The combination of imaging technologies and luciferase-based bioluminescent bacterial reporter strains provide a sensitive and simple non-invasive detection method (photonic bioimaging) for the study of diverse biological processes, as well as efficacy of therapeutic interventions, in live animal models of disease. The engineering of bioluminescent bacteria required for photonic bioimaging is frequently hampered by lack of promoters suitable for strong, yet stable luciferase gene expression.
We devised a novel method for identification of constitutive native promoters in Gram-negative bacteria. The method is based on a Tn5/7 transposon that exploits the unique features of Tn5 (random transposition) and Tn7 (site-specific transposition). The transposons are designed such that Tn5 transposition will allow insertion of a promoter-less bacterial luxCDABE operon downstream of a bacterial gene promoter. Cloning of DNA fragments from luminescent isolates results in a plasmid that replicates in pir+ hosts. Sequencing of the lux-chromosomal DNA junctions on the plasmid reveals transposon insertion sites within genes or operons. The plasmid is also a mini-Tn7-lux delivery vector that can be used to introduce the promoter-lux operon fusion into other derivatives of the bacterium of interest in an isogenic fashion. Alternatively, promoter-containing sequences can be PCR-amplified from plasmid or chromosomal DNA and cloned into a series of accompanying mini-Tn7-lux vectors. The mini-Tn5/7-lux and mini-Tn7-lux vectors are equipped with diverse selection markers and thus applicable in numerous Gram-negative bacteria. Various mini-Tn5/7-lux vectors were successfully tested for transposition and promoter identification by imaging in Acinetobacter baumannii, Escherichia coli, and Burkholderia pseudomallei. Strong promoters were captured for lux expression in E. coli and A. baumannii. Some mini-Tn7-lux vectors are also equipped with attB sites for swapping of the lux operon with other reporter genes using Gateway technology.
Although mini-Tn5-lux and mini-Tn7-lux elements have previously been developed and used for bacterial promoter identification and chromosomal insertion of promoter-lux gene fusions, respectively, the newly developed mini-Tn5/7-lux and accompanying accessory plasmids streamline and accelerate the promoter discovery and bioluminescent strain engineering processes. Availability of vectors with diverse selection markers greatly extend the host-range of promoter probe and lux gene fusion vectors.
Electronic supplementary material
The online version of this article (doi:10.1186/s12866-015-0354-3) contains supplementary material, which is available to authorized users.
Imaging; Luciferase; Bioluminescent bacteria; Host range; Mini-Tn5/7-Lux vectors; Lux fusion vectors; Gram-negative bacteria
World Melioidosis Congress 2013; Burkholderia pseudomallei; melioidosis; current challenges; environmental saprophyte; bacterium; resource sharing
Pacidamycins (or uridyl peptide antibiotics) possess selective in vivo activity against Pseudomonas aeruginosa. An important limitation for the therapeutic use of pacidamycins with P. aeruginosa is the high frequency (10−6 to 10−7) at which resistant mutants emerge. To elucidate the mechanism(s) of this resistance, pacidamycin-resistant P. aeruginosa mutants were isolated. Two types of mutants were obtained. Type 1, or high-level resistance mutants with a pacidamycin MIC of 512 μg/ml, were more abundant, with a frequency of ∼2 × 10−6, and did not show cross-resistance with other antibiotics. Type 2, low-level resistance mutants, were isolated with a frequency of ∼10−8 and had a pacidamycin MIC of 64 μg/ml (the MIC for the wild-type strain was 4 to 16 μg/ml). These mutants were cross-resistant to levofloxacin, tetracycline, and erythromycin and were shown to overexpress either the MexAB-OprM or MexCD-OprJ multidrug resistance efflux pumps. High-level resistant mutants were isolated by transposon mutagenesis and one insertion was localized to oppB, one of two periplasmic binding protein components of an oligopeptide transport system which is encoded by the opp-fabI operon. The Opp system is required for uptake of pacidamycin across the inner membrane, since various opp, but not fabI, mutants were resistant to high levels of pacidamycin. Both of the two putative Opp periplasmic binding proteins, OppA and OppB, were required for pacidamycin uptake. Although both impaired uptake into and efflux from the cell can cause pacidamycin resistance in P. aeruginosa, our data suggest that impaired uptake is the primary reason for the high-frequency and high-level pacidamycin resistance.
