Structure and function of TolC: the bacterial exit duct for proteins and drugs

Crystal structure of an antigenic outer-membrane protein from Salmonella Typhi suggests a potential antigenic loop and an efflux mechanism

ST50, an outer-membrane component of the multi-drug efflux system from Salmonella enterica serovar Typhi, is an obligatory diagnostic antigen for typhoid fever. ST50 is an excellent and unique diagnostic antigen with 95% specificity and 90% sensitivity and is used in the commercial diagnosis test kit (TYPHIDOT(TM)). The crystal structure of ST50 at a resolution of 2.98 Å reveals a trimer that forms an α-helical tunnel and a β-barrel transmembrane channel traversing the periplasmic space and outer membrane. Structural investigations suggest significant conformational variations in the extracellular loop regions, especially extracellular loop 2. This is the location of the most plausible antibody-binding domain that could be used to target the design of new antigenic epitopes for the development of better diagnostics or drugs for the treatment of typhoid fever. A molecule of the detergent n-octyl-β-D-glucoside is observed in the D-cage, which comprises three sets of Asp361 and Asp371 residues at the periplasmic entrance. These structural insights suggest a possible substrate transport mechanism in which the substrate first binds at the periplasmic entrance of ST50 and subsequently, via iris-like structural movements to open the periplasmic end, penetrates the periplasmic domain for efflux pumping of molecules, including poisonous metabolites or xenobiotics, for excretion outside the pathogen.

Structure, Function, and Assembly of Adhesive Organelles by Uropathogenic Bacteria

Bacteria assemble a wide range of adhesive proteins, termed adhesins, to mediate binding to receptors and colonization of surfaces. For pathogenic bacteria, adhesins are critical for early stages of infection, allowing the bacteria to initiate contact with host cells, colonize different tissues, and establish a foothold within the host. The adhesins expressed by a pathogen are also critical for bacterial-bacterial interactions and the formation of bacterial communities, including biofilms. The ability to adhere to host tissues is particularly important for bacteria that colonize sites such as the urinary tract, where the flow of urine functions to maintain sterility by washing away non-adherent pathogens. Adhesins vary from monomeric proteins that are directly anchored to the bacterial surface to polymeric, hair-like fibers that extend out from the cell surface. These latter fibers are termed pili or fimbriae, and were among the first identified virulence factors of uropathogenic Escherichia coli. Studies since then have identified a range of both pilus and non-pilus adhesins that contribute to bacterial colonization of the urinary tract, and have revealed molecular details of the structures, assembly pathways, and functions of these adhesive organelles. In this review, we describe the different types of adhesins expressed by both Gram-negative and Gram-positive uropathogens, what is known about their structures, how they are assembled on the bacterial surface, and the functions of specific adhesins in the pathogenesis of urinary tract infections.

Bordetella adenylate cyclase toxin: a unique combination of a pore-forming moiety with a cell-invading adenylate cyclase enzyme

The adenylate cyclase toxin-hemolysin (CyaA, ACT or AC-Hly) is a key virulence factor of the whooping cough agent Bordetella pertussis. CyaA targets myeloid phagocytes expressing the complement receptor 3 (CR3, known as αMβ2 integrin CD11b/CD18 or Mac-1) and translocates by a poorly understood mechanism directly across the cytoplasmic membrane into cell cytosol of phagocytes an adenylyl cyclase(AC) enzyme. This binds intracellular calmodulin and catalyzes unregulated conversion of cytosolic ATP into cAMP. Among other effects, this yields activation of the tyrosine phosphatase SHP-1, BimEL accumulation and phagocyte apoptosis induction. In parallel, CyaA acts as a cytolysin that forms cation-selective pores in target membranes. Direct penetration of CyaA into the cytosol of professional antigen-presenting cells allows the use of an enzymatically inactive CyaA toxoid as a tool for delivery of passenger antigens into the cytosolic pathway of processing and MHC class I-restricted presentation, which can be exploited for induction of antigen-specific CD8(+) cytotoxic T-lymphocyte immune responses.

Structural models of intrinsically disordered and calcium-bound folded states of a protein adapted for secretion

Many Gram-negative bacteria use Type I secretion systems, T1SS, to secrete virulence factors that contain calcium-binding Repeat-in-ToXin (RTX) motifs. Here, we present structural models of an RTX protein, RD, in both its intrinsically disordered calcium-free Apo-state and its folded calcium-bound Holo-state. Apo-RD behaves as a disordered polymer chain comprising several statistical elements that exhibit local rigidity with residual secondary structure. Holo-RD is a folded multi-domain protein with an anisometric shape. RTX motifs thus appear remarkably adapted to the structural and mechanistic constraints of the secretion process. In the low calcium environment of the bacterial cytosol, Apo-RD is an elongated disordered coil appropriately sized for transport through the narrow secretion machinery. The progressive folding of Holo-RD in the extracellular calcium-rich environment as it emerges form the T1SS may then favor its unidirectional export through the secretory channel. This process is relevant for hundreds of bacterial species producing virulent RTX proteins.

Effect of iron on expression of efflux pump (adeABC) and quorum sensing (luxI, luxR) genes in clinical isolates of Acinetobacter baumannii

Resistance-nodulation-division efflux system (RND) adeABC contributes to intrinsic resistance to various drug classes in Acinetobacter baumannii. Similarly, quorum sensing (QS) plays an important role in the biofilm formation and pathogenicity of this bacterium. The aims of this study were to evaluate the influence of iron limitation on the expression of efflux pump (adeABC) genes and QS (luxI, luxR) system by relative quantitative real-time polymerase chain reaction (qRT-PCR). In addition, DNA sequence and phylogenetic relatedness of biofilm-associated protein (Bap) gene was also investigated. Sixty-five multidrug-resistant isolates of A. baumannii were recovered from ICU patients of three hospitals in Kerman, Iran. The isolates were highly resistant to at least 11 antibiotics (MIC ≥64 μg/mL); however, 87% and 89% were susceptible to colistin and tigecycline, respectively (MIC 0.05 μg/mL) (p ≤ 0.05). We detected the presence of RND efflux pump, QS, and bap genes with the frequencies of 92% (adeA), 61.5% (adeB), 84.6% (adeC), 80% (luxI), 61% (luxR), and 66% (bap), respectively. qRT-PCR analysis showed that in some isolates, expression of both adeABC and luxI/R was increased more than fourfold in the presence of low iron (20 μm), suggesting the additional regulatory role of iron on both efflux pump and QS system. Alignment and phylogenetic analysis on the strong biofilm forming isolates confirmed that the fragments amplified were indeed part of bap gene and deduced sequence was similar to A. baumannii K9B410.

