Structure of the Ligand-blocked Periplasmic Entrance of the Bacterial Multidrug Efflux Protein TolC

Targeting the outer membrane of gram-negative foodborne pathogens for food safety: compositions, functions, and disruption strategies

Foodborne pathogens are a major threat to both food safety and public health. The current trend toward fresh and less processed foods and the misuse of antibiotics in food production have made controlling these pathogens even more challenging. The outer membrane has been employed as a practical target to combat foodborne Gram-negative pathogens due to its accessibility and importance. In this review, the compositions of the outer membrane are extensively described firstly, to offer a thorough overview of this target. Current strategies for disrupting the outer membrane are also discussed, with emphasized on their mechanism of action. The disruption of the outer membrane structure, whether caused by severe damage of the lipid bilayer or by interference with the biosynthesis pathway, has been demonstrated to represent an effective antimicrobial strategy. Interference with the outer membrane-mediated functions of barrier, efflux and adhesion also contributes to the fight against Gram-negative pathogens. Their potential for control of foodborne pathogens in the production chain are also proposed. However, it is possible that multiple components in the food matrix may act as a protective barrier against microorganisms, and it is often the case that contamination is not caused by a single microorganism. Further investigation is needed to determine the effectiveness and safety of these methods in more complex systems, and it may be advisable to consider a multi-technology combined approach. Additionally, further studies on outer membranes are necessary to discover more promising mechanisms of action.

Escherichia coli resistance mechanism AcrAB-TolC efflux pump interactions with commonly used antibiotics: a molecular dynamics study

While antibiotic resistance poses a threat from both Gram-positive bacteria (GPB) and Gram-negative bacteria (GNB), GNB pose a more imminent public health hazard globally. GNB are a threat to growing antibiotic resistance because of the complex makeup of the membrane. The AcrAB-TolC efflux pump is a known resistance mechanism of Escherichia coli (E. coli) cells. This study utilized molecular dynamics modeling to visualize some of the changes occurring at a molecular level when airborne bacteria are exposed to stress and antibiotics. This study was conducted to build upon previous experimental research showing that there is an increase in antibiotic resistance and efflux pump activity when exposed to aerosolization. AcrB and AcrAB-TolC proteins were simulated under standard and increased pressure to compare the effect of aerosolization on the binding to the three different antibiotics (puromycin (PUY), ampicillin (AMP) and sulfamethoxazole-trimethoprim (SXT)) to the AcrB binding site. Analysis such as root-mean-square deviation of atomic positions and root-mean-square fluctuation, the opening of TolC, and the significant molecular mechanics with generalized Born and surface area solvation (MM-GBSA) scores associated with specific ligands were recorded. Resistance in experimental data indicated a relationship between the docking scores and some ligand-protein interactions. Results showed that there was more flexibility in the proteins within simulations conducted under standard pressure for the AcrB protein and the full tripartite complex AcrAB-TolC, showing that increased pressure causes more rigidity. MM-GBSA scores, used to calculate the free energy of ligand-protein binding, did not show a significant change, but interestingly, the strongest MM-GBSA scores were for ligands that moved to another binding pocket and did not result in resistance or opening of the efflux pump. However, the ligand moved from the binding site and did not cause the opening of TolC to increase significantly, whereas PUY and AMP were bound to the binding site for the duration of all simulations. AMP ligands under increased pressure showed the largest change in opening of the TolC efflux pump and aligns with experimental data showing E. coli cells had the most resistance to AMP after aerosolization. These results, in addition to other real-time changes such as OM proteins and mutations of targets within the cell, could be used to delineate and mitigate antibiotic resistance mechanisms.

Molecular mechanisms of antibiotic resistance revisited

Antibiotic resistance is a global health emergency, with resistance detected to all antibiotics currently in clinical use and only a few novel drugs in the pipeline. Understanding the molecular mechanisms that bacteria use to resist the action of antimicrobials is critical to recognize global patterns of resistance and to improve the use of current drugs, as well as for the design of new drugs less susceptible to resistance development and novel strategies to combat resistance. In this Review, we explore recent advances in understanding how resistance genes contribute to the biology of the host, new structural details of relevant molecular events underpinning resistance, the identification of new resistance gene families and the interactions between different resistance mechanisms. Finally, we discuss how we can use this information to develop the next generation of antimicrobial therapies.

TolC-AcrA complex formation monitored by time dependent single-channel electrophysiology

Characterizing protein-protein interaction on a single molecular level is a challenge, experimentally as well as interpretation of the data. For example, Gram-negative bacteria contain protein complexes spanning the outer and inner cell wall devoted to efflux effectively cell toxic substances. Recent seminal work revealed the high-resolution structure of such a tripartic composition TolC-AcrA-AcrB suggesting to design inhibitors preventing efflux of antibiotics. To show that electrophysiology can provide supporting information here, we reconstitute single TolC homotrimer into a planar lipid membrane, apply a transmembrane voltage and follow the assembly of AcrA to TolC using the modulation of the ion current through TolC channel during binding. In particular, the presence of AcrA in solution increases the average ionic current through TolC and, moreover, reduces the ion-current fluctuations caused by flickering of TolC. Here, we show that statistical properties of ion-current fluctuations (the power spectral density) provide a complementary measure of the interaction of the TolC-AcrA complex in presence of putative efflux pump inhibitors. Both characteristics, the average ion current across TolC and the current noise, taken into consideration together, point to a stiffening of the tip of TolC which might reduce the formation of the complex.

Conformational dynamics and putative substrate extrusion pathways of the N-glycosylated outer membrane factor CmeC from Campylobacter jejuni

The outer membrane factor CmeC of the efflux machinery CmeABC plays an important role in conferring antibiotic and bile resistance to Campylobacter jejuni. Curiously, the protein is N-glycosylated, with the glycans playing a key role in the effective function of this system. In this work we have employed atomistic equilibrium molecular dynamics simulations of CmeC in a representative model of the C. jejuni outer membrane to characterise the dynamics of the protein and its associated glycans. We show that the glycans are more conformationally labile than had previously been thought. The extracellular loops of CmeC visit the open and closed states freely suggesting the absence of a gating mechanism on this side, while the narrow periplasmic entrance remains tightly closed, regulated via coordination to solvated cations. We identify several cation binding sites on the interior surface of the protein. Additionally, we used steered molecular dynamics simulations to elucidate translocation pathways for a bile acid and a macrolide antibiotic. These, and additional equilibrium simulations suggest that the anionic bile acid utilises multivalent cations to climb the ladder of acidic residues that line the interior surface of the protein.