Trimethoprim-sulfamethoxazole (co-trimoxazole) is the primary drug used for oral eradication therapy of Burkholderia pseudomallei infections (melioidosis). Here, we demonstrate that trimethoprim resistance is widespread in clinical and environmental isolates from northeast Thailand and northern Australia. This resistance was shown to be due to BpeEF-OprC efflux pump expression. No dihydrofolate reductase target mutations were involved, although frequent insertion of ISBma2 was noted within the putative folA transcriptional terminator. All isolates tested remained susceptible to trimethoprim-sulfamethoxazole, suggesting that resistance to trimethoprim alone in these strains probably does not affect the efficacy of co-trimoxazole therapy.
We have made significant improvements to a broad-host-range system for the cloning and manipulation of large bacterial genomic regions based on site-specific recombination between directly repeated oriT sites during conjugation. Using two suicide capture vectors carrying flanking homology regions, oriT sites are recombined on either side of the target region. Using a broad-host-range conjugation helper plasmid, the region between the oriT sites is conjugated into an Escherichia coli recipient strain, where it is circularized and maintained as a chimeric mini-F vector. The cloned target region is functionalized in multiple ways to accommodate downstream manipulation. The target region is flanked with Gateway attB sites for recombination into other vectors and by rare 18-bp I-SceI restriction sites for subcloning. The Tn7-functionalized target can also be inserted at a naturally occurring chromosomal attTn7 site(s) or maintained as a broad-host-range plasmid for complementation or heterologous expression studies. We have used the oriTn7 capture technique to clone and complement Burkholderia pseudomallei genomic regions up to 140 kb in size and have created isogenic Burkholderia strains with various combinations of genomic islands. We believe this system will greatly aid the cloning and genetic analysis of genomic islands, biosynthetic gene clusters, and large open reading frames.
Identification of a novel class of anti-Burkholderia compounds is key in addressing antimicrobial resistance to current therapies as well as naturally occurring resistance. The FabI enoyl-ACP reductase in Burkholderia is an underexploited target that presents an opportunity for development of a new class of inhibitors. A library of substituted diphenyl ethers was used to identify FabI1-specific inhibitors for assessment in
Burkholderia pseudomallei ex vivo and murine efficacy models. Active FabI1 inhibitors were identified in a two-stage format consisting of percent inhibition screening and MIC determination by the broth microdilution method. Each compound was evaluated against the B. pseudomallei 1026b (efflux-proficient) and Bp400 (efflux-compromised) strains. In vitro screening identified candidate substituted diphenyl ethers that exhibited MICs of less than 1 μg/ml, and enzyme kinetic assays were used to assess potency and specificity against the FabI1 enzyme. These compounds demonstrated activity in a
Burkholderia ex vivo efficacy model, and two demonstrated efficacy in an acute B. pseudomallei mouse infection model. This work establishes substituted diphenyl ethers as a suitable platform for development of novel anti-Burkholderia compounds that can be used for treatment of melioidosis.
The bacterial fatty acid biosynthesis pathway is a validated target for the development of novel chemotherapeutics. However, since Burkholderia pseudomallei carries genes that encode both FabI and FabV enoyl-acyl carrier protein (ACP) reductase homologues, the enoyl-ACP reductase that is essential for in vivo growth needs to be defined so that the correct drug target can be chosen for development. Accordingly, ΔfabI1, ΔfabI2, and ΔfabV knockout strains were constructed and tested in a mouse model of infection. Mice infected with a ΔfabI1 strain did not show signs of morbidity, mortality, or dissemination after 30 days of infection compared to the wild-type and ΔfabI2 and ΔfabV mutant strains that had times to mortality of 60 to 84 h. Although signs of morbidity and mortality of ΔfabI2 and ΔfabV strains were not significantly different from those of the wild-type strain, a slight delay was observed. A FabI1-specific inhibitor was used to confirm that inhibition of FabI1 results in reduced bacterial burden and efficacy in an acute B. pseudomallei murine model of infection. This work establishes that FabI1 is required for growth of Burkholderia pseudomallei
in vivo and is a potential molecular target for drug development.