Hopanoids as functional analogues of cholesterol in bacterial membranes

The functionality of cellular membranes relies on the molecular order imparted by lipids. In eukaryotes, sterols such as cholesterol modulate membrane order, yet they are not typically found in prokaryotes. The structurally similar bacterial hopanoids exhibit similar ordering properties as sterols in vitro, but their exact physiological role in living bacteria is relatively uncharted. We present evidence that hopanoids interact with glycolipids in bacterial outer membranes to form a highly ordered bilayer in a manner analogous to the interaction of sterols with sphingolipids in eukaryotic plasma membranes. Furthermore, multidrug transport is impaired in a hopanoid-deficient mutant of the gram-negative Methylobacterium extorquens, which introduces a link between membrane order and an energy-dependent, membrane-associated function in prokaryotes. Thus, we reveal a convergence in the architecture of bacterial and eukaryotic membranes and implicate the biosynthetic pathways of hopanoids and other order-modulating lipids as potential targets to fight pathogenic multidrug resistance.

Heavy metal transport by the CusCFBA efflux system

It is widely accepted that the increased use of antibiotics has resulted in bacteria with developed resistance to such treatments. These organisms are capable of forming multi-protein structures that bridge both the inner and outer membrane to expel diverse toxic compounds directly from the cell. Proteins of the resistance nodulation cell division (RND) superfamily typically assemble as tripartite efflux pumps, composed of an inner membrane transporter, a periplasmic membrane fusion protein, and an outer membrane factor channel protein. These machines are the most powerful antimicrobial efflux machinery available to bacteria. In Escherichia coli, the CusCFBA complex is the only known RND transporter with a specificity for heavy metals, detoxifying both Cu(+) and Ag(+) ions. In this review, we discuss the known structural information for the CusCFBA proteins, with an emphasis on their assembly, interaction, and the relationship between structure and function.

Directionality of substrate translocation of the hemolysin A Type I secretion system

Type 1 secretion systems (T1SS) of Gram-negative bacteria are responsible for the secretion of various proteases, lipases, S-layer proteins or toxins into the extracellular space. The paradigm of these systems is the hemolysin A (HlyA) T1SS of Escherichia coli. This multiple membrane protein complex is able to secrete the toxin HlyA in one step across both E. coli membranes. Common to all secreted T1SS substrates is a C-terminal secretion sequence being necessary as well as sufficient for secretion. However, it is not known whether transport occurs directionally, i.e. the N- or the C-terminus of T1SS substrates is secreted first. We have addressed this question by constructing HlyA fusions with the rapidly folding eGFP resulting in a stalled T1SS. Differential labeling and subsequent fluorescence microscopic detection of C- and N-terminal parts of the fusions allowed us to demonstrate vectorial transport of HlyA through the T1SS with the C-terminus appearing first outside the bacterial cells.

Transposon Mutagenesis Paired with Deep Sequencing of Caulobacter crescentus under Uranium Stress Reveals Genes Essential for Detoxification and Stress Tolerance

The ubiquitous aquatic bacterium Caulobacter crescentus is highly resistant to uranium (U) and facilitates U biomineralization and thus holds promise as an agent of U bioremediation. To gain an understanding of how C. crescentus tolerates U, we employed transposon (Tn) mutagenesis paired with deep sequencing (Tn-seq) in a global screen for genomic elements required for U resistance. Of the 3,879 annotated genes in the C. crescentus genome, 37 were found to be specifically associated with fitness under U stress, 15 of which were subsequently tested through mutational analysis. Systematic deletion analysis revealed that mutants lacking outer membrane transporters (rsaFa and rsaFb), a stress-responsive transcription factor (cztR), or a ppGpp synthetase/hydrolase (spoT) exhibited a significantly lower survival rate under U stress. RsaFa and RsaFb, which are homologues of TolC in Escherichia coli, have previously been shown to mediate S-layer export. Transcriptional analysis revealed upregulation of rsaFa and rsaFb by 4- and 10-fold, respectively, in the presence of U. We additionally show that rsaFa mutants accumulated higher levels of U than the wild type, with no significant increase in oxidative stress levels. Our results suggest a function for RsaFa and RsaFb in U efflux and/or maintenance of membrane integrity during U stress. In addition, we present data implicating CztR and SpoT in resistance to U stress. Together, our findings reveal novel gene targets that are key to understanding the molecular mechanisms of U resistance in C. crescentus.

IMPORTANCE

Caulobacter crescentus is an aerobic bacterium that is highly resistant to uranium (U) and has great potential to be used in U bioremediation, but its mechanisms of U resistance are poorly understood. We conducted a Tn-seq screen to identify genes specifically required for U resistance in C. crescentus. The genes that we identified have previously remained elusive using other omics approaches and thus provide significant insight into the mechanisms of U resistance by C. crescentus. In particular, we show that outer membrane transporters RsaFa and RsaFb, previously known as part of the S-layer export machinery, may confer U resistance by U efflux and/or by maintaining membrane integrity during U stress.

Interaction of antibacterial compounds with RND efflux pumps in Pseudomonas aeruginosa

Pseudomonas aeruginosa infections are becoming increasingly difficult to treat due to intrinsic antibiotic resistance and the propensity of this pathogen to accumulate diverse resistance mechanisms. Hyperexpression of efflux pumps of the Resistance-Nodulation-Cell Division (RND)-type multidrug efflux pumps (e.g., MexAB-OprM), chromosomally encoded by mexAB-oprM, mexCD-oprJ, mexEF-oprN, and mexXY (-oprA) is often detected in clinical isolates and contributes to worrying multi-drug resistance phenotypes. Not all antibiotics are affected to the same extent by the aforementioned RND efflux pumps. The impact of efflux on antibiotic activity varies not only between different classes of antibiotics but also between members of the same family of antibiotics. Subtle differences in physicochemical features of compound-pump and compound-solvent interactions largely determine how compounds are affected by efflux activity. The combination of different high-resolution techniques helps to gain insight into the functioning of these molecular machineries. This review discusses substrate recognition patterns based on experimental evidence and computer simulations with a focus on MexB, the pump subunit of the main RND transporter in P. aeruginosa.