Colicin E1 opens its hinge to plug TolC

The double membrane architecture of Gram-negative bacteria forms a barrier that is impermeable to most extracellular threats. Bacteriocin proteins evolved to exploit the accessible, surface-exposed proteins embedded in the outer membrane to deliver cytotoxic cargo. Colicin E1 is a bacteriocin produced by, and lethal to, Escherichia coli that hijacks the outer membrane proteins (OMPs) TolC and BtuB to enter the cell. Here, we capture the colicin E1 translocation domain inside its membrane receptor, TolC, by high-resolution cryo-electron microscopy to obtain the first reported structure of a bacteriocin bound to TolC. Colicin E1 binds stably to TolC as an open hinge through the TolC pore—an architectural rearrangement from colicin E1’s unbound conformation. This binding is stable in live E. coli cells as indicated by single-molecule fluorescence microscopy. Finally, colicin E1 fragments binding to TolC plug the channel, inhibiting its native efflux function as an antibiotic efflux pump, and heightening susceptibility to three antibiotic classes. In addition to demonstrating that these protein fragments are useful starting points for developing novel antibiotic potentiators, this method could be expanded to other colicins to inhibit other OMP functions.

Bacteria are constantly warring with each other for space and resources. As a result, they have developed a range of molecular weapons to poison, damage or disable other cells. For instance, bacteriocins are proteins that can latch onto structures at the surface of enemy bacteria and push toxins through their outer membrane.

Bacteria are increasingly resistant to antibiotics, representing a growing concern for modern healthcare. One way that they are able to survive is by using ‘efflux pumps’ studded through their external membranes to expel harmful drugs before these can cause damage. Budiardjo et al. wanted to test whether bacteriocins could interfere with this defence mechanism by blocking efflux pumps.

Bacteriocins are usually formed of binding elements (which recognise specific target proteins) and of a ‘killer tail’ that can stab the cell. Experiments showed that the binding parts of a bacteriocin could effectively ‘plug’ efflux pumps in Escherichia coli bacteria: high-resolution molecular microscopy revealed how the bacteriocin fragment binds to the pump, while fluorescent markers showed that it attached to the surface of E. coli and stopped the efflux pumps from working. As a result, lower amounts of antibiotics were necessary to kill the bacteria when bacteriocins were present.

The work by Budiardjo et al. could lead to new ways to combat bacteria that will reduce the need for current antibiotics. In the future, bacteriocins could also be harnessed to target other proteins than efflux pumps, allowing scientists to manipulate a range of bacterial processes.

[Antibacterial mechanism of Brevinin-2GHk, an antimicrobial peptide from Fejervarya limnocharis skin]

OBJECTIVE

To investigate the secondary structure, physicochemical properties and antibacterial activity of Brevinin- 2GHk (BR2GK), an antimicrobial peptide from frog skin, and explore its antibacterial mechanism.

METHODS

BR2GK was synthesized, purified with high performance liquid chromatography (HPLC) and identified using mass spectrometry. Circular dichroism was used to determine the secondary structure and physicochemical properties of BR2GK. Two-fold dilution method was used to determine the antibacterial activity of BR2GK, and its antibacterial mechanism was explored using laser scanning confocal microscopy (LSCM) and scanning electron microscopy (SEM). The hemolytic activity of BR2GK was analyzed in red blood cells. Isothermal titration calorimetry (ITC) and surface plasmon resonance imaging (SPRi) were employed to detect the binding of BR2GK to lipopolysaccharide (LPS), and the antioxidant activity of BR2GK was evaluated using biochemical kits.

RESULTS

The synthesized BR2GK, with a purity exceeding 95% after purification, had the basic characteristics of cationic antimicrobial peptides. BR2GK consisted mainly of α-helical structure in SDS solution and exhibited a broad-spectrum antibacterial activity. Antibacterial activity assay showed that BR2GK had inhibitory and killing activity against a variety of strains with a minimum inhibitory concentration (MIC) of 2.76 μmol/L against Staphylococcus aureus. Observation with LSCM and SEM showed that BR2GK at an active concentration caused bacterial cell membrane damage, cell swelling, contraction, deformation, and massive exudation of intracellular contents without causing hemolysis. ITC showed that the binding affinity K_D of BR2GK to LPS was 18.2±0.8 μmol/L. The antioxidant test showed that BR2GK was capable of effectively scavenging NO, ABTS and DPPH.

CONCLUSION

BR2GK, as a broad-spectrum antibacterial peptide with also a strong antioxidant capacity, does not cause hemolysis and is capable of binding to LPS. BR2GK has an important value for future design and synthesis of antimicrobial peptides with stronger antibacterial activity and lower cytotoxicity.

Structural Modeling of the Treponema pallidum Outer Membrane Protein Repertoire: a Road Map for Deconvolution of Syphilis Pathogenesis and Development of a Syphilis Vaccine