Pseudomonas aeruginosa can grow to very high-cell-density (HCD) during infection of the cystic fibrosis (CF) lung. Phosphatidylcholine (PC), the major component of lung surfactant, has been hypothesized to support HCD growth of P. aeruginosa in vivo. The phosphorylcholine headgroup, a glycerol molecule, and two long-chain fatty acids (FAs) are released by enzymatic cleavage of PC by bacterial phospholipase C and lipases. Three different bacterial pathways, the choline, glycerol, and fatty acid degradation pathways, are then involved in the degradation of these PC components. Here, we identified five potential FA degradation (Fad) related fadBA-operons (fadBA1-5, each encoding 3-hydroxyacyl-CoA dehydrogenase and acyl-CoA thiolase). Through mutagenesis and growth analyses, we showed that three (fadBA145) of the five fadBA-operons are dominant in medium-chain and long-chain Fad. The triple fadBA145 mutant also showed reduced ability to degrade PC in vitro. We have previously shown that by partially blocking Fad, via mutagenesis of fadBA5 and fadDs, we could significantly reduce the ability of P. aeruginosa to replicate on FA and PC in vitro, as well as in the mouse lung. However, no studies have assessed the ability of mutants, defective in choline and/or glycerol degradation in conjunction with Fad, to grow on PC or in vivo. Hence, we constructed additional mutants (ΔfadBA145ΔglpD, ΔfadBA145ΔbetAB, and ΔfadBA145ΔbetABΔglpD) significantly defective in the ability to degrade FA, choline, and glycerol and, therefore, PC. The analysis of these mutants in the BALB/c mouse lung infection model showed significant inability to utilize PC in vitro, resulted in decreased replication fitness and competitiveness in vivo compared to the complement strain, although there was little to no variation in typical virulence factor production (e.g., hemolysin, lipase, and protease levels). This further supports the hypothesis that lung surfactant PC serves as an important nutrient for P. aeruginosa during CF lung infection.
Infections with the Gram-negative bacterium Burkholderia pseudomallei (melioidosis) are associated with high mortality, and there is currently no approved vaccine to prevent the development of melioidosis in humans. Infected patients also do not develop protective immunity to reinfection, and some individuals will develop chronic, subclinical infections with B. pseudomallei. At present, our understanding of what constitutes effective protective immunity against B. pseudomallei infection remains incomplete. Therefore, we conducted a study to elucidate immune correlates of vaccine-induced protective immunity against acute B. pseudomallei infection. BALB/c and C57BL/6 mice were immunized subcutaneously with a highly attenuated, Select Agent-excluded purM deletion mutant of B. pseudomallei (strain Bp82) and then subjected to intranasal challenge with virulent B. pseudomallei strain 1026b. Immunization with Bp82 generated significant protection from challenge with B. pseudomallei, and protection was associated with a significant reduction in bacterial burden in lungs, liver, and spleen of immunized mice. Humoral immunity was critically important for vaccine-induced protection, as mice lacking B cells were not protected by immunization and serum from Bp82-vaccinated mice could transfer partial protection to nonvaccinated animals. In contrast, vaccine-induced protective immunity was found to be independent of both CD4 and CD8 T cells. Tracking studies demonstrated uptake of the Bp82 vaccine strain predominately by neutrophils in vaccine-draining lymph nodes and by smaller numbers of dendritic cells (DC) and monocytes. We concluded that protection following cutaneous immunization with a live attenuated Burkholderia vaccine strain was dependent primarily on generation of effective humoral immune responses.
