhoeae causes a significant disease burden worldwide, and a meteoric rise in its multidrug resistance has reduced the efficacy of antibiotics previously or currently approved for therapy of gonorrheal infections. The multidrug efflux pump MtrCDE transports multiple drugs and host-derived antimicrobials from the bacterial cell and confers survival advantage on the pathogen within the host. Transcription of the pump is repressed by MtrR but relieved by the cytosolic influx of antimicrobials. Here, we describe the structure of induced MtrR and use this structure to identify bile salts as physiological inducers of MtrR. These findings provide a mechanistic basis for antimicrobial sensing and gonococcal protection by MtrR through the derepression of mtrCDE expression after exposure to intrinsic and clinically applied antimicrobials.

Mutations within the mtrR gene are commonly found amongst multidrug resistant clinical isolates of Neisseria gonorrhoeae, which has been labelled a superbug by the Centers for Disease Control and Prevention. These mutations appear to contribute to antibiotic resistance by interfering with the ability of MtrR to bind to and repress expression of its target genes, which include the mtrCDE multidrug efflux transporter genes and the rpoH oxidative stress response sigma factor gene. However, the DNA-recognition mechanism of MtrR and the consensus sequence within these operators to which MtrR binds has remained unknown. In this work, we report the crystal structures of MtrR bound to the mtrCDE and rpoH operators, which reveal a conserved, but degenerate, DNA consensus binding site 5′-MCRTRCRN4YGYAYGK-3′. We complement our structural data with a comprehensive mutational analysis of key MtrR-DNA contacts to reveal their importance for MtrR-DNA binding both in vitro and in vivo. Furthermore, we model and generate common clinical mutations of MtrR to provide plausible biochemical explanations for the contribution of these mutations to multidrug resistance in N. gonorrhoeae. Collectively, our findings unveil key biological mechanisms underlying the global stress responses of N. gonorrhoeae.

There has been a rapid spread of multidrug‐resistant (MDR) bacteria across the world. MDR efflux transporters are an important mechanism of antibiotic resistance in many pathogens among both Gram positive and Gram negative bacteria. These pumps can recognize a variety of chemically and structurally different compounds, including innate and clinically administered antibiotics. Intriguingly, these efflux pumps are often regulated by transcription factors that themselves bind a diverse set of substrates thereby allowing them to regulate the expression of their cognate MDR efflux pumps. One significant family of such transcription factors is the Multiple antibiotic resistance Repressor (MarR) family. Members of this family are well conserved across different bacterial species and in some cases are known to regulate vital bacterial functions. This review focusses on the role of MarR family transcriptional factors in antibiotic resistance within a select group of clinically relevant pathogens.

The authors describe the construction and use of tactile models for demonstrating intermolecular forces. These models are composed of inexpensive materials and can be used in college chemistry classrooms of varying levels including general and organic chemistry. The models were designed to be paired with an accompanying activity to encourage students to compare and contrast the three types of intermolecular forces (hydrogen bonding, dipole−dipole forces, and London dispersion forces) as well as determine the relationship between intermolecular forces and the physical properties of specific molecules. More importantly, the handheld models allow for visualization of molecular interactions to promote multisensory learning. In this work, the authors discuss the development, implementation, and survey results of a modeling activity for conceptualization and review of intermolecular forces in introductory, general, and organic chemistry classrooms.

Most bacteria in the biosphere are predicted to be polylysogens harbouring multiple prophages1–5. In studied systems, prophage induction from lysogeny to lysis is near-universally driven by DNA-damaging agents6. Thus, how co-residing prophages compete for cell resources if they respond to an identical trigger is unknown. Here we discover regulatory modules that control prophage induction independently of the DNA-damage cue. The modules bear little resemblance at the sequence level but share a regulatory logic by having a transcription factor that activates the expression of a neighbouring gene that encodes a small protein. The small protein inactivates the master repressor of lysis, which leads to induction. Polylysogens that harbour two prophages exposed to DNA damage release mixed populations of phages. Single-cell analyses reveal that this blend is a consequence of discrete subsets of cells producing one, the other or both phages. By contrast, induction through the DNA-damage-independent module results in cells producing only the phage sensitive to that specific cue. Thus, in the polylysogens tested, the stimulus used to induce lysis determines phage productivity. Considering the lack of potent DNA-damaging agents in natural habitats, additional phage-encoded sensory pathways to lysis likely have fundamental roles in phage–host biology and inter-prophage competition.

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