Treponema pallidum, an obligate human pathogen, has an outer membrane (OM) whose physical properties, ultrastructure, and composition differ markedly from those of phylogenetically distant Gram-negative bacteria. We developed structural models for the outer membrane protein (OMP) repertoire (OMPeome) of T. pallidum Nichols using solved Gram-negative structures, computational tools, and small-angle X-ray scattering (SAXS) of selected recombinant periplasmic domains. The T. pallidum "OMPeome" harbors two "stand-alone" proteins (BamA and LptD) involved in OM biogenesis and four paralogous families involved in the influx/efflux of small molecules: 8-stranded β-barrels, long-chain-fatty-acid transporters (FadLs), OM factors (OMFs) for efflux pumps, and T. pallidum repeat proteins (Tprs). BamA (TP0326), the central component of a β-barrel assembly machine (BAM)/translocation and assembly module (TAM) hybrid, possesses a highly flexible polypeptide-transport-associated (POTRA) 1-5 arm predicted to interact with TamB (TP0325). TP0515, an LptD ortholog, contains a novel, unstructured C-terminal domain that models inside the β-barrel. T. pallidum has four 8-stranded β-barrels, each containing positively charged extracellular loops that could contribute to pathogenesis. Three of five FadL-like orthologs have a novel α-helical, presumptively periplasmic C-terminal extension. SAXS and structural modeling further supported the bipartite membrane topology and tridomain architecture of full-length members of the Tpr family. T. pallidum's two efflux pumps presumably extrude noxious small molecules via four coexpressed OMFs with variably charged tunnels. For BamA, LptD, and OMFs, we modeled the molecular machines that deliver their substrates into the OM or external milieu. The spirochete's extended families of OM transporters collectively confer a broad capacity for nutrient uptake. The models also furnish a structural road map for vaccine development. IMPORTANCE The unusual outer membrane (OM) of T. pallidum, the syphilis spirochete, is the ultrastructural basis for its well-recognized capacity for invasiveness, immune evasion, and persistence. In recent years, we have made considerable progress in identifying T. pallidum's repertoire of OMPs. Here, we developed three-dimensional (3D) models for the T. pallidum Nichols OMPeome using structural modeling, bioinformatics, and solution scattering. The OM contains three families of OMP transporters, an OMP family involved in the extrusion of noxious molecules, and two "stand-alone" proteins involved in OM biogenesis. This work represents a major advance toward elucidating host-pathogen interactions during syphilis; understanding how T. pallidum, an extreme auxotroph, obtains a wide array of biomolecules from its obligate human host; and developing a vaccine with global efficacy.

The antibiotic efflux protein TolC is a highly evolvable target under colicin E1 or TLS phage selection

Bacteriophages and bacterial toxins are promising antibacterial agents to treat infections caused by multidrug-resistant (MDR) bacteria. In fact, bacteriophages have recently been successfully used to treat life-threatening infections caused by MDR bacteria (Schooley RT, Biswas B, Gill JJ, Hernandez-Morales A, Lancaster J, Lessor L, Barr JJ, Reed SL, Rohwer F, Benler S, et al. 2017. Development and use of personalized bacteriophage-based therapeutic cocktails to treat a patient with a disseminated resistant Acinetobacter baumannii infection. Antimicrob Agents Chemother. 61(10); Chan BK, Turner PE, Kim S, Mojibian HR, Elefteriades JA, Narayan D. 2018. Phage treatment of an aortic graft infected with Pseudomonas aeruginosa. Evol Med Public Health. 2018(1):60–66; Petrovic Fabijan A, Lin RCY, Ho J, Maddocks S, Ben Zakour NL, Iredell JR, Westmead Bacteriophage Therapy Team. 2020. Safety of bacteriophage therapy in severe Staphylococcus aureus infection. Nat Microbiol. 5(3):465–472). One potential problem with using these antibacterial agents is the evolution of resistance against them in the long term. Here, we studied the fitness landscape of the Escherichia coli TolC protein, an outer membrane efflux protein that is exploited by a pore forming toxin called colicin E1 and by TLS phage (Pagie L, Hogeweg P. 1999. Colicin diversity: a result of eco-evolutionary dynamics. J Theor Biol. 196(2):251–261; Andersen C, Hughes C, Koronakis V. 2000. Chunnel vision. Export and efflux through bacterial channel-tunnels. EMBO Rep. 1(4):313–318; Koronakis V, Andersen C, Hughes C. 2001. Channel-tunnels. Curr Opin Struct Biol. 11(4):403–407; Czaran TL, Hoekstra RF, Pagie L. 2002. Chemical warfare between microbes promotes biodiversity. Proc Natl Acad Sci U S A. 99(2):786–790; Cascales E, Buchanan SK, Duché D, Kleanthous C, Lloubès R, Postle K, Riley M, Slatin S, Cavard D. 2007. Colicin biology. Microbiol Mol Biol Rev. 71(1):158–229). By systematically assessing the distribution of fitness effects of ∼9,000 single amino acid replacements in TolC using either positive (antibiotics and bile salts) or negative (colicin E1 and TLS phage) selection pressures, we quantified evolvability of the TolC. We demonstrated that the TolC is highly optimized for the efflux of antibiotics and bile salts. In contrast, under colicin E1 and TLS phage selection, TolC sequence is very sensitive to mutations. Finally, we have identified a large set of mutations in TolC that increase resistance of E. coli against colicin E1 or TLS phage without changing antibiotic susceptibility of bacterial cells. Our findings suggest that TolC is a highly evolvable target under negative selection which may limit the potential clinical use of bacteriophages and bacterial toxins if evolutionary aspects are not taken into account.

Structure, Assembly, and Function of Tripartite Efflux and Type 1 Secretion Systems in Gram-Negative Bacteria

Tripartite efflux pumps and the related type 1 secretion systems (T1SSs) in Gram-negative organisms are diverse in function, energization, and structural organization. They form continuous conduits spanning both the inner and the outer membrane and are composed of three principal components-the energized inner membrane transporters (belonging to ABC, RND, and MFS families), the outer membrane factor channel-like proteins, and linking the two, the periplasmic adaptor proteins (PAPs), also known as the membrane fusion proteins (MFPs). In this review we summarize the recent advances in understanding of structural biology, function, and regulation of these systems, highlighting the previously undescribed role of PAPs in providing a common architectural scaffold across diverse families of transporters. Despite being built from a limited number of basic structural domains, these complexes present a staggering variety of architectures. While key insights have been derived from the RND transporter systems, a closer inspection of the operation and structural organization of different tripartite systems reveals unexpected analogies between them, including those formed around MFS- and ATP-driven transporters, suggesting that they operate around basic common principles. Based on that we are proposing a new integrated model of PAP-mediated communication within the conformational cycling of tripartite systems, which could be expanded to other types of assemblies.