Phage amplification detected by MALDI-TOF MS was investigated for rapid and simultaneous Burkholderia pseudomallei identification and ceftazidime resistance determination. B. pseudomallei ceftazidime susceptible and resistant ΔpurM mutant strains Bp82 and Bp82.3 were infected with broadly targeting B. pseudomallei phage ϕX216 and production of the m/z 37.6 kDa phage capsid protein observed by MALDI-TOF MS over the course of 3 h infections. This allowed for repoducible phage-based bacterial ID within 2 h of the onset of infection. MALDI-TOF MS-measured time to detection correlated with in silico modeling, which predicted an approximate 2 h detection time. Ceftazidime susceptible strain Bp82, while detectable in the absence of the drug, owing to the reliance of phage amplification on a viable host, was not detectable when 10 μg/mL ceftazidime was added at the onset of infection. In contrast, resistant strain Bp82.3 was detected in the same 2 h timeframe both with and without the addition of ceftazidime.
phage amplification; bacterial identification; antibiotic resistance; Burkholderia; MALDI-TOF MS
Burkholderia pseudomallei is the etiological agent of melioidosis. Because of the bacterium’s intrinsic resistance and propensity to establish latent infections, melioidosis therapy is complicated and prolonged. Newer generation β-lactams, specifically ceftazidime, are used for acute phase therapy, but resistance to this cephalosporin has been observed. The chromosomally encoded penA gene encodes a putative twin arginine translocase (TAT)-secreted β-lactamase, and penA mutations have been implicated in ceftazidime resistance in clinical isolates. However, the role of PenA in resistance has not yet been systematically studied in isogenetic B. pseudomallei mutant backgrounds. We investigated the effects of penA deletion, point mutations, and up-regulation, as well as tat operon deletion and PenA TAT-signal sequence mutations. These experiments were made possible by employing a B. pseudomallei strain that is excluded from Select Agent regulations. Deletion of penA significantly (>4-fold) reduced the susceptibility to six of the nine β-lactams tested and ≥16-fold for ampicillin, amoxicillin, and carbenicillin. Overexpression of penA by single-copy, chromosomal expression of the gene under control of the inducible Ptac promoter, increased resistance levels for all β-lactams tested 2- to 10-fold. Recreation of the C69Y and P167S PenA amino acid substitutions previously observed in resistant clinical isolates increased resistance to ceftazidime by ≥85- and 5- to 8-fold, respectively. Similarly, a S72F substitution resulted in a 4-fold increase in resistance to amoxicillin and clavulanic acid. Susceptibility assays with PenA TAT-signal sequence and ΔtatABC mutants, as well as Western blot analysis, confirmed that PenA is a TAT secreted enzyme and not periplasmic but associated with the spheroplastic cell fraction. Lastly, we determined that two LysR-family regulators encoded by genes adjacent to penA do not play a role in transcriptional regulation of penA expression.
Burkholderia pseudomallei; melioidosis; antibiotic resistance; β-lactams; β-lactamase; TAT secretion
We describe the construction of mini-Tn7-based broad-host-range vectors encoding lux genes as bioluminescent reporters. These constructs can be mobilized into the desired host(s) by conjugation for chromosomal mini-Tn7-lux integration and are useful for localization of bacteria during infections or for characterizing regulation of promoters of interest in Gram-negative bacteria.
Melioidosis is a disease in tropical and subtropical regions of the world that is caused by Burkholderia pseudomallei. In endemic regions the disease occurs primarily in humans and goats. In the present study, we used the goat as a model to dissect the polar lipids of B. pseudomallei to identify lipid molecules that could be used for adjuvants/vaccines or as diagnostic tools. We showed that the lipidome of B. pseudomallei and its fractions contain several polar lipids with the capacity to elicit different immune responses in goats, namely rhamnolipids and ornithine lipids which induced IFN-γ, whereas phospholipids and an undefined polar lipid induced strong IL-10 secretion in CD4+ T cells. Autologous T cells co-cultured with caprine dendritic cells (cDCs) and polar lipids of B. pseudomallei proliferated and up-regulated the expression of CD25 (IL-2 receptor) molecules. Furthermore, we demonstrated that polar lipids were able to up-regulate CD1w2 antigen expression in cDCs derived from peripheral blood monocytes. Interestingly, the same polar lipids had only little effect on the expression of MHC class II DR antigens in the same caprine dendritic cells. Finally, antibody blocking of the CD1w2 molecules on cDCs resulted in decreased expression for IFN-γ by CD4+ T cells. Altogether, these results showed that polar lipids of B. pseudomallei are recognized by the caprine immune system and that their recognition is primarily mediated by the CD1 antigen cluster.
