An RND-type efflux system in Borrelia burgdorferi is involved in virulence and resistance to antimicrobial compounds

Specificity and role of the Borrelia burgdorferi CtpA protease in outer membrane protein processing

To further characterize the function of the Borrelia burgdorferi C-terminal protease CtpA, we used site-directed mutagenesis to alter the putative CtpA cleavage site of one of its known substrates, the outer membrane (OM) porin P13. These mutations resulted in only partial blockage of P13 processing. Ectopic expression of a C-terminally truncated P13 in B. burgdorferi indicated that the C-terminal peptide functions as a safeguard against misfolding or mislocalization prior to its proteolytic removal by CtpA. In a parallel study of Borrelia burgdorferi lipoprotein sorting mechanisms, we observed a lower-molecular-weight variant of surface lipoprotein OspC that was particularly prominent with OspC mutants that mislocalized to the periplasm or contained C-terminal epitope tags. Further investigation revealed that the variant resulted from C-terminal proteolysis by CtpA. Together, these findings indicate that CtpA rather promiscuously targets polypeptides that lack structurally constrained C termini, as proteolysis appears to occur independently of a specific peptide recognition sequence. Low-level processing of surface lipoproteins such as OspC suggests the presence of a CtpA-dependent quality control mechanism that may sense proper translocation of integral outer membrane proteins and surface lipoproteins by detecting the release of C-terminal peptides.

Global proteome analysis of Leptospira interrogans

Comparative global proteome analyses were performed on Leptospira interrogans serovar Copenhageni grown under conventional in vitro conditions and those mimicking in vivo conditions (iron limitation and serum presence). Proteomic analyses were conducted using iTRAQ and LC-ESI-tandem mass spectrometry complemented with two-dimensional gel electrophoresis and MALDI-TOF mass spectrometry. A total of 563 proteins were identified in this study. Altered expression of 65 proteins, including upregulation of the L. interrogans virulence factor Loa22 and 5 novel proteins with homology to virulence factors found in other pathogens, was observed between the comparative conditions. Immunoblot analyses confirmed upregulation of 5 of the known or putative virulence factors in L. interrogans exposed to the in vivo-like environmental conditions. Further, ELISA analyses using serum from patients with leptospirosis and immunofluorescence studies performed on liver sections derived from L. interrogans-infected hamsters verified expression of all but one of the identified proteins during infection. These studies, which represent the first documented comparative global proteome analysis of Leptospira, demonstrated proteome alterations under conditions that mimic in vivo infection and allowed for the identification of novel putative L. interrogans virulence factors.

The L. interrogans proteome was analyzed using iTRAQ and 2DGE. These analyses identified 563 proteins and altered expression of 65 proteins upon growth of L. interrogans under in vivo-like conditions, including upregulation of the L. interrogans virulence factor Loa22, a putative lipoprotein with primary amino acid sequence similarity to the outer surface protein ErpY of B. burgdorferi, and 4 additional proteins with homology to virulence factors found in other pathogens.

RamA, a member of the AraC/XylS family, influences both virulence and efflux in Salmonella enterica serovar Typhimurium

The transcriptomes of Salmonella enterica serovar Typhimurium SL1344 lacking a functional ramA or ramR or with plasmid-mediated high-level overexpression of ramA were compared to those of the wild-type parental strain. Inactivation of ramA led to increased expression of 14 SPI-1 genes and decreased expression of three SPI-2 genes, and it altered expression of ribosomal biosynthetic genes and several amino acid biosynthetic pathways. Furthermore, disruption of ramA led to decreased survival within RAW 264.7 mouse macrophages and attenuation within the BALB/c ByJ mouse model. Highly overexpressed ramA led to increased expression of genes encoding multidrug resistance (MDR) efflux pumps, including acrAB, acrEF, and tolC. Decreased expression of 34 Salmonella pathogenicity island (SPI) 1 and 2 genes, decreased SipC production, decreased adhesion to and survival within macrophages, and decreased colonization of Caenorhabditis elegans were also seen. Disruption of ramR led to the increased expression of ramA, acrAB, and tolC, but not to the same level as when ramA was overexpressed on a plasmid. Inactivation of ramR had a more limited effect on pathogenicity gene expression. In silico analysis of a suggested RamA-binding consensus sequence identified target genes, including ramR, acrA, tolC, sipABC, and ssrA. This study demonstrates that the regulation of a mechanism of MDR and expression of virulence genes show considerable overlap, and we postulate that such a mechanism is dependent on transcriptional activator concentration and promoter sensitivity. However, we have no evidence to support the hypothesis that increased MDR via RamA regulation of AcrAB-TolC gives rise to a hypervirulent strain.