Multidrug Efflux Pumps and the Two-Faced Janus of Substrates and Inhibitors

Antibiotics are miracle drugs that can cure infectious bacterial diseases. However, their utility is challenged by antibiotic-resistant bacteria emerging in clinics and straining modern medicine and our ways of life. Certain bacteria such as Gram-negative (Gram(-)) and Mycobacteriales species are intrinsically resistant to most clinical antibiotics and can further gain multidrug resistance through mutations and plasmid acquisition. These species stand out by the presence of an additional external lipidic membrane, the outer membrane (OM), that is composed of unique glycolipids. Although formidable, the OM is a passive permeability barrier that can reduce penetration of antibiotics but cannot affect intracellular steady-state concentrations of drugs. The two-membrane envelopes are further reinforced by active efflux transporters that expel antibiotics from cells against their concentration gradients. The major mechanism of antibiotic resistance in Gram(-) pathogens is the active efflux of drugs, which acts synergistically with the low permeability barrier of the OM and other mutational and plasmid-borne mechanisms of antibiotic resistance.The synergy between active efflux and slow uptake offers Gram(-) bacteria an impressive degree of protection from potentially harmful chemicals, but it is also their Achilles heel. Kinetic studies have revealed that even small changes in the efficiency of either of the two factors can have dramatic effects on drug penetration into the cell. In line with these expectations, two major approaches to overcome this antibiotic resistance mechanism are currently being explored: (1) facilitation of antibiotic penetration across the outer membranes and (2) avoidance and inhibition of clinically relevant multidrug efflux pumps. Herein we summarize the progress in the latter approach with a focus on efflux pumps from the resistance-nodulation-division (RND) superfamily. The ability to export various substrates across the OM at the expense of the proton-motive force acting on the inner membrane and the engagement of accessory proteins for their functions are the major mechanistic advantages of these pumps. Both the RND transporters and their accessory proteins are being targeted in the discovery of efflux pump inhibitors, which in combination with antibiotics can potentiate antibacterial activities. We discuss intriguing relationships between substrates and inhibitors of efflux pumps, as these two types of ligands face similar barriers and binding sites in the transporters and accessory proteins and both types of activities often occur with the same chemical scaffold. Several distinct chemical classes of efflux inhibitors have been discovered that are as structurally diverse as the substrates of efflux pumps. Recent mechanistic insights, both empirical and computational, have led to the identification of features that distinguish OM permeators and efflux pump avoiders as well as efflux inhibitors from substrates. These findings suggest a path forward for optimizing the OM permeation and efflux-inhibitory activities in antibiotics and other chemically diverse compounds.

Molecular Modeling of Multidrug Properties of Resistance Nodulation Division (RND) Transporters

Efflux pumps of the resistance nodulation division (RND) superfamily are among the major contributors to intrinsic and acquired multidrug resistance in Gram-negative bacteria. Structural information on AcrAB-TolC and MexAB-OprM, major efflux pumps of Escherichia coli and Pseudomonas aeruginosa respectively, boosted intensive research aimed at understanding the molecular mechanisms ruling the active extrusion processes. In particular, several studies were devoted to the understanding of the determinants behind the extraordinary broad specificity of the RND transporters AcrB and MexB. In this chapter, we discuss the ever-growing role computational methods have been playing in deciphering key structural and dynamical features of these transporters and of their interaction with substrates and inhibitors. We further discuss and illustrate examples from our lab of how molecular docking, homology modeling, all-atom molecular dynamics simulations and in silico free energy estimations can all together give precious insights into the processes of recognition and extrusion of substrates, as well as on the possible inhibition strategies.

Modulation of the Neisseria gonorrhoeae drug efflux conduit MtrE

Widespread antibiotic resistance, especially of Gram-negative bacteria, has become a severe concern for human health. Tripartite efflux pumps are one of the major contributors to resistance in Gram-negative pathogens, by efficiently expelling a broad spectrum of antibiotics from the organism. In Neisseria gonorrhoeae, one of the first bacteria for which pan-resistance has been reported, the most expressed efflux complex is MtrCDE. Here we present the electrophysiological characterisation of the outer membrane component MtrE and the membrane fusion protein MtrC, obtained by a combination of planar lipid bilayer recordings and in silico techniques. Our in vitro results show that MtrE can be regulated by periplasmic binding events and that the interaction between MtrE and MtrC is sufficient to stabilize this complex in an open state. In contrast to other efflux conduits, the open complex only displays a slight preference for cations. The maximum conductance we obtain in the in vitro recordings is comparable to that seen in our computational electrophysiology simulations conducted on the MtrE crystal structure, indicating that this state may reflect a physiologically relevant open conformation of MtrE. Our results suggest that the MtrC/E binding interface is an important modulator of MtrE function, which could potentially be targeted by new efflux inhibitors.

A lower isoelectric point increases signal sequence-mediated secretion of recombinant proteins through a bacterial ABC transporter

Efficient protein production for industrial and academic purposes often involves engineering microorganisms to produce and secrete target proteins into the culture. Pseudomonas fluorescens has a TliDEF ATP-binding cassette transporter, a type I secretion system, which recognizes C-terminal LARD3 signal sequence of thermostable lipase TliA. Many proteins are secreted by TliDEF in vivo when recombined with LARD3, but there are still others that cannot be secreted by TliDEF even when LARD3 is attached. However, the factors that determine whether or not a recombinant protein can be secreted through TliDEF are still unknown. Here, we recombined LARD3 with several proteins and examined their secretion through TliDEF. We found that the proteins secreted via LARD3 are highly negatively charged with highly-acidic isoelectric points (pI) lower than 5.5. Attaching oligo-aspartate to lower the pI of negatively-charged recombinant proteins improved their secretion, and attaching oligo-arginine to negatively-charged proteins blocked their secretion by LARD3. In addition, negatively supercharged green fluorescent protein (GFP) showed improved secretion, whereas positively supercharged GFP did not secrete. These results disclosed that proteins' acidic pI and net negative charge are major factors that determine their secretion through TliDEF. Homology modeling for TliDEF revealed that TliD dimer forms evolutionarily-conserved positively-charged clusters in its pore and substrate entrance site, which also partially explains the pI dependence of the TliDEF-dependent secretions. In conclusion, lowering the isoelectric point improved LARD3-mediated protein secretion, both widening the range of protein targets for efficient production via secretion and signifying an important aspect of ABC transporter-mediated secretions.