Burkholderia pseudomallei is a Gram-negative environmental bacterium found in tropical climates that causes melioidosis. Culture remains the diagnostic gold standard, but isolation of B. pseudomallei from heavily contaminated sites, such as fecal specimens, can be difficult. We recently reported that B. pseudomallei is capable of infecting the gastrointestinal tract of mice and suggested that the same may be true in humans. Thus, there is a strong need for new culture techniques to allow for efficient detection of B. pseudomallei in fecal and other specimens. We found that the addition of norfloxacin, ampicillin, and polymyxin B to Ashdown's medium (NAP-A) resulted in increased specificity without affecting the growth of 25 B. pseudomallei strains. Furthermore, recovery of B. pseudomallei from human clinical specimens was not affected by the three additional antibiotics. Therefore, we conclude that NAP-A medium provides a new tool for more sensitive isolation of B. pseudomallei from heavily contaminated sites.
Burkholderia pseudomallei is the etiologic agent of melioidosis. This multifaceted disease is difficult to treat, resulting in high morbidity and mortality. Treatment of B. pseudomallei infections is lengthy and necessitates an intensive phase (parenteral ceftazidime, amoxicillin–clavulanic acid or meropenem) and an eradication phase (oral trimethoprim–sulfamethoxazole). The main resistance mechanisms affecting these antibiotics include enzymatic inactivation, target deletion and efflux from the cell, and are mediated by chromosomally encoded genes. Overproduction and mutations in the class A PenA β-lactamase cause ceftazidime and amoxicillin–clavulanic acid resistance. Deletion of the penicillin binding protein 3 results in ceftazidime resistance. BpeEF–OprC efflux pump expression causes trimethoprim and trimethoprim–sulfamethoxazole resistance. Although resistance is still relatively rare, therapeutic efficacies may be compromised by resistance emergence due to increased use of antibiotics in endemic regions. Novel agents and therapeutic strategies are being tested and, in some instances, show promise as anti-B. pseudomallei infectives.
antibiotics; Burkholderia pseudomallei; melioidosis; resistance; therapy
Genetic research into these select agents is hampered by lack of permitted markers.
Genetic research into the select agents Burkholderia pseudomallei and B. mallei is currently hampered by a paucity of approved antimicrobial drug–selection markers. The strict regulations imposed on researchers in the United States but not in other parts of the world lead to discrepancies in practice, hinder distribution of genetically modified strains, and impede progress in the field. Deliberation and decisions regarding alternative selection markers (antimicrobial and nonantimicrobial drugs) by the international community, regulatory authorities, and funding agencies are needed.
antimicrobial drug–selection markers; select agents; Burkholderia pseudomallei; Burkholderia mallei; melioidosis; glanders; perspective
We describe a mini-Tn7-based broad-host-range expression cassette for arabinose-inducible gene expression from the PBAD promoter. This delivery vector, pTJ1, can integrate a single copy of a gene into the chromosome of Gram-negative bacteria for diverse genetic applications, of which several are discussed, using Pseudomonas aeruginosa as the model host.