Comparison of in vitro activities of tigecycline, doxycycline, and tetracycline against the spirochete Borrelia burgdorferi

Tigecycline is a new glycylcycline that has recently been revealed to be very effective in vitro against a variety of Gram-negative and Gram-positive bacteria including multi-drug resistant microorganisms. Using a standardized microdilution susceptibility testing method, we determined the minimal inhibitory concentrations (MICs) and the minimal bactericidal concentrations (MBCs) of tigecycline, in parallel with doxycycline, tetracycline, and other antibiotic agents relevant for Lyme borreliosis treatment such as ceftriaxone and cefotaxime. The activity of all agents against 16 different Borrelia isolates belonging to all borrelial genospecies known to be pathogenic for humans was investigated and analyzed under standardized conditions. The overall rank order of MIC(90)s was tigecycline (≤0.016 mg/L) > ceftriaxone (0.03 mg/L) > cefotaxime (≤0.125 mg/L) > doxycycline (0.25 mg/L) > tetracycline (0.25 mg/L). The rank order of MBC(90)s was tigecycline (0.5 mg/L) > ceftriaxone (2 mg/L) > tetracycline (16 mg/L) > doxycycline (16 mg/L) > cefotaxime (>16 mg/L). High in vitro activity of the new glycylcycline against Borrelia was further substantiated by time-kill experiments performed with B. afzelii isolate EB1. Parallel testing of tigecycline and ceftriaxone demonstrated a bacteriostatic effect for 0.016 mg/L of tigecycline and for 0.03 mg/L for ceftriaxone after 72 h of incubation. Moreover, tigecycline was bactericidal at a concentration of 0.25 mg/L showing a >3 log(10) unit reduction of the initial inoculum, whereas for ceftriaxone a concentration of 2 mg/L was needed.

Borrelia burgdorferi locus BB0795 encodes a BamA orthologue required for growth and efficient localization of outer membrane proteins

The outer membrane (OM) of the pathogenic diderm spirochete, Borrelia burgdorferi, contains integral beta-barrel outer membrane proteins (OMPs) in addition to its numerous outer surface lipoproteins. Very few OMPs have been identified in B. burgdorferi, and the protein machinery required for OMP assembly and OM localization is currently unknown. Essential OM BamA proteins have recently been characterized in Gram-negative bacteria that are central components of an OM beta-barrel assembly machine and are required for proper localization and insertion of bacterial OMPs. In the present study, we characterized a putative B. burgdorferi BamA orthologue encoded by open reading frame bb0795. Structural model predictions and cellular localization data indicate that the B. burgdorferi BB0795 protein contains an N-terminal periplasmic domain and a C-terminal, surface-exposed beta-barrel domain. Additionally, assays with an IPTG-regulatable bb0795 mutant revealed that BB0795 is required for B. burgdorferi growth. Furthermore, depletion of BB0795 results in decreased amounts of detectable OMPs in the B. burgdorferi OM. Interestingly, a decrease in the levels of surface-exposed lipoproteins was also observed in the mutant OMs. Collectively, our structural, cellular localization and functional data are consistent with the characteristics of other BamA proteins, indicating that BB0795 is a B. burgdorferi BamA orthologue.