Xenon for tunnelling analysis of the efflux pump component OprN

Tripartite efflux pumps are among the main actors responsible for antibiotics resistance in Gram-negative bacteria. In the last two decades, structural studies gave crucial information about the assembly interfaces and the mechanistic motions. Thus rigidifying the assembly seems to be an interesting way to hamper the drug efflux. In this context, xenon is a suitable probe for checking whether small ligands could act as conformational lockers by targeting hydrophobic cavities. Here we focus on OprN, the outer membrane channel of the MexEF efflux pump from Pseudomonas aeruginosa. After exposing OprN crystals to xenon gas pressure, 14 binding sites were observed using X-ray crystallography. These binding sites were unambiguously characterized in hydrophobic cavities of OprN. The major site is observed in the sensitive iris-like region gating the channel at the periplasmic side, built by the three key-residues Leu 405, Asp 109, and Arg 412. This arrangement defines along the tunnel axis a strong hydrophobic/polar gradient able to enhance the passive efflux mechanism of OprN. The other xenon atoms reveal strategic hydrophobic regions of the channel scaffold to target, with the aim to freeze the dynamic movements responsible of the open/close conformational equilibrium in OprN.

Discovery of Novel Leptospirosis Vaccine Candidates Using Reverse and Structural Vaccinology

Leptospira spp. are diderm (two membranes) bacteria that infect mammals causing leptospirosis, a public health problem with global implications. Thousands of people die every year due to leptospirosis, especially in developing countries with tropical climates. Prophylaxis is difficult due to multiple factors, including the large number of asymptomatic hosts that transmit the bacteria, poor sanitation, increasing numbers of slum dwellers, and the lack of an effective vaccine. Several leptospiral recombinant antigens were evaluated as a replacement for the inactivated (bacterin) vaccine; however, success has been limited. A prospective vaccine candidate is likely to be a surface-related protein that can stimulate the host immune response to clear leptospires from blood and organs. In this study, a comprehensive bioinformatics approach based on reverse and structural vaccinology was applied toward the discovery of novel leptospiral vaccine candidates. The Leptospira interrogans serovar Copenhageni strain L1-130 genome was mined in silico for the enhanced identification of conserved β-barrel (βb) transmembrane proteins and outer membrane (OM) lipoproteins. Orthologs of the prospective vaccine candidates were screened in the genomes of 20 additional Leptospira spp. Three-dimensional structural models, with a high degree of confidence, were created for each of the surface-exposed proteins. Major histocompatibility complex II (MHC-II) epitopes were identified, and their locations were mapped on the structural models. A total of 18 βb transmembrane proteins and 8 OM lipoproteins were identified. These proteins were conserved among the pathogenic Leptospira spp. and were predicted to have epitopes for several variants of MHC-II receptors. A structural and functional analysis of the sequence of these surface proteins demonstrated that most βb transmembrane proteins seem to be TonB-dependent receptors associated with transportation. Other proteins identified included, e.g., TolC efflux pump proteins, a BamA-like OM component of the βb transmembrane protein assembly machinery, and the LptD-like LPS assembly protein. The structural mapping of the immunodominant epitopes identified the location of conserved, surface-exposed, immunogenic regions for each vaccine candidate. The proteins identified in this study are currently being evaluated for experimental evidence for their involvement in virulence, disease pathogenesis, and physiology, in addition to vaccine development.

Computer simulations suggest direct and stable tip to tip interaction between the outer membrane channel TolC and the isolated docking domain of the multidrug RND efflux transporter AcrB

One way by which bacteria achieve antibiotics resistance is preventing drug access to its target molecule for example through an overproduction of multi-drug efflux pumps of the resistance nodulation division (RND) protein super family of which AcrAB-TolC in Escherichia coli is a prominent example. Although representing one of the best studied efflux systems, the question of how AcrB and TolC interact is still unclear as the available experimental data suggest that either both proteins interact in a tip to tip manner or do not interact at all but are instead connected by a hexamer of AcrA molecules. Addressing the question of TolC-AcrB interaction, we performed a series of 100 ns - 1 µs-molecular dynamics simulations of membrane-embedded TolC in presence of the isolated AcrB docking domain (AcrB(DD)). In 5/6 simulations we observe direct TolC-AcrB(DD) interaction that is only stable on the simulated time scale when both proteins engage in a tip to tip manner. At the same time we find TolC opening and closing freely on extracellular side while remaining closed at the inner periplasmic bottleneck region, suggesting that either the simulated time is too short or additional components are required to unlock TolC.

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.

Focus on the Outer Membrane Factor OprM, the Forgotten Player from Efflux Pumps Assemblies

Antibiotics have been used extensively during several decades and we are now facing the emergence of multidrug resistant strains. It has become a major public concern, urging the need to discover new strategies to combat them. Among the different ways used by bacteria to resist antibiotics, the active efflux is one of the main mechanisms. In Gram-negative bacteria the efflux pumps are comprised of three components forming a long edifice crossing the complete cell wall from the inside to the outside of the cell. Blocking these pumps would permit the restoration of the effectiveness of the current antibiotherapy which is why it is important to increase our knowledge on the different proteins involved in these complexes. A tremendous number of experiments have been performed on the inner membrane protein AcrB from Escherichia coli and, to a lesser extent, the protein partners forming the AcrAB-TolC pump, but less information is available concerning the efflux pumps from other virulent Gram-negative bacteria. The present review will focus on the OprM outer membrane protein from the MexAB-OprM pump of Pseudomonas aeruginosa, highlighting similarities and differences compare to the archetypal AcrAB-TolC in terms of structure, function, and assembly properties.

The ins and outs of RND efflux pumps in Escherichia coli

Infectious diseases remain one of the principal causes of morbidity and mortality in the world. Relevant authorities including the WHO and CDC have expressed serious concern regarding the continued increase in the development of multidrug resistance among bacteria. They have also reaffirmed the urgent need for investment in the discovery and development of new antibiotics and therapeutic approaches to treat multidrug resistant (MDR) bacteria. The extensive use of antimicrobial compounds in diverse environments, including farming and healthcare, has been identified as one of the main causes for the emergence of MDR bacteria. Induced selective pressure has led bacteria to develop new strategies of defense against these chemicals. Bacteria can accomplish this by several mechanisms, including enzymatic inactivation of the target compound; decreased cell permeability; target protection and/or overproduction; altered target site/enzyme and increased efflux due to over-expression of efflux pumps. Efflux pumps can be specific for a single substrate or can confer resistance to multiple antimicrobials by facilitating the extrusion of a broad range of compounds including antibiotics, heavy metals, biocides and others, from the bacterial cell. To overcome antimicrobial resistance caused by active efflux, efforts are required to better understand the fundamentals of drug efflux mechanisms. There is also a need to elucidate how these mechanisms are regulated and how they respond upon exposure to antimicrobials. Understanding these will allow the development of combined therapies using efflux inhibitors together with antibiotics to act on Gram-negative bacteria, such as the emerging globally disseminated MDR pathogen Escherichia coli ST131 (O25:H4). This review will summarize the current knowledge on resistance-nodulation-cell division efflux mechanisms in E. coli, a bacteria responsible for community and hospital-acquired infections, as well as foodborne outbreaks worldwide.