Prevention and control of infectious diseases remains a major public health challenge and a number of highly virulent pathogens are emerging both in and beyond the hospital setting. Despite beneficial aspects such as use in biocontrol and bioremediation exhibited by members of the Burkholderia cepacia complex (Bcc) some members of this group have recently gained attention as significant bacterial pathogens due to their high levels of intrinsic antibiotic resistance, transmissibility in nosocomial settings, persistence in the presence of antimicrobials and intracellular survival capabilities. The Bcc are opportunistic pathogens and their arsenal of virulence factors includes proteases, lipases and other secreted exoproducts, including secretion system-associated effectors. Deciphering the function of virulence factors and assessment of novel therapeutic strategies has been facilitated by use of diverse non-vertebrate hosts (the fly Drosophila melanogaster, the microscopic nematode Caenorhabditis elegans, the zebrafish and the greater Galleria mellonella wax moth caterpillar larvae). Researchers are now employing sophisticated approaches to dissect the virulence determinants of Bcc with the ultimate goal being the development of novel anti-infective countermeasures. This editorial will highlight selected recent research endeavors aimed at dissecting adaptive responses and the virulence factor portfolio of Burkholderia species.
Burkholderia cepacia complex; hemolysis; non-ribosomal peptide synthetase; non-vertebrate hosts; toxins; virulence factors
The Pseudomonas aeruginosa PAO1 thiol peroxidase homolog (Tpx) belongs to a family of enzymes implicated in the removal of toxic peroxides. We have shown the expression of tpx to be highly inducible with redox cycling/superoxide generators and diamide and weakly inducible with organic hydroperoxides and hydrogen peroxide (H2O2). The PAO1 tpx pattern is unlike the patterns for other peroxide-scavenging genes in P. aeruginosa. Analysis of the tpx promoter reveals the presence of a putative IscR binding site located near the promoter. The tpx expression profiles in PAO1 and the iscR mutant, together with results from gel mobility shift assays showing that purified IscR specifically binds the tpx promoter, support the role of IscR as a transcriptional repressor of tpx that also regulates the oxidant-inducible expression of the gene. Recombinant Tpx has been purified and biochemically characterized. The enzyme catalyzes thioredoxin-dependent peroxidation and can utilize organic hydroperoxides and H2O2 as substrates. The Δtpx mutant demonstrates differential sensitivity to H2O2 only at moderate concentrations (0.5 mM) and not at high (20 mM) concentrations, suggesting a novel protective role of tpx against H2O2 in P. aeruginosa. Altogether, P. aeruginosa tpx is a novel member of the IscR regulon and plays a primary role in protecting the bacteria from submillimolar concentrations of H2O2.
Burkholderia pseudomallei and B. mallei are closely related Category B Select Agents of bioterrorism and the causative agents of the diseases melioidosis and glanders, respectively. Rapid phage-based diagnostic tools would greatly benefit early recognition and treatment of these diseases. There is extensive strain-to-strain variation in B. pseudomallei genome content due in part to the presence or absence of integrated prophages. Several phages have previously been isolated from B. pseudomallei lysogens, for example φK96243, φ1026b and φ52237.
We have isolated a P2-like bacteriophage, φX216, which infects 78% of all B. pseudomallei strains tested. φX216 also infects B. mallei, but not other Burkholderia species, including the closely related B. thailandensis and B. oklahomensis. The nature of the φX216 host receptor remains unclear but evidence indicates that in B. mallei φX216 uses lipopolysaccharide O-antigen but a different receptor in B. pseudomallei. The 37,637 bp genome of φX216 encodes 47 predicted open reading frames and shares 99.8% pairwise identity and an identical strain host range with bacteriophage φ52237. Closely related P2-like prophages appear to be widely distributed among B. pseudomallei strains but both φX216 and φ52237 readily infect prophage carrying strains.
The broad strain infectivity and high specificity for B. pseudomallei and B. mallei indicate that φX216 will provide a good platform for the development of phage-based diagnostics for these bacteria.