Ineffectiveness of tigecycline against persistent Borrelia burgdorferi

The effectiveness of a new first-in-class antibiotic, tigecycline (glycylcycline), was evaluated during the early dissemination (1 week), early immune (3 weeks), or late persistent (4 months) phases of Borrelia burgdorferi infection in C3H mice. Mice were treated with high or low doses of tigecycline, saline (negative-effect controls), or a previously published regimen of ceftriaxone (positive-effect controls). Infection status was assessed at 3 months after treatment by culture, quantitative ospA real-time PCR, and subcutaneous transplantation of joint and heart tissue into SCID mice. Tissues from all saline-treated mice were culture and ospA PCR positive, tissues from all antibiotic-treated mice were culture negative, and some of the tissues from most of the mice treated with antibiotics were ospA PCR positive, although the DNA marker load was markedly decreased compared to that in saline-treated mice. Antibiotic treatment during the early stage of infection appeared to be more effective than treatment that began during later stages of infection. The viability of noncultivable spirochetes in antibiotic-treated mice (demonstrable by PCR) was confirmed by transplantation of tissue allografts from treated mice into SCID mice, with dissemination of spirochetal DNA to multiple recipient tissues, and by xenodiagnosis, including acquisition by ticks, transmission by ticks to SCID mice, and survival through molting into nymphs and then into adults. Furthermore, PCR-positive heart base tissue from antibiotic-treated mice revealed RNA transcription of several B. burgdorferi genes. These results extended previous studies with ceftriaxone, indicating that antibiotic treatment is unable to clear persisting spirochetes, which remain viable and infectious, but are nondividing or slowly dividing.

The TolC protein of Legionella pneumophila plays a major role in multi-drug resistance and the early steps of host invasion

Pneumonia associated with Iegionnaires's disease is initiated in humans after inhalation of contaminated aerosols. In the environment, Legionella pneumophila is thought to survive and multiply as an intracellular parasite within free-living amoeba. In the genome of L. pneumophila Lens, we identified a unique gene, tolC, encoding a protein that is highly homologous to the outer membrane protein TolC of Escherichia coli. Deletion of tolC by allelic exchange in L. pneumophila caused increased sensitivity to various drugs. The complementation of the tolC mutation in trans restored drug resistance, indicating that TolC is involved in multi-drug efflux machinery. In addition, deletion of tolC caused a significant attenuation of virulence towards both amoebae and macrophages. Thus, the TolC protein appears to play a crucial role in virulence which could be mediated by its involvement in efflux pump mechanisms. These findings will be helpful in unraveling the pathogenic mechanisms of L. pneumophila as well as in developing new therapeutic agents affecting the efflux of toxic compounds.

Klebsiella pneumoniae AcrAB efflux pump contributes to antimicrobial resistance and virulence

Respiratory infections caused by Klebsiella pneumoniae are characterized by high rates of mortality and morbidity. Management of these infections is often difficult, due to the high frequency of strains that are resistant to multiple antimicrobial agents. Multidrug efflux pumps play a major role as a mechanism of antimicrobial resistance in Gram-negative pathogens. In the present study, we investigated the role of the K. pneumoniae AcrRAB operon in antimicrobial resistance and virulence by using isogenic knockouts deficient in the AcrB component and the AcrR repressor, both derived from the virulent strain 52145R. We demonstrated that the AcrB knockout was more susceptible, not only to quinolones, but also to other antimicrobial agents, including beta-lactams, than the wild-type strain and the AcrR knockout. We further showed that the AcrB knockout was more susceptible to antimicrobial agents present in human bronchoalveolar lavage fluid and to human antimicrobial peptides than the wild-type strain and the AcrR knockout. Finally, the AcrB knockout exhibited a reduced capacity to cause pneumonia in a murine model, in contrast to the wild-type strain. The results of this study suggest that, in addition to contributing to the multidrug resistance phenotype, the AcrAB efflux pump may represent a novel virulence factor required for K. pneumoniae to resist innate immune defense mechanisms of the lung, thus facilitating the onset of pneumonia.