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.

A novel packing arrangement of AcrB in the lipid bilayer membrane

The central component AcrB of the Escherichia coli drug efflux complex AcrA-AcrB-TolC has been extensively investigated by X-ray crystallography of detergent-protein 3-D crystals. In these crystals, AcrB packs as trimers - the functional unit. We visualized the AcrB-AcrB interaction in its native environment by examining E. coli lipid reconstituted 2-D crystals, which were overwhelmingly formed by asymmetric trimers stabilized by strongly-interacting monomers from adjacent trimers. Most interestingly, we observed lattices formed by an arrangement of AcrB monomers distinct from that in traditional trimers. This hitherto unobserved packing, might play a role in the biogenesis of trimeric AcrB.

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.

Stealth carriers for low-resolution structure determination of membrane proteins in solution

Structural studies of membrane proteins remain a great experimental challenge. Functional reconstitution into artificial nanoscale bilayer disc carriers that mimic the native bilayer environment allows the handling of membrane proteins in solution. This enables the use of small-angle scattering techniques for fast and reliable structural analysis. The difficulty with this approach is that the carrier discs contribute to the measured scattering intensity in a highly nontrivial fashion, making subsequent data analysis challenging. Here, an elegant solution to circumvent the intrinsic complexity brought about by the presence of the carrier disc is presented. In combination with small-angle neutron scattering (SANS) and the D2O/H2O-based solvent contrast-variation method, it is demonstrated that it is possible to prepare specifically deuterated carriers that become invisible to neutrons in 100% D2O at the length scales relevant to SANS. These `stealth' carrier discs may be used as a general platform for low-resolution structural studies of membrane proteins using well established data-analysis tools originally developed for soluble proteins.

Structure and operation of bacterial tripartite pumps

In bacteria such as Pseudomonas aeruginosa and Escherichia coli, tripartite membrane machineries, or pumps, determine the efflux of small noxious molecules, such as detergents, heavy metals, and antibiotics, and the export of large proteins including toxins. They are therefore influential in bacterial survival, particularly during infections caused by multidrug-resistant pathogens. In these tripartite pumps an inner membrane transporter, typically an ATPase or proton antiporter, binds and translocates export or efflux substrates. In cooperation with a periplasmic adaptor protein it recruits and opens a TolC family cell exit duct, which is anchored in the outer membrane and projects across the periplasmic space between inner and outer membranes. Assembled tripartite pumps thus span the entire bacterial cell envelope. We review the atomic structures of each of the three pump components and discuss how these have allowed high-resolution views of tripartite pump assembly, operation, and possible inhibition.

Molecular Dynamics Computer Simulations of Multidrug RND Efflux Pumps

Over-expression of multidrug efflux pumps of the Resistance Nodulation Division (RND) protein super family counts among the main causes for microbial resistance against pharmaceuticals. Understanding the molecular basis of this process is one of the major challenges of modern biomedical research, involving a broad range of experimental and computational techniques. Here we review the current state of RND transporter investigation employing molecular dynamics simulations providing conformational samples of transporter components to obtain insights into the functional mechanism underlying efflux pump-mediated antibiotics resistance in Escherichia coli and Pseudomonas aeruginosa.

On the role of TolC in multidrug efflux: the function and assembly of AcrAB-TolC tolerate significant depletion of intracellular TolC protein

TolC channel provides a route for the expelled drugs and toxins to cross the outer membrane of Escherichia coli. The puzzling feature of TolC structure is that the periplasmic entrance of the channel is closed by dense packing of 12 α-helices. Efflux pumps exemplified by AcrAB are proposed to drive the opening of TolC channel. How interactions with AcrAB promote the close-to-open transition in TolC remains unclear. In this study, we investigated in vivo the functional and physical interactions of AcrAB with the closed TolC and its conformer opened by mutations in the periplasmic entrance. We found that the two conformers of TolC are readily distinguishable in vivo by characteristic drug susceptibility, thiol modification and proteolytic profiles. However, these profiles of TolC variants respond neither to the in vivo stoichiometry of AcrAB:TolC nor to the presence of vancomycin, which is used often to assess the permeability of TolC channel. We further found that the activity and assembly of AcrAB-TolC tolerates significant changes in amounts of TolC and that only a small fraction of intracellular TolC is likely used to support efflux needs of E. coli. Our findings explain why TolC is not a good target for inhibition of multidrug efflux.

Locked on one side only: ground state dynamics of the outer membrane efflux duct TolC

Playing a major role in the expulsion of antibiotics and the secretion of cell toxins in conjunction with inner membrane transporters of three protein superfamilies, the outer membrane channel TolC occurs in at least two states blocking or permitting the passage of substrates. The details of the underlying gating mechanism are not fully understood. Addressing the questions of extracellular access control and periplasmic gating mechanism, we conducted a series of independent, unbiased 150-300 ns molecular dynamics simulations of wild-type TolC in a phospholipid membrane/150 mM NaCl water environment. We find that TolC opens and closes freely on the extracellular side, suggesting the absence of a gating mechanism on this side in the isolated protein. On the periplasmic side, we observe the outer periplasmic bottleneck region adopting in all simulations a conformation more open than the TolC wild-type crystal structures until in one run the successive binding of two sodium ions induces the transition to a conformation more closed than any of the available TolC X-ray structures. Concurrent with a heightened sodium residence probability near Asp374, the inner periplasmic bottleneck region at Asp374 remains closed throughout the simulations unless all NaCl is removed from the system, inducing a reopening of the outer and inner bottleneck. Our findings suggest that TolC is locked only on the periplasmic side in a sodium-dependent manner.