Structure of a bacterial toxin-activating acyltransferase

Secreted pore-forming toxins of pathogenic Gram-negative bacteria such as Escherichia coli hemolysin (HlyA) insert into host-cell membranes to subvert signal transduction and induce apoptosis and cell lysis. Unusually, these toxins are synthesized in an inactive form that requires posttranslational activation in the bacterial cytosol. We have previously shown that the activation mechanism is an acylation event directed by a specialized acyl-transferase that uses acyl carrier protein (ACP) to covalently link fatty acids, via an amide bond, to specific internal lysine residues of the protoxin. We now reveal the 2.15-Å resolution X-ray structure of the 172-aa ApxC, a toxin-activating acyl-transferase (TAAT) from pathogenic Actinobacillus pleuropneumoniae. This determination shows that bacterial TAATs are a structurally homologous family that, despite indiscernible sequence similarity, form a distinct branch of the Gcn5-like N-acetyl transferase (GNAT) superfamily of enzymes that typically use acyl-CoA to modify diverse bacterial, archaeal, and eukaryotic substrates. A combination of structural analysis, small angle X-ray scattering, mutagenesis, and cross-linking defined the solution state of TAATs, with intermonomer interactions mediated by an N-terminal α-helix. Superposition of ApxC with substrate-bound GNATs, and assay of toxin activation and binding of acyl-ACP and protoxin peptide substrates by mutated ApxC variants, indicates the enzyme active site to be a deep surface groove.

Architecture and roles of periplasmic adaptor proteins in tripartite efflux assemblies

Recent years have seen major advances in the structural understanding of the different components of tripartite efflux assemblies, which encompass the multidrug efflux (MDR) pumps and type I secretion systems. The majority of these investigations have focused on the role played by the inner membrane transporters and the outer membrane factor (OMF), leaving the third component of the system – the Periplasmic Adaptor Proteins (PAPs) – relatively understudied. Here we review the current state of knowledge of these versatile proteins which, far from being passive linkers between the OMF and the transporter, emerge as active architects of tripartite assemblies, and play diverse roles in the transport process. Recognition between the PAPs and OMFs is essential for pump assembly and function, and targeting this interaction may provide a novel avenue for combating multidrug resistance. With the recent advances elucidating the drug efflux and energetics of the tripartite assemblies, the understanding of the interaction between the OMFs and PAPs is the last piece remaining in the complete structure of the tripartite pump assembly puzzle.

Microbial Uptake, Toxicity, and Fate of Biofabricated ZnS:Mn Nanocrystals

Despite their importance in nano-environmental health and safety, interactions between engineered nanomaterials and microbial life remain poorly characterized. Here, we used the model organism E. coli to study the penetration requirements, subcellular localization, induction of stress responses, and long-term fate of luminescent Mn-doped ZnS nanocrystals fabricated under "green" processing conditions with a minimized ZnS-binding protein. We find that such protein-coated quantum dots (QDs) are unable to penetrate the envelope of unmodified E. coli but readily translocate to the cytoplasm of cells that have been made competent by chemical treatment. The process is dose-dependent and reminiscent of bacterial transformation. Cells that have internalized up to 0.5 μg/mL of nanocrystals do not experience a significant activation of the unfolded protein or SOS responses but undergo oxidative stress when exposed to high QD doses (2.5 μg/mL). Finally, although they are stable in quiescent cells over temperatures ranging from 4 to 42°C, internalized QDs are rapidly diluted by cell division in a process that does not involve TolC-dependent efflux. Taken together, our results suggest that biomimetic QDs based on low toxicity inorganic cores capped by a protein shell are unlikely to cause significant damage to the microbial ecosystem.

Functional analysis of Vibrio vulnificus RND efflux pumps homologous to Vibrio cholerae VexAB and VexCD, and to Escherichia coli AcrAB

Resistance-nodulation-division (RND) efflux pumps are associated with multidrug resistance in many gram-negative pathogens. The genome of Vibrio vulnificus encodes 11 putative RND pumps homologous to those of Vibrio cholerae and Escherichia coli. In this study, we analyzed three putative RND efflux pumps, showing homology to V. cholerae VexAB and VexCD and to E. coli AcrAB, for their functional roles in multidrug resistance of V. vulnificus. Deletion of the vexAB homolog resulted in increased susceptibility of V. vulnificus to bile acid, acriflavine, ethidium bromide, and erythromycin, whereas deletion of acrAB homologs rendered V. vulnificus more susceptible to acriflavine only. Deletion of vexCD had no effect on susceptibility of V. vulnificus to these chemicals. Upon exposure to these antibacterial chemicals, expression of tolCV1 and tolCV2, which are putative outer membrane factors of RND efflux pumps, was induced, whereas expression levels of vexAB, vexCD, and acrAB homologs were not significantly changed. Our results show that the V. vulnificus homologs of VexAB largely contributed to in vitro antimicrobial resistance with a broad substrate specificity that was partially redundant with the AcrAB pump homologs.

Vibrio cholerae MARTX toxin heterologous translocation of beta-lactamase and roles of individual effector domains on cytoskeleton dynamics

The Vibrio cholerae MARTXVc toxin delivers three effector domains to eukaryotic cells. To study toxin delivery and function of individual domains, the rtxA gene was modified to encode toxin with an in-frame beta-lactamase (Bla) fusion. The hybrid RtxA::Bla toxin was Type I secreted from bacteria; and then Bla was translocated into eukaryotic cells and delivered by autoprocessing, demonstrating that the MARTXVc toxin is capable of heterologous protein transfer. Strains that produce hybrid RtxA::Bla toxins that carry one effector domain in addition to Bla were found to more efficiently translocate Bla. In cell biological assays, the actin cross-linking domain (ACD) and Rho-inactivation domain (RID) are found to cross-link actin and inactivate RhoA, respectively, when other effector domains are absent, with toxin autoprocessing required for high efficiency. The previously unstudied alpha-beta hydrolase domain (ABH) is shown here to activate CDC42, although the effect is ameliorated when RID is also present. Despite all effector domains acting on cytoskeleton assembly, the ACD was sufficient to rapidly inhibit macrophage phagocytosis. Both the ACD and RID independently disrupted polarized epithelial tight junction integrity. The sufficiency of ACD but strong selection for retention of RID and ABH suggests these two domains may primarily function by modulating cell signaling.