Borrelia burgdorferi resistance to a major skin antimicrobial peptide is independent of outer surface lipoprotein content

We hypothesize a potential role for Borrelia burgdorferi OspC in innate immune evasion at the initial stage of mammalian infection. We demonstrate that B. burgdorferi is resistant to high levels (>200 microg/ml) of cathelicidin and that this antimicrobial peptide exhibits limited binding to the spirochetal outer membrane, irrespective of OspC or other abundant surface lipoproteins. We conclude that the essential role of OspC is unrelated to resistance to this component of innate immunity.

Efflux-mediated drug resistance in bacteria: an update

Drug efflux pumps play a key role in drug resistance and also serve other functions in bacteria. There has been a growing list of multidrug and drug-specific efflux pumps characterized from bacteria of human, animal, plant and environmental origins. These pumps are mostly encoded on the chromosome, although they can also be plasmid-encoded. A previous article in this journal provided a comprehensive review regarding efflux-mediated drug resistance in bacteria. In the past 5 years, significant progress has been achieved in further understanding of drug resistance-related efflux transporters and this review focuses on the latest studies in this field since 2003. This has been demonstrated in multiple aspects that include but are not limited to: further molecular and biochemical characterization of the known drug efflux pumps and identification of novel drug efflux pumps; structural elucidation of the transport mechanisms of drug transporters; regulatory mechanisms of drug efflux pumps; determining the role of the drug efflux pumps in other functions such as stress responses, virulence and cell communication; and development of efflux pump inhibitors. Overall, the multifaceted implications of drug efflux transporters warrant novel strategies to combat multidrug resistance in bacteria.

The outer membrane protein TolC is required for phytoalexin resistance and virulence of the fire blight pathogen Erwinia amylovora

Erwinia amylovora causes fire blight on several plant species such as apple and pear, which produce diverse phytoalexins as defence mechanisms. An evolutionary successful pathogen thus must develop resistance mechanisms towards these toxic compounds. The E. amylovora outer membrane protein, TolC, might mediate phytoalexin resistance through its interaction with the multidrug efflux pump, AcrAB. To prove this, a tolC mutant and an acrB/tolC double mutant were constructed. The minimal inhibitory concentrations of diverse antimicrobials and phytoalexins were determined for these mutants and compared with that of a previously generated acrB mutant. The tolC and arcB/tolC mutants were considerably more susceptible than the wild type but showed similar levels as the acrB mutant. The results clearly indicated that neither TolC nor AcrAB significantly interacted with other transport systems during the efflux of the tested toxic compounds. Survival and virulence assays on inoculated apple plants showed that pathogenicity and the ability of E. amylovora to colonize plant tissue were equally impaired by mutations of tolC and acrB/tolC. Our results allowed the conclusion that TolC plays an important role as a virulence and fitness factor of E. amylovora by mediating resistance towards phytoalexins through its exclusive interaction with AcrAB.

A comprehensive approach to identification of surface-exposed, outer membrane-spanning proteins of Leptospira interrogans

Leptospirosis is a zoonosis with worldwide distribution caused by pathogenic spirochetes belonging to the genus Leptospira. The leptospiral life cycle involves transmission via fresh water and colonization of the renal tubules of their reservoir hosts or infection of accidental hosts, including humans. Bacterial outer membrane proteins (OMPs), particularly those with surface-exposed regions, play crucial roles in virulence mechanisms of pathogens and the adaptation to various environmental conditions, including those of the mammalian host. Little is known about the surface-exposed OMPs in Leptospira, particularly those with outer membrane-spanning domains. Herein, we describe a comprehensive strategy for identification and characterization of leptospiral transmembrane OMPs. The genomic sequence of L. interrogans serovar Copenhageni strain Fiocruz L1–130 allowed us to employ the β-barrel prediction programs, PRED-TMBB and TMBETA-NET, to identify potential transmembrane OMPs. Several complementary methods were used to characterize four novel OMPs, designated OmpL36, OmpL37, OmpL47 and OmpL54. In addition to surface immunofluorescence and surface biotinylation, we describe surface proteolysis of intact leptospires as an improved method for determining the surface exposure of leptospiral proteins. Membrane integration was confirmed using techniques for removal of peripheral membrane proteins. We also demonstrate deficiencies in the Triton X-114 fractionation method for assessing the outer membrane localization of transmembrane OMPs. Our results establish a broadly applicable strategy for the elucidation of novel surface-exposed outer membrane-spanning proteins of Leptospira, an essential step in the discovery of potential virulence factors, diagnostic antigens and vaccine candidates.