Effect of the F610A mutation on substrate extrusion in the AcrB transporter: explanation and rationale by molecular dynamics simulations

The tripartite efflux pump AcrAB-TolC is responsible for the intrinsic and acquired multidrug resistance in Escherichia coli. Its active part, the homotrimeric transporter AcrB, is in charge of the selective binding of substrates and energy transduction. The mutation F610A has been shown to significantly reduce the minimum inhibitory concentration of doxorubicin and many other substrates, although F610 does not appear to interact strongly with them. Biochemical study of transport kinetics in AcrB is not yet possible, except for some β-lactams, and other techniques should supply this important information. Therefore, in this work, we assess the impact of the F610A mutation on the functionality of AcrB by means of computational techniques, using doxorubicin as substrate. We found that the compound slides deeply inside the binding pocket after mutation, increasing the strength of the interaction. During subsequent conformational alterations of the transporter, doxorubicin was either not extruded from the binding site or displaced along a direction other than the one associated with extrusion. Our study indicates how subtle interactions determine the functionality of multidrug transporters, since decreased transport might not be simplistically correlated to decreased substrate binding affinity.

Structural and functional studies of a 50 kDa antigenic protein from Salmonella enterica serovar Typhi

The high typhoid incidence rate in developing and under-developed countries emphasizes the need for a rapid, affordable and accessible diagnostic test for effective therapy and disease management. TYPHIDOT®, a rapid dot enzyme immunoassay test for typhoid, was developed from the discovery of a ∼50 kDa protein specific for Salmonella enterica serovar Typhi. However, the structure of this antigen remains unknown till today. Studies on the structure of this antigen are important to elucidate its function, which will in turn increase the efficiency of the development and improvement of the typhoid detection test. This paper described the predictive structure and function of the antigenically specific protein. The homology modeling approach was employed to construct the three-dimensional structure of the antigen. The built structure possesses the features of TolC-like outer membrane protein. Molecular docking simulation was also performed to further probe the functionality of the antigen. Docking results showed that hexamminecobalt, Co(NH(3))(6)(3+), as an inhibitor of TolC protein, formed favorable hydrogen bonds with D368 and D371 of the antigen. The single point (D368A, D371A) and double point (D368A and D371A) mutations of the antigen showed a decrease (single point mutation) and loss (double point mutations) of binding affinity towards hexamminecobalt. The architecture features of the built model and the docking simulation reinforced and supported that this antigen is indeed the variant of outer membrane protein, TolC. As channel proteins are important for the virulence and survival of bacteria, therefore this ∼50 kDa channel protein is a good specific target for typhoid detection test.

Structures of sequential open states in a symmetrical opening transition of the TolC exit duct

In bacterial drug resistance and virulence pumps, an inner membrane (IM) transporter and periplasmic adaptor recruit an outer membrane (OM) trimeric TolC exit duct that projects an α-helical tunnel across the periplasm. The TolC periplasmic entrance is closed by densely packed α-helical coiled coils, inner H7/H8, and outer H3/H4, constrained by a hydrogen bond network. On recruitment, these coiled coils must undergo transition to the open state. We present 2.9 Å resolution crystal structures of two sequential TolC open states in which the network is incrementally disrupted and channel conductances defined in lipid bilayers. Superimposition of TolC(RS) (370 pS) and TolC(YFRS) (1,000 pS) on the TolC(WT) closed state (80 pS) showed that in the initial open-state TolC(RS), relaxation already causes approximately 14° twisting and expansion of helix H7 at the periplasmic tip, increasing interprotomer distances from 12.2 Å in TolC(WT) to 18.9 Å. However, in the crystal structure, the weakened Asp(374) pore constriction was maintained at the closed state 11.3 Å(2). In the advanced open-state TolC(YFRS), there was little further expansion at the tip, to interprotomer 21.3 Å, but substantial movement of inner and outer coiled coils dilated the pore constriction. In particular, upon abolition of the TolC(YFRS) intraprotomer Tyr(362)-Asp(153) link, a redirection of Tyr(362) and "bulge" in H3 allowed a simple movement outward of H8, establishing a 50.3 Å(2) opening. Root mean square deviations (rmsds) over the coiled coils of the three protomers of TolC(RS) and TolC(YFRS) illustrate that, whereas independent movement at the periplasmic tips may feature in the initial stages of opening, full dilation of the pore constriction is entirely symmetrical.

Assembly and transport mechanism of tripartite drug efflux systems

Multidrug efflux (MDR) pumps remove a variety of compounds from the cell into the external environment. There are five different classes of MDR pumps in bacteria, and quite often a single bacterial species expresses multiple classes of pumps. Although under normal circumstances MDR pumps confer low-level intrinsic resistance to drugs, the presence of drugs and mutations in regulatory genes lead to high level expression of MDR pumps that can pose problems with therapeutic treatments. This review focuses on the resistance nodulation cell division (RND)-class of MDR pumps that assemble from three proteins. Significant recent advancement in structural aspects of the three pump components has shed new light on the mechanism by which the tripartite efflux pumps extrude drugs. This new information will be critical in developing inhibitors against MDR pumps to improve the potency of prescribed drugs.

The Enterobacter aerogenes outer membrane efflux proteins TolC and EefC have different channel properties

The outer membrane proteins TolC and EefC from Enterobacter aerogenes are involved in multidrug resistance as part of two resistance-nodulation-division efflux systems. To gain more understanding in the molecular mechanism underlying drug efflux, we have undertaken an electrophysiological characterization of the channel properties of these two proteins. TolC and EefC were purified in their native trimeric form and then reconstituted in proteoliposomes for patch-clamp experiments and in planar lipid bilayers. Both proteins generated a small single channel conductance of about 80 pS in 0.5 M KCl, indicating a common gated structure. The resultant pores were stable, and no voltage-dependent openings or closures were observed. EefC has a low ionic selectivity (P(K)/P(Cl)= approximately 3), whereas TolC is more selective to cations (P(K)/P(Cl)= approximately 30). This may provide a possible explanation for the difference in drug selectivity between the AcrAB-TolC and EefABC efflux systems observed in vivo. The pore-forming activity of both TolC and EefC was severely inhibited by divalent cations entering from the extracellular side. Another characteristic of the TolC and EefC channels was the systematic closure induced by acidic pH. These results are discussed in respect to the physiological functions and structural models of TolC and EefC.