Assessing the Outer Membrane Insertion and Folding of Multimeric Transmembrane β-Barrel Proteins

In addition to the cytoplasmic membrane, Gram-negative bacteria have a second lipid bilayer, the outer membrane, which is the de facto barrier between the cell and the extracellular milieu. Virtually all integral proteins of the outer membrane form β-barrels, which are inserted into the outer membrane by the BAM complex. Some outer membrane proteins, like the porins and trimeric autotransporter adhesins, are multimeric. In the former case, the porin trimer consists of three individual β-barrels, whereas in the latter, the single autotransporter β-barrel domain is formed by three separate polypeptides. This chapter reviews methods to investigate the folding and membrane insertion of multimeric OMPs and further explains the use of a BamA depletion strain to study the effects of the BAM complex on multimeric OMPs in E. coli.

Membrane defects accelerate outer membrane β-barrel protein folding

Outer membrane β-barrel proteins spontaneously fold into lipid bilayers with rates of folding that are strongly influenced by the physical properties of the membrane. We show that folding is accelerated when the bilayer is at the phase transition temperature, because of the coexistence of lipid phase domains and the high degree of defects present at domain boundaries. These results are consistent with previous observations of faster folding into thin and highly curved membranes, which also contain a higher prevalence of defects. The importance of defects in β-barrel folding provides insight into the intrinsic folding process and the biological assembly pathway.

Computational and experimental analysis of the secretome of Methylococcus capsulatus (Bath)

The Gram-negative methanotroph Methylococcus capsulatus (Bath) was recently demonstrated to abrogate inflammation in a murine model of inflammatory bowel disease, suggesting interactions with cells involved in maintaining mucosal homeostasis and emphasizing the importance of understanding the many properties of M. capsulatus. Secreted proteins determine how bacteria may interact with their environment, and a comprehensive knowledge of such proteins is therefore vital to understand bacterial physiology and behavior. The aim of this study was to systematically analyze protein secretion in M. capsulatus (Bath) by identifying the secretion systems present and the respective secreted substrates. Computational analysis revealed that in addition to previously recognized type II secretion systems and a type VII secretion system, a type Vb (two-partner) secretion system and putative type I secretion systems are present in M. capsulatus (Bath). In silico analysis suggests that the diverse secretion systems in M.capsulatus transport proteins likely to be involved in adhesion, colonization, nutrient acquisition and homeostasis maintenance. Results of the computational analysis was verified and extended by an experimental approach showing that in addition an uncharacterized protein and putative moonlighting proteins are released to the medium during exponential growth of M. capsulatus (Bath).

Secretome of obligate intracellular Rickettsia

The genus Rickettsia (Alphaproteobacteria, Rickettsiales, Rickettsiaceae) is comprised of obligate intracellular parasites, with virulent species of interest both as causes of emerging infectious diseases and for their potential deployment as bioterrorism agents. Currently, there are no effective commercially available vaccines, with treatment limited primarily to tetracycline antibiotics, although others (e.g. josamycin, ciprofloxacin, chloramphenicol, and azithromycin) are also effective. Much of the recent research geared toward understanding mechanisms underlying rickettsial pathogenicity has centered on characterization of secreted proteins that directly engage eukaryotic cells. Herein, we review all aspects of the Rickettsia secretome, including six secretion systems, 19 characterized secretory proteins, and potential moonlighting proteins identified on surfaces of multiple Rickettsia species. Employing bioinformatics and phylogenomics, we present novel structural and functional insight on each secretion system. Unexpectedly, our investigation revealed that the majority of characterized secretory proteins have not been assigned to their cognate secretion pathways. Furthermore, for most secretion pathways, the requisite signal sequences mediating translocation are poorly understood. As a blueprint for all known routes of protein translocation into host cells, this resource will assist research aimed at uniting characterized secreted proteins with their apposite secretion pathways. Furthermore, our work will help in the identification of novel secreted proteins involved in rickettsial 'life on the inside'.

Functional type 1 secretion system involved in Legionella pneumophila virulence

Legionella pneumophila is a Gram-negative pathogen found mainly in water, either in a free-living form or within infected protozoans, where it replicates. This bacterium can also infect humans by inhalation of contaminated aerosols, causing a severe form of pneumonia called legionellosis or Legionnaires' disease. The involvement of type II and IV secretion systems in the virulence of L. pneumophila is now well documented. Despite bioinformatic studies showing that a type I secretion system (T1SS) could be present in this pathogen, the functionality of this system based on the LssB, LssD, and TolC proteins has never been established. Here, we report the demonstration of the functionality of the T1SS, as well as its role in the infectious cycle of L. pneumophila. Using deletion mutants and fusion proteins, we demonstrated that the repeats-in-toxin protein RtxA is secreted through an LssB-LssD-TolC-dependent mechanism. Moreover, fluorescence monitoring and confocal microscopy showed that this T1SS is required for entry into the host cell, although it seems dispensable to the intracellular cycle. Together, these results underline the active participation of L. pneumophila, via its T1SS, in its internalization into host cells.

Structure-function of cyanobacterial outer-membrane protein, Slr1270: homolog of Escherichia coli drug export/colicin import protein, TolC

Compared to thylakoid and inner membrane proteins in cyanobacteria, no structure-function information is available presently for integral outer-membrane proteins (OMPs). The Slr1270 protein from the cyanobacterium Synechocystis 6803, over-expressed in Escherichia coli, was refolded, and characterized for molecular size, secondary structure, and ion-channel function. Refolded Slr1270 displays a single band in native-electrophoresis, has an α-helical content of 50-60%, as in E. coli TolC with which it has significant secondary-structure similarity, and an ion-channel function with a single-channel conductance of 80-200pS, and a monovalent ion (K(+):Cl(-)) selectivity of 4.7:1. The pH-dependence of channel conductance implies a role for carboxylate residues in channel gating, analogous to that in TolC.