Bacteriophage; Burkholderia pseudomallei; B. mallei; P2; Prophage distribution; Phage-based diagnostics
The US Public Health Emergency Medical Countermeasures Enterprise convened subject matter experts at the 2010 HHS Burkholderia Workshop to develop consensus recommendations for postexposure prophylaxis against and treatment for Burkholderia pseudomallei and B. mallei infections, which cause melioidosis and glanders, respectively. Drugs recommended by consensus of the participants are ceftazidime or meropenem for initial intensive therapy, and trimethoprim/sulfamethoxazole or amoxicillin/clavulanic acid for eradication therapy. For postexposure prophylaxis, recommended drugs are trimethoprim/sulfamethoxazole or co-amoxiclav. To improve the timely diagnosis of melioidosis and glanders, further development and wide distribution of rapid diagnostic assays were also recommended. Standardized animal models and B. pseudomallei strains are needed for further development of therapeutic options. Training for laboratory technicians and physicians would facilitate better diagnosis and treatment options.
Burkholderia pseudomallei; melioidosis; Burkholderia mallei; glanders; drug therapy; postexposure prophylaxis; ceftazidime; carbapenems; trimethoprim/sulfamethoxazole; combination; amoxicillin/potassium clavulanate; clavulanic acid bacteria; antibiotic; antibacterial drugs; antimicrobial drugs; bacteria; Suggested citation for this article: Lipsitz R; Garges S; Aurigemma R; Baccam P; Blaney DD; Cheng AC; et al. Workshop on treatment of and postexposure prophylaxis for Burkholderia pseudomallei and B. mallei infection; 2010. Emerg Infect Dis [Internet]. 2012 Dec [date cited]. http://dx.doi.org/10.3201/eid1812.120638
d-boroAla was previously characterized as an inhibitor of bacterial alanine racemase and d-Ala-d-Ala ligase enzymes [Duncan, K., et al Biochemistry 1989, 28:3541–9]. In the present study, d-boroAla was identified and characterized as an antibacterial agent. d-boroAla has activity against both Gram-positive and Gram-negative organisms, with MICs down to 8 µg/mL. A structure-function study on the alkyl side chain (NH2-CHR-B(OR’)2) revealed that d-boroAla is the most effective agent in a series including boroGly, d-boroHomoAla, and d-boroVal. l-boroAla was much less active, and N-acetylation completely abolished activity. An LC-MS/MS assay was used to demonstrate that d-boroAla exerts its antibacterial activity by inhibition of d-Ala-d-Ala ligase (DDL). d-boroAla is bactericidal at 1× MIC against Staphylococcus aureus and Bacillus subtilis – which each encode one copy of DDL, and at 4× MIC against Escherichia coli and Salmonella enterica serovar Typhimurium – which each encode two copies of DDL. d-boroAla demonstrated a frequency of resistance of 8×10−8 at 4× MIC in S. aureus. These results demonstrate that d-boroAla has promising antibacterial activity, and could serve as the lead agent in a new class of DDL targeted antibacterial agents. This study also demonstrates d-boroAla as a possible probe for DDL function.
antibacterial; cell wall; alanine branch; broad spectrum; d-Ala-d-Ala ligase
The FabAB pathway is one of the unsaturated fatty acid (UFA) synthesis pathways for Pseudomonas aeruginosa. It was previously noted that this operon was upregulated in biofilms and repressed by exogenous UFAs. Deletion of a 30 nt fabA upstream sequence, which is conserved in P. aeruginosa, P. putida, and P. syringae, led to a significant decrease in fabA transcription, suggesting positive regulation by an unknown positive regulatory mechanism.
Here, genetic and biochemical approaches were employed to identify a potential fabAB activator. Deletion of candidate genes such as PA1611 or PA1627 was performed to determine if any of these gene products act as a fabAB activator. However, none of these genes were involved in the regulation of fabAB transcription. Use of mariner-based random mutagenesis to screen for fabA activator(s) showed that several genes encoding unknown functions, rpoN and DesA may be involved in fabA regulation, but probably via indirect mechanisms. Biochemical attempts performed did fail to isolate an activator of fabAB operon.
The data suggest that fabA expression might not be regulated by protein-binding, but by a distinct mechanism such as a regulatory RNA-based mechanism.