Protein Secretion and Membrane Insertion Systems in Gram-Negative Bacteria

In contrast to other organisms, gram-negative bacteria have evolved numerous systems for protein export. Eight types are known that mediate export across or insertion into the cytoplasmic membrane, while eight specifically mediate export across or insertion into the outer membrane. Three of the former secretory pathway (SP) systems, type I SP (ISP, ABC), IIISP (Fla/Path) and IVSP (Conj/Vir), can export proteins across both membranes in a single energy-coupled step. A fourth generalized mechanism for exporting proteins across the two-membrane envelope in two distinct steps (which we here refer to as type II secretory pathways [IISP]) utilizes either the general secretory pathway (GSP or Sec) or the twin-arginine targeting translocase for translocation across the inner membrane, and either the main terminal branch or one of several protein-specific export systems for translocation across the outer membrane. We here survey the various well-characterized protein translocation systems found in living organisms and then focus on the systems present in gram-negative bacteria. Comparisons between these systems suggest specific biogenic, mechanistic and evolutionary similarities as well as major differences.

Crystal structure of a divalent metal ion transporter CorA at 2.9 angstrom resolution

CorA family members are ubiquitously distributed transporters of divalent metal cations and are considered to be the primary Mg2+ transporter of Bacteria and Archaea. We have determined a 2.9 angstrom resolution structure of CorA from Thermotoga maritima that reveals a pentameric cone-shaped protein. Two potential regulatory metal binding sites are found in the N-terminal domain that bind both Mg2+ and Co2+. The structure of CorA supports an efflux system involving dehydration and rehydration of divalent metal ions potentially mediated by a ring of conserved aspartate residues at the cytoplasmic entrance and a carbonyl funnel at the periplasmic side of the pore.

Antibacterial drug discovery and structure-based design

Bacterial resistance continues to develop and pose a significant threat, both in hospitals and, more recently, in the community. A focus on other therapeutic areas by the larger pharmaceutical companies has left a shortfall in the pipeline of novel antibacterials. Recently, many new structures have been studied by structure-genomics initiatives, delivering a wealth of targets to consider. Using the tools of structure-based design, antibacterial discovery must exploit these targets to accelerate the process of drug discovery.

Efflux-mediated antimicrobial resistance

Antibiotic resistance continues to plague antimicrobial chemotherapy of infectious disease. And while true biocide resistance is as yet unrealized, in vitro and in vivo episodes of reduced biocide susceptibility are common and the history of antibiotic resistance should not be ignored in the development and use of biocidal agents. Efflux mechanisms of resistance, both drug specific and multidrug, are important determinants of intrinsic and/or acquired resistance to these antimicrobials, with some accommodating both antibiotics and biocides. This latter raises the spectre (as yet generally unrealized) of biocide selection of multiple antibiotic-resistant organisms. Multidrug efflux mechanisms are broadly conserved in bacteria, are almost invariably chromosome-encoded and their expression in many instances results from mutations in regulatory genes. In contrast, drug-specific efflux mechanisms are generally encoded by plasmids and/or other mobile genetic elements (transposons, integrons) that carry additional resistance genes, and so their ready acquisition is compounded by their association with multidrug resistance. While there is some support for the latter efflux systems arising from efflux determinants of self-protection in antibiotic-producing Streptomyces spp. and, thus, intended as drug exporters, increasingly, chromosomal multidrug efflux determinants, at least in Gram-negative bacteria, appear not to be intended as drug exporters but as exporters with, perhaps, a variety of other roles in bacterial cells. Still, given the clinical significance of multidrug (and drug-specific) exporters, efflux must be considered in formulating strategies/approaches to treating drug-resistant infections, both in the development of new agents, for example, less impacted by efflux and in targeting efflux directly with efflux inhibitors.

TolC but not AcrB is essential for multidrug-resistant Salmonella enterica serotype Typhimurium colonization of chicks

OBJECTIVES

To study the role of the multidrug efflux system AcrAB-TolC in resistance of multidrug-resistant Salmonella enterica serotype Typhimurium (S. Typhimurium) phage type DT104 and DT204 strains to detergents and bile salts. To evaluate the importance of the inner membrane transporter AcrB and the outer membrane component TolC of this efflux system in the colonization of two multidrug-resistant S. Typhimurium DT104 and one DT204 strain in chicks.

METHODS

acrB and tolC mutants of multidrug-resistant S. Typhimurium DT104 and DT204 strains were constructed by insertional inactivation of the acrB gene and deletion of the tolC gene. MICs of detergent and bile salts were determined for the wild-type strains, the acrB and the tolC mutant strains, in presence and in absence of the efflux pump inhibitor Phe-Arg beta-naphthylamide. The effect of sodium choleate on the in vitro growth of these strains was also evaluated. The LD50s of the strains were measured in a day-old chicken model, inoculated with several doses (1 x 10(3) to 1 x 10(8) cfu) by the oral route, for 7 days post-inoculation. The colonization levels were assessed at the sublethal dose 7 days post-inoculation by determining the number of cfu of Salmonella in the faeces, caecum, spleen and liver.

RESULTS

The decrease in resistance levels to bile salts was 64- to 256-fold higher for the tolC mutants than for the acrB mutants relative to those of the parental strains. Addition of choleate in culture medium did not affect the growth of the wild-type strains or that of the acrB mutants, but inhibited completely the growth of the tolC mutants. The LD50s were 1.0 x 10(6) and 1.2 x 10(7) cfu for one wild-type S. Typhimurium DT104 strain and the acrB mutant, respectively, and were >1.0 x 10(8) for the tolC mutants or the S. Typhimurium DT204 strains. In contrast to the acrB mutants, the tolC mutants were unable to colonize the caecum, spleen and liver after 1 week of infection. Moreover, in most chicks, no intestinal excretion was detected for the tolC mutants. The colonization levels of the acrB mutants were not significantly different from those of the wild-type strains.

CONCLUSIONS

TolC but not AcrB appears to be essential for multidrug-resistant S. Typhimurium DT104 and DT204 colonization of chicks, which is in accordance with their respective roles in resistance to detergents and bile salts. Therefore, TolC could be a better target than AcrB for the development of efflux pump inhibitors.