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.

In vitro activity of tigecycline against multiple strains of Borrelia burgdorferi

OBJECTIVES

To compare the antimicrobial activity of tigecycline and doxycycline against multiple isolates of Borrelia burgdorferi.

METHODS

In vitro antimicrobial assays were carried out using a microdilution assay. The time needed to inhibit, immobilize and kill the B31 strain of B. burgdorferi was determined. The MIC, MBC and concentration needed to immobilize the organism were determined for each antimicrobial for various strains of B. burgdorferi.

RESULTS

Tigecycline inhibited the growth of and killed the organism more rapidly than doxycycline. Tigecycline was able to kill B. burgdorferi within 24 h at clinically achievable concentrations (<1 mg/L). In contrast, doxycycline was bacteriostatic and required 48-72 h to achieve its maximal inhibitory effect. The anti-Borrelia activity of the antibiotics was tested against 20 different isolates from three species. Tigecycline was 16- to 1000-fold more active than doxycycline at immobilizing Borrelia for the 20 isolates tested.

CONCLUSIONS

We demonstrate that the in vitro activity of tigecycline against B. burgdorferi is superior to that of doxycycline. Tigecycline acted more rapidly and was bactericidal, whereas doxycycline was bacteriostatic and required a more prolonged co-incubation to achieve its maximal inhibitory effect.

In vitro susceptibility of Borrelia spielmanii to antimicrobial agents commonly used for treatment of Lyme disease

Ten isolates of the recently delineated genospecies Borrelia spielmanii were tested against antimicrobial agents used to treat Lyme disease and compared to eight isolates of the other three human-pathogenic borrelial genospecies. Despite some small but significant differences in four out of eight antibiotic agents, the susceptibility pattern of B. spielmanii mainly parallels that of the other known human-pathogenic members of the B. burgdorferi sensu lato complex.

sodA is essential for virulence of Borrelia burgdorferi in the murine model of Lyme disease

Borrelia burgdorferi, the causative agent of Lyme disease, has a limited set of genes to combat oxidative/nitrosative stress encountered in its tick vector or mammalian hosts. We inactivated the gene encoding for superoxide dismutase A (sodA, bb0153), an enzyme mediating the dismutation of superoxide anions and examined the in vitro and in vivo phenotype of the mutant. There were no significant differences in the in vitro growth characteristics of the sodA mutant compared with the control strains. Microscopic analysis of viability of spirochaetes revealed greater percentage of cell death upon treatment of sodA mutant with superoxide generators compared with its controls. Infectivity analysis in C3H/HeN mice following intradermal needle inoculation of 10(3) or 10(5) spirochaetes per mouse revealed complete attenuation of infectivity for the sodA mutant compared with control strains at 21 days post infection. The sodA mutant was more susceptible to the effects of activated macrophages and neutrophils, suggesting that its in vivo phenotype is partly due to the killing effects of activated immune cells. These studies indicate that SodA plays an important role in combating oxidative stress and is essential for the colonization and dissemination of B. burgdorferi in the murine model of Lyme disease.