The CpxR/CpxA two-component regulatory system up-regulates the multidrug resistance cascade to facilitate Escherichia coli resistance to a model antimicrobial peptide

A genome-wide susceptibility assay was used to identify specific CpxR-dependent genes that facilitate Escherichia coli resistance to a model cationic antimicrobial peptide, protamine. A total of 115 strains from the Keio Collection, each of which contained a deletion at a demonstrated or predicted CpxR/CpxA-dependent locus, were tested for protamine susceptibility. One strain that exhibited high susceptibility carried a deletion of tolC, a gene that encodes the outer membrane component of multiple tripartite multidrug transporters. Concomitantly, two of these efflux systems, AcrAB/TolC and EmrAB/TolC, play major roles in protamine resistance. Activation of the CpxR/CpxA system stimulates mar transcription, suggesting a new regulatory circuit that enhances the multidrug resistance cascade. Tripartite multidrug efflux systems contribute to bacterial resistance to protamine differently from the Tat system. DNase I footprinting analysis demonstrated that the CpxR protein binds to a sequence located in the -35 and -10 regions of mar promoter. This sequence resembles the consensus CpxR binding site, however, on the opposite strand. aroK, a CpxR-dependent gene that encodes a shikimate kinase in the tryptophan biosynthesis pathway, was also found to facilitate protamine resistance. Specific aromatic metabolites from this pathway, such as indole, can stimulate expression of well studied CpxR-dependent genes degP and cpxP, which are not components of the tripartite multidrug transporters. Thus, we propose a novel mechanism for E. coli to modulate resistance to protamine and likely other cationic antimicrobial peptides in which the CpxR/CpxA system up-regulates mar transcription in response to specific aromatic metabolites, subsequently stimulating the multidrug resistance cascade.

A novel high-throughput cell-based assay aimed at identifying inhibitors of DNA metabolism in bacteria

Bacterial biosensor strains can be useful tools for the discovery and characterization of antibacterial compounds. A plasmid-based reporter vector containing a transcriptional fusion between the recA promoter and green fluorescence protein gene was introduced into an Escherichia coli ΔtolC strain to create a biosensor strain that selectively senses inhibitors of DNA metabolism via the SOS response. The strain was used to develop a high-throughput assay to identify new inhibitors of DNA metabolism. Screening of the AstraZeneca compound library with this strain identified known inhibitors of DNA metabolism, as well as novel chemotypes. The cellular target of one novel series was elucidated as DNA gyrase through genetic characterization of laboratory-generated resistant mutants followed by 50% inhibitory concentration measurements in a DNA gyrase activity assay. These studies validated the use of this antibiotic biosensor strain to identify novel selective inhibitors of DNA metabolism by high-throughput screening.

Characterisation of SalRAB a salicylic acid inducible positively regulated efflux system of Rhizobium leguminosarum bv viciae 3841

Salicylic acid is an important signalling molecule in plant-microbe defence and symbiosis. We analysed the transcriptional responses of the nitrogen fixing plant symbiont, Rhizobium leguminosarum bv viciae 3841 to salicylic acid. Two MFS-type multicomponent efflux systems were induced in response to salicylic acid, rmrAB and the hitherto undescribed system salRAB. Based on sequence similarity salA and salB encode a membrane fusion and inner membrane protein respectively. salAB are positively regulated by the LysR regulator SalR. Disruption of salA significantly increased the sensitivity of the mutant to salicylic acid, while disruption of rmrA did not. A salA/rmrA double mutation did not have increased sensitivity relative to the salA mutant. Pea plants nodulated by salA or rmrA strains did not have altered nodule number or nitrogen fixation rates, consistent with weak expression of salA in the rhizosphere and in nodule bacteria. However, BLAST analysis revealed seventeen putative efflux systems in Rlv3841 and several of these were highly differentially expressed during rhizosphere colonisation, host infection and bacteroid differentiation. This suggests they have an integral role in symbiosis with host plants.

Secretome analysis of Anabaena sp. PCC 7120 and the involvement of the TolC-homologue HgdD in protein secretion

Secretion of proteins is a central strategy of bacteria to influence and respond to their environment. Until now, there has been very few discoveries regarding the cyanobacterial secrotome or the secretion machineries involved. For a mutant of the outer membrane channel TolC-homologue HgdD of Anabaena sp. PCC 7120, a filamentous and heterocyst-forming cyanobacterium, an altered secretome profile was reported. To define the role of HgdD in protein secretion, we have developed a method to isolate extracellular proteins of Anabaena sp. PCC 7120 wild type and an hgdD loss-of-function mutant. We identified 51 proteins of which the majority is predicted to have an extracellular secretion signal, while few seem to be localized in the periplasmic space. Eight proteins were exclusively identified in the secretome of wild-type cells, which coincides with the distribution of type I secretion signal. We selected three candidates and generated hemagglutinin-tagged fusion proteins which could be exclusively detected in the extracellular protein fraction. However, these proteins are not secreted in the hgdD-mutant background, where they are rapidly degraded. This confirms a direct function of HgdD in protein secretion and points to the existence of a quality control mechanism at least for proteins secreted in an HgdD-dependent pathway.

Structure of the periplasmic adaptor protein from a major facilitator superfamily (MFS) multidrug efflux pump

Periplasmic adaptor proteins are key components of bacterial tripartite efflux pumps. The 2.85 Å resolution structure of an MFS (major facilitator superfamily) pump adaptor, Aquifex aeolicus EmrA, shows linearly arranged α-helical coiled-coil, lipoyl, and β-barrel domains, but lacks the fourth membrane-proximal domain shown in other pumps to interact with the inner membrane transporter. The adaptor α-hairpin, which binds outer membrane TolC, is exceptionally long at 127 Å, and the β-barrel contains a conserved disordered loop. The structure extends the view of adaptors as flexible, modular components that mediate diverse pump assembly, and suggests that in MFS tripartite pumps a hexamer of adaptors could provide a periplasmic seal.

Free energy profiles of ion permeation and doxorubicin translocation in TolC

The outer membrane protein TolC of Escherichia coli forms a channel-tunnel pore spanning the periplasmic space and outer membrane, serving as the main exit duct for bacteria multidrug resistance and protein export. Many aspects of the transport mechanism of TolC are still unclear. Here, we have investigated the substrate permeability and gating mechanism of TolC by calculating the potential of mean forces (PMFs) for transporting sodium ion and doxorubicin through TolC using the adaptive biasing force (ABF) method. The transport mechanism is turned out to be substrate dependent. It is found that the periplasmic gate is required to open for the passage of both Na + and doxorubicin, but the conformational gating does not lead to permeation barrier for Na + at this region. The extracellular loops and K283 residues cause permeation barriers for Na + at the extracellular entrance, but not for doxorubicin due to the extensive interactions between the drug molecule and the protein. TolC exhibits high conformational flexibility during the transport of Na +, while doxorubicin seems to be able to stabilize TolC in the resting state with the periplasmic gate closed. The association of the TolC docking domain of AcrB does not lower the permeation barrier for doxorubicin at the periplasmic gate, while the gate opening induces the dissociation of the TolC–AcrB complex.

Interaction mediated by the putative tip regions of MdsA and MdsC in the formation of a Salmonella-specific tripartite efflux pump

To survive in the presence of a wide range of toxic compounds, gram-negative bacteria expel such compounds via tripartite efflux pumps that span both the inner and outer membranes. The Salmonella-specific MdsAB pump consists of MdsB, a resistance-nodulation-division (RND)-type inner membrane transporter (IMT) that requires the membrane fusion protein (MFP) MdsA, and an outer membrane protein (OMP; MdsC or TolC) to form a tripartite efflux complex. In this study, we investigated the role of the putative tip regions of MdsA and its OMPs, MdsC and TolC, in the formation of a functional MdsAB-mediated efflux pump. Comparative analysis indicated that although sequence homologies of MdsA and MdsC with other MFPs and OMPs, respectively, are extremely low, key residues in the putative tip regions of these proteins are well conserved. Mutagenesis studies on these conserved sites demonstrated their importance for the physical and functional interactions required to form an MdsAB-mediated pump. Our studies suggest that, despite differences in the primary amino acid sequences and functions of various OMPs and MFPs, interactions mediated by the conserved tip regions of OMP and MFP are required for the formation of functional tripartite efflux pumps in gram-negative bacteria.

Reciprocal regulation of resistance-nodulation-division efflux systems and the Cpx two-component system in Vibrio cholerae

The Cpx two-component regulatory system has been shown in Escherichia coli to alleviate stress caused by misfolded cell envelope proteins. The Vibrio cholerae Cpx system was previously found to respond to cues distinct from those in the E. coli system, suggesting that this system fulfills a different physiological role in the cholera pathogen. Here, we used microarrays to identify genes that were regulated by the V. cholerae Cpx system. Our observations suggest that the activation of the V. cholerae Cpx system does not induce expression of genes involved in the mitigation of stress generated by misfolded cell envelope proteins but promotes expression of genes involved in antimicrobial resistance. In particular, activation of the Cpx system induced expression of the genes encoding the VexAB and VexGH resistance-nodulation-division (RND) efflux systems and their cognate outer membrane pore protein TolC. The promoters for these loci contained putative CpxR consensus binding sites, and ectopic cpxR expression activated transcription from the promoters for the RND efflux systems. CpxR was not required for intrinsic antimicrobial resistance, but CpxR activation enhanced resistance to antimicrobial substrates of VexAB and VexGH. Mutations that inactivated VexAB or VexGH efflux activity resulted in the activation of the Cpx response, suggesting that vexAB and vexGH and the cpxP-cpxRA system are reciprocally regulated. We speculate that the reciprocal regulation of the V. cholerae RND efflux systems and the Cpx two-component system is mediated by the intracellular accumulation of an endogenously produced metabolic by-product that is normally extruded from the cell by the RND efflux systems.

TolC-dependent modulation of host cell death by the Francisella tularensis live vaccine strain

Francisella tularensis is a facultative intracellular, Gram-negative pathogen and the causative agent of tularemia. We previously identified TolC as a virulence factor of the F. tularensis live vaccine strain (LVS) and demonstrated that a ΔtolC mutant exhibits increased cytotoxicity toward host cells and elicits increased proinflammatory responses compared to those of the wild-type (WT) strain. TolC is the outer membrane channel component used by the type I secretion pathway to export toxins and other bacterial virulence factors. Here, we show that the LVS delays activation of the intrinsic apoptotic pathway in a TolC-dependent manner, both during infection of primary macrophages and during organ colonization in mice. The TolC-dependent delay in host cell death is required for F. tularensis to preserve its intracellular replicative niche. We demonstrate that TolC-mediated inhibition of apoptosis is an active process and not due to defects in the structural integrity of the ΔtolC mutant. These findings support a model wherein the immunomodulatory capacity of F. tularensis relies, at least in part, on TolC-secreted effectors. Finally, mice vaccinated with the ΔtolC LVS are protected from lethal challenge and clear challenge doses faster than WT-vaccinated mice, demonstrating that the altered host responses to primary infection with the ΔtolC mutant led to altered adaptive immune responses. Taken together, our data demonstrate that TolC is required for temporal modulation of host cell death during infection by F. tularensis and highlight how shifts in the magnitude and timing of host innate immune responses may lead to dramatic changes in the outcome of infection.

In and out: an analysis of epibiotic vs periplasmic bacterial predators

Bdellovibrio and like organisms (BALO) are obligate predators of Gram-negative bacteria, belonging to the α- and δ-proteobacteria. BALO prey using either a periplasmic or an epibiotic predatory strategy, but the genetic background underlying these phenotypes is not known. Here we compare the epibiotic Bdellovibrio exovorus and Micavibrio aeruginosavorus to the periplasmic B. bacteriovorus and Bacteriovorax marinus. Electron microscopy showed that M. aeruginosavorus, but not B. exovorus, can attach to prey cells in a non-polar manner through its longitudinal side. Both these predators were resistant to a surprisingly high number of antibiotic compounds, possibly via 26 and 19 antibiotic-resistance genes, respectively, most of them encoding efflux pumps. Comparative genomic analysis of all the BALOs revealed that epibiotic predators have a much smaller genome (ca. 2.5 Mbp) than the periplasmic predators (ca. 3.5 Mbp). Additionally, periplasmic predators have, on average, 888 more proteins, at least 60% more peptidases, and one more rRNA operon. Fifteen and 219 protein families were specific to the epibiotic and the periplasmic predators, respectively, the latter clearly forming the core of the periplasmic 'predatome', which is upregulated during the growth phase. Metabolic deficiencies of epibiotic genomes include the synthesis of inosine, riboflavin, vitamin B6 and the siderophore aerobactin. The phylogeny of the epibiotic predators suggests that they evolved by convergent evolution, with M. aeruginosavorus originating from a non-predatory ancestor while B. exovorus evolved from periplasmic predators by gene loss.