Structural and functional diversity of bacterial membrane fusion proteins

Membrane Fusion Proteins (MFPs) are functional subunits of multi-component transporters that perform diverse physiological functions in both Gram-positive and Gram-negative bacteria. MFPs associate with transporters belonging to Resistance-Nodulation-cell Division (RND), ATP-Binding Cassette (ABC) and Major Facilitator (MF) superfamilies of proteins. Recent studies suggested that MFPs interact with substrates and play an active role in transport reactions. In addition, the MFP-dependent transporters from Gram-negative bacteria recruit the outer membrane channels to expel various substrates across the outer membrane into external medium. This review is focused on the diversity, structure and molecular mechanism of MFPs that function in multidrug efflux. Using phylogenetic approaches we analyzed diversity and representation of multidrug MFPs in sequenced bacterial genomes. In addition to previously characterized MFPs from Gram-negative bacteria, we identified MFPs that associate with RND-, MF- and ABC-type transporters in Gram-positive bacteria. Sequence analyses showed that MFPs vary significantly in size (200-650 amino acid residues) with some of them lacking the signature alpha-helical domain of multidrug MFPs. Furthermore, many transport operons contain two- or three genes encoding distinct MFPs. We further discuss the diversity of MFPs in the context of current views on the mechanism and structure of MFP-dependent transporters.

MacB ABC transporter is a dimer whose ATPase activity and macrolide-binding capacity are regulated by the membrane fusion protein MacA

Gram-negative bacteria utilize specialized machinery to translocate drugs and protein toxins across the inner and outer membranes, consisting of a tripartite complex composed of an inner membrane secondary or primary active transporter (IMP), a periplasmic membrane fusion protein, and an outer membrane channel. We have investigated the assembly and function of the MacAB/TolC system that confers resistance to macrolides in Escherichia coli. The membrane fusion protein MacA not only stabilizes the tripartite assembly by interacting with both the inner membrane protein MacB and the outer membrane protein TolC, but also has a role in regulating the function of MacB, apparently increasing its affinity for both erythromycin and ATP. Analysis of the kinetic behavior of ATP hydrolysis indicated that MacA promotes and stabilizes the ATP-binding form of the MacB transporter. For the first time, we have established unambiguously the dimeric nature of a noncanonic ABC transporter, MacB that has an N-terminal nucleotide binding domain, by means of nondissociating mass spectrometry, analytical ultracentrifugation, and atomic force microscopy. Structural studies of ABC transporters indicate that ATP is bound between a pair of nucleotide binding domains to stabilize a conformation in which the substrate-binding site is outward-facing. Consequently, our data suggest that in the presence of ATP the same conformation of MacB is promoted and stabilized by MacA. Thus, MacA would facilitate the delivery of drugs by MacB to TolC by enhancing the binding of drugs to it and inducing a conformation of MacB that is primed and competent for binding TolC. Our structural studies are an important first step in understanding how the tripartite complex is assembled.

Oms38 is the first identified pore-forming protein in the outer membrane of relapsing fever spirochetes

Relapsing fever is a worldwide, endemic disease caused by several spirochetal species belonging to the genus Borrelia. During the recurring fever peaks, borreliae proliferate remarkably quickly compared to the slow dissemination of Lyme disease Borrelia and therefore require efficient nutrient uptake from the blood of their hosts. This study describes the identification and characterization of the first relapsing fever porin, which is present in the outer membranes of B. duttonii, B. hermsii, B. recurrentis, and B. turicatae. The pore-forming protein was purified by hydroxyapatite chromatography and designated Oms38, for outer membrane-spanning protein of 38 kDa. Biophysical characterization of Oms38 was done by using the black lipid bilayer method, demonstrating that Oms38 forms small, water-filled channels of 80 pS in 1 M KCl that did not exhibit voltage-dependent closure. The Oms38 channel is slightly selective for anions and shows a ratio of permeability for cations over anions of 0.41 in KCl. Analysis of the deduced amino acid sequences demonstrated that Oms38 contains an N-terminal signal sequence which is processed under in vivo conditions. Oms38 is highly conserved within the four studied relapsing fever species, sharing an overall amino acid identity of 58% and with a strong indication for the presence of amphipathic beta-sheets.