Functional analysis of TolC homologs in Vibrio vulnificus

Gram-negative bacteria use tripartite pumps to transport antibacterial drugs and other toxic compounds across the inner and outer membranes, which are separated by the periplasmic space. The TolC protein is an outer membrane factor that participates in the formation of tripartite efflux pumps. The genome of Vibrio vulnificus encodes two E. coli TolC homologs, TolCV1 and TolCV2. Here, we show that both TolCV1 and TolCV2 are involved in the efflux of antimicrobial agents. Deletion of tolCV1 resulted in increased susceptibility of V. vulnificus to chemical detergents, DNA intercalating agents, and antibiotics including erythromycin, novobiocin, and tetracycline, whereas deletion of tolCV2 rendered V. vulnificus more susceptible to the above mentioned antibiotics only. We also observed that tolCV1 deletion resulted in reduced motility of V. vulnificus. Our results indicate active roles for TolCV1 and TolCV2 in the physiology of V. vulnificus.

Interaction between the α-barrel tip of Vibrio vulnificus TolC homologs and AcrA implies the adapter bridging model

The AcrAB-TolC multidrug efflux pump confers resistance to Escherichia coli against many antibiotics and toxic compounds. The TolC protein is an outer membrane factor that participates in the formation of type I secretion systems. The genome of Vibrio vulnificus encodes two proteins homologous to the E. coli TolC, designated TolCV1 and TolCV2. Here, we show that both TolCV1 and TolCV2 partially complement the E. coli TolC function and physically interact with the membrane fusion protein AcrA, a component of the E. coli AcrAB-TolC efflux pump. Using site-directed mutational analyses and an in vivo cross-linking assay, we demonstrated that the α-barrel tip region of TolC homologs plays a critical role in the formation of functional AcrAB-TolC efflux pumps. Our findings suggest the adapter bridging model as a general assembly mechanism for tripartite drug efflux pumps in Gram-negative bacteria.

Structure-function analysis of the ATP-driven glycolipid efflux pump DevBCA reveals complex organization with TolC/HgdD

In Gram-negative bacteria, trans-envelope efflux pumps have periplasmic membrane fusion proteins (MFPs) as essential components. MFPs act as mediators between outer membrane factors (OMFs) and inner membrane factors (IMFs). In this study, structure-function relations of the ATP-driven glycolipid efflux pump DevBCA-TolC/HgdD from the cyanobacterium Anabaena sp. PCC 7120 were analyzed. The binding of the MFP DevB to the OMF TolC absolutely required the respective tip-regions. The interaction of DevB with the IMF DevAC mainly involved the β-barrel and the lipoyl domain. Efficient binding to DevAC and TolC, substrate recognition and export activity by DevAC were dependent on stable DevB hexamers.

Accumulation of periplasmic enterobactin impairs the growth and morphology of Escherichia coli tolC mutants

TolC is the outer membrane component of tripartite efflux pumps, which expel proteins, toxins and antimicrobial agents from Gram-negative bacteria. Escherichia coli tolC mutants grow well and are slightly elongated in rich media but grow less well than wild-type cells in minimal media. These phenotypes have no physiological explanation as yet. Here, we find that tolC mutants have highly aberrant shapes when grown in M9-glucose medium but that adding iron restores wild-type morphology. When starved for iron, E. coli tolC mutants synthesize but cannot secrete the siderophore enterobactin, which collects in the periplasm. tolC mutants unable to synthesize enterobactin display no growth or morphological defects, and adding exogenous enterobactin recreates these aberrations, implicating this compound as the causative agent. Cells unable to import enterobactin across the outer membrane grow normally, whereas cells that import enterobactin only to the periplasm become morphologically aberrant. Thus, tolC mutants grown in low iron conditions accumulate periplasmic enterobactin, which impairs bacterial morphology, possibly by sequestering iron and inhibiting an iron-dependent reaction involved in cell division or peptidoglycan synthesis. The results also highlight the need to supply sufficient iron when studying TolC-directed export or efflux, to eliminate extraneous physiological effects.

Metagenomic profiles of antibiotic resistance genes (ARGs) between human impacted estuary and deep ocean sediments

Knowledge of the origins and dissemination of antibiotic resistance genes (ARGs) is essential for understanding modern resistomes in the environment. The mechanisms of the dissemination of ARGs can be revealed through comparative studies on the metagenomic profiling of ARGs between relatively pristine and human-impacted environments. The deep ocean bed of the South China Sea (SCS) is considered to be largely devoid of anthropogenic impacts, while the Pearl River Estuary (PRE) in south China has been highly impacted by intensive human activities. Commonly used antibiotics (sulfamethazine, norfloxacin, ofloxacin, tetracycline, and erythromycin) have been detected through chemical analysis in the PRE sediments, but not in the SCS sediments. In the relatively pristine SCS sediments, the most prevalent and abundant ARGs are those related to resistance to macrolides and polypeptides, with efflux pumps as the predominant mechanism. In the contaminated PRE sediments, the typical ARG profiles suggest a prevailing resistance to antibiotics commonly used in human health and animal farming (including sulfonamides, fluoroquinolones, and aminoglycosides), and higher diversity in both genotype and resistance mechanism than those in the SCS. In particular, antibiotic inactivation significantly contributed to the resistance to aminoglycosides, β-lactams, and macrolides observed in the PRE sediments. There was a significant correlation in the levels of abundance of ARGs and those of mobile genetic elements (including integrons and plasmids), which serve as carriers in the dissemination of ARGs in the aquatic environment. The metagenomic results from the current study support the view that ARGs naturally originate in pristine environments, while human activities accelerate the dissemination of ARGs so that microbes would be able to tolerate selective environmental stress in response to anthropogenic impacts.

The Type 1 secretion pathway - the hemolysin system and beyond

Type 1 secretion systems (T1SS) are wide-spread among Gram-negative bacteria. An important example is the secretion of the hemolytic toxin HlyA from uropathogenic strains. Secretion is achieved in a single step directly from the cytosol to the extracellular space. The translocation machinery is composed of three indispensable membrane proteins, two in the inner membrane, and the third in the outer membrane. The inner membrane proteins belong to the ABC transporter and membrane fusion protein families (MFPs), respectively, while the outer membrane component is a porin-like protein. Assembly of the three proteins is triggered by accumulation of the transport substrate (HlyA) in the cytoplasm, to form a continuous channel from the inner membrane, bridging the periplasm and finally to the exterior. Interestingly, the majority of substrates of T1SS contain all the information necessary for targeting the polypeptide to the translocation channel - a specific sequence at the extreme C-terminus. Here, we summarize our current knowledge of regulation, channel assembly, translocation of substrates, and in the case of the HlyA toxin, its interaction with host membranes. We try to provide a complete picture of structure function of the components of the translocation channel and their interaction with the substrate. Although we will place the emphasis on the paradigm of Type 1 secretion systems, the hemolysin A secretion machinery from E. coli, we also cover as completely as possible current knowledge of other examples of these fascinating translocation systems. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

Structure of an atypical periplasmic adaptor from a multidrug efflux pump of the spirochete Borrelia burgdorferi

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Periplasmic adaptors are essential to tripartite drug efflux pump assembly.

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We present the structure of the periplasmic adaptor BesA from Borrelia burgdorferi.

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BesA lacks the α-hairpin shown to underpin exit duct recruitment and pump assembly.

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Recruitment of the TolC exit duct must be different in this pump.

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The BesA structure has implications for proposed models of pump assembly.

Periplasmic adaptor proteins are essential components of bacterial tripartite multidrug efflux pumps. Here we report the 2.35 Å resolution crystal structure of the BesA adaptor from the spirochete Borrelia burgdorferi solved using selenomethionine derivatized protein. BesA shows the archetypal linear, flexible, multi-domain architecture evident among proteobacteria and retains the lipoyl, β-barrel and membrane-proximal domains that interact with the periplasmic domains of the inner membrane transporter. However, it lacks the α-hairpin domain shown to establish extensive coiled-coil interactions with the periplasmic entrance helices of the outer membrane-anchored TolC exit duct. This has implications for the modelling of assembled tripartite efflux pumps.

The α-barrel tip region of Escherichia coli TolC homologs of Vibrio vulnificus interacts with the MacA protein to form the functional macrolide-specific efflux pump MacAB-TolC

TolC and its homologous family of proteins are outer membrane factors that are essential for exporting small molecules and toxins across the outer membrane in Gram-negative bacteria. Two open reading frames in the Vibrio vulnificus genome that encode proteins homologous to Escherichia coli TolC, designated TolCV1 and TolCV2, have 51.3% and 29.6% amino acid identity to TolC, respectively. In this study, we show that TolCV1 and TolCV2 functionally and physically interacted with the membrane fusion protein, MacA, a component of the macrolide-specific MacAB-TolC pump of E. coli. We further show that the conserved residues located at the aperture tip region of the α-hairpin of TolCV1 and TolCV2 played an essential role in the formation of the functional MacAB-TolC pump using site-directed mutational analyses. Our findings suggest that these outer membrane factors have conserved tip-to-tip interaction with the MacA membrane fusion protein for action of the drug efflux pump in Gram-negative bacteria.

Structure, Function and Regulation of Outer Membrane Proteins Involved in Drug Transport in Enterobactericeae: the OmpF/C - TolC Case

Antibiotic translocation across membranes of Gram-negative bacteria is a key step for the activity on their specific intracellular targets. Resistant bacteria control their membrane permeability as a first line of defense to protect themselves against external toxic compounds such as antibiotics and biocides. On one hand, resistance to small hydrophilic antibiotics such as ß-lactams and fluoroquinolones frequently results from the « closing » of their way in: the general outer membrane porins. On the other hand, an effective way out for a wide range of antibiotics is provided by TolC-like proteins, which are outer membrane components of multidrug efflux pumps. Accordingly, altered membrane permeability, including porin modifications and/or efflux pumps’ overexpression, is always associated to multidrug resistance (MDR) in a number of clinical isolates.

Several recent studies have highlighted our current understanding of porins/TolC structures and functions in Enterobacteriaceae. Here, we review the transport of antibiotics through the OmpF/C general porins and the TolC-like channels with regards to recent data on their structure, function, assembly, regulation and contribution to bacterial resistance.

Because MDR strains have evolved global strategies to identify and fight our antibiotic arsenal, it is important to constantly update our global knowledge on antibiotic transport.

Central metabolism controls transcription of a virulence gene regulator in Vibrio cholerae

ToxT is the central regulatory protein involved in activation of the main virulence genes in Vibrio cholerae. We have identified transposon insertions in central metabolism genes, whose disruption increases toxT transcription. These disrupted genes encode the primary respiration-linked sodium pump (NADH:ubiquinone oxidoreductase or NQR) and certain tricarboxylic acid (TCA) cycle enzymes. Observations made following stimulation of respiration in the nqr mutant or chemical inhibition of NQR activity in the TCA cycle mutants led to the hypothesis that NQR affects toxT transcription via the TCA cycle. That toxT transcription increased when the growth medium was supplemented with citrate, but decreased with oxaloacetate, focused our attention on the TCA cycle substrate acetyl-CoA and its non-TCA cycle metabolism. Indeed, both the nqr and the TCA cycle mutants increased acetate excretion. A similar correlation between acetate excretion and toxT transcription was observed in a tolC mutant and upon amino acid (NRES) supplementation. As acetate and its tendency to decrease pH exerted no strong effect on toxT transcription, and because disruption of the major acetate excretion pathway increased toxT transcription, we propose that toxT transcription is regulated by either acetyl-CoA or some close derivative.

Creation and screening of a multi-family bacterial oxidoreductase library to discover novel nitroreductases that efficiently activate the bioreductive prodrugs CB1954 and PR-104A

Two potentially complementary approaches to improve the anti-cancer strategy gene-directed enzyme prodrug therapy (GDEPT) are discovery of more efficient prodrug-activating enzymes, and development of more effective prodrugs. Here we demonstrate the utility of a flexible screening system based on the Escherichia coli SOS response to evaluate novel nitroreductase enzymes and prodrugs in concert. To achieve this, a library of 47 candidate genes representing 11 different oxidoreductase families was created and screened to identify the most efficient activators of two different nitroaromatic prodrugs, CB1954 and PR-104A. The most catalytically efficient nitroreductases were found in the NfsA and NfsB enzyme families, with NfsA homologues generally more active than NfsB. Some members of the AzoR, NemA and MdaB families also exhibited low-level activity with one or both prodrugs. The results of SOS screening in our optimised E. coli reporter strain SOS-R2 were generally predictive of the ability of nitroreductase candidates to sensitise E. coli to CB1954, and of the kcat/Km for each prodrug substrate at a purified protein level. However, we also found that not all nitroreductases express stably in human (HCT-116 colon carcinoma) cells, and that activity at a purified protein level did not necessarily predict activity in stably transfected HCT-116. These results highlight a need for all enzyme-prodrug partners for GDEPT to be assessed in the specific context of the vector and cell line that they are intended to target. Nonetheless, our oxidoreductase library and optimised screens provide valuable tools to identify preferred nitroreductase-prodrug combinations to advance to preclinical evaluation.