Monika Bokori-Brown

Conjugative transfer of clostridial shuttle vectors from Escherichia coli to Clostridium difficile through circumvention of the restriction barrier

Progress towards understanding the molecular basis of virulence in Clostridium difficile has been hindered by the lack of effective gene transfer systems. We have now, for the first time, developed procedures that may be used to introduce autonomously replicating vectors into this organism through their conjugative, oriT‐based mobilization from Escherichia coli donors. Successful transfer was achieved through the use of a plasmid replicon isolated from an indigenous C. difficile plasmid, pCD6, and through the characterization and subsequent circumvention of host restriction/modification (RM) systems. The characterized replicon is the first C. difficile plasmid replicon to be sequenced and encodes a large replication protein (RepA) and a repetitive region composed of a 35 bp iteron sequence repeated seven times. Strain CD6 has two RM systems, CdiCD6I/M.CdiCD6I and CdiCD6II/M. CdiCD6II, with equivalent specificities to Sau96I/M. Sau96I (5′‐GGNMCC‐3′) and MboI/M. MboI (5′‐GMATC‐3′) respectively. A second strain (CD3) possesses a type IIs restriction enzyme, Cdi I, which cleaves the sequence 5′‐CATCG‐3′ between the fourth and fifth nucleotide to give a blunt‐ended fragment. This is the first time that an enzyme with this specificity has been reported. The sequential addition of this site to vectors showed that each site caused between a five‐ and 16‐fold reduction in transfer efficiency. The transfer efficiencies achieved with both strains equated to between 1.0 × 10−6 and 5.5 × 10−5 transconjugants per donor.

A Burkholderia pseudomallei toxin inhibits helicase activity of translation factor eIF4A.

A toxin associated with a disease often observed in Vietnam veterans is identified and characterized. The structure of BPSL1549, a protein of unknown function from Burkholderia pseudomallei, reveals a similarity to Escherichia coli cytotoxic necrotizing factor 1. We found that BPSL1549 acted as a potent cytotoxin against eukaryotic cells and was lethal when administered to mice. Expression levels of bpsl1549 correlate with conditions expected to promote or suppress pathogenicity. BPSL1549 promotes deamidation of glutamine-339 of the translation initiation factor eIF4A, abolishing its helicase activity and inhibiting translation. We propose to name BPSL1549 Burkholderia lethal factor 1.

Molecular basis of toxicity of Clostridium perfringens epsilon toxin

Clostridium perfringensε‐toxin is produced by toxinotypes B and D strains. The toxin is the aetiological agent of dysentery in newborn lambs but is also associated with enteritis and enterotoxaemia in goats, calves and foals. It is considered to be a potential biowarfare or bioterrorism agent by the US Government Centers for Disease Control and Prevention. The relatively inactive 32.9 kDa prototoxin is converted to active mature toxin by proteolytic cleavage, either by digestive proteases of the host, such as trypsin and chymotrypsin, or by C. perfringensλ‐protease. In vivo, the toxin appears to target the brain and kidneys, but relatively few cell lines are susceptible to the toxin, and most work has been carried out using Madin–Darby canine kidney (MDCK) cells. The binding of ε‐toxin to MDCK cells and rat synaptosomal membranes is associated with the formation of a stable, high molecular weight complex. The crystal structure of ε‐toxin reveals similarity to aerolysin from Aeromonas hydrophila, parasporin‐2 from Bacillus thuringiensis and a lectin from Laetiporus sulphureus. Like these toxins, ε‐toxin appears to form heptameric pores in target cell membranes. The exquisite specificity of the toxin for specific cell types suggests that it binds to a receptor found only on these cells.

Molecular architecture and functional analysis of NetB, a pore-forming toxin from Clostridium perfringens

Background: Clostridium perfringens toxin NetB is a key factor in avian necrotic enteritis. Results: NetB forms heptameric pores structurally similar to Staphylococcus aureus toxins but lacks a phosphocholine binding pocket. NetB activity is enhanced by cholesterol. Conclusion: NetB has distinct binding specificity, and cholesterol may act as a receptor. Significance: The structure of NetB will facilitate development of control measures against necrotic enteritis. NetB is a pore-forming toxin produced by Clostridium perfringens and has been reported to play a major role in the pathogenesis of avian necrotic enteritis, a disease that has emerged due to the removal of antibiotics in animal feedstuffs. Here we present the crystal structure of the pore form of NetB solved to 3.9 Å. The heptameric assembly shares structural homology to the staphylococcal α-hemolysin. However, the rim domain, a region that is thought to interact with the target cell membrane, shows sequence and structural divergence leading to the alteration of a phosphocholine binding pocket found in the staphylococcal toxins. Consistent with the structure we show that NetB does not bind phosphocholine efficiently but instead interacts directly with cholesterol leading to enhanced oligomerization and pore formation. Finally we have identified conserved and non-conserved amino acid positions within the rim loops that significantly affect binding and toxicity of NetB. These findings present new insights into the mode of action of these pore-forming toxins, enabling the design of more effective control measures against necrotic enteritis and providing potential new tools to the field of bionanotechnology.

Cryo-EM structure of lysenin pore elucidates membrane insertion by an aerolysin family protein

Lysenin from the coelomic fluid of the earthworm Eisenia fetida belongs to the aerolysin family of small β-pore-forming toxins (β-PFTs), some members of which are pathogenic to humans and animals. Despite efforts, a high-resolution structure of a channel for this family of proteins has been elusive and therefore the mechanism of activation and membrane insertion remains unclear. Here we determine the pore structure of lysenin by single particle cryo-EM, to 3.1 Å resolution. The nonameric assembly reveals a long β-barrel channel spanning the length of the complex that, unexpectedly, includes the two pre-insertion strands flanking the hypothetical membrane-insertion loop. Examination of other members of the aerolysin family reveals high structural preservation in this region, indicating that the membrane-insertion pathway in this family is conserved. For some toxins, proteolytic activation and pro-peptide removal will facilitate unfolding of the pre-insertion strands, allowing them to form the β-barrel of the channel.

Galleria mellonella as a model system to test the pharmacokinetics and efficacy of antibiotics against Burkholderia pseudomallei

Mammalian models of infection are paramount to elucidating the mechanisms of bacterial pathogenesis and are also used for evaluating the efficacy of novel antimicrobials before the commencement of human trials. In this study, Galleria mellonella was used to determine the efficacy of antibiotics towards a Burkholderia thailandensis infection in G. mellonella larvae. Kanamycin, imipenem, ceftazidime, doxycycline and ciprofloxacin could all provide some protection when given 1 h before challenge with B. thailandensis; however, at 2 h or 6 h post challenge, imipenem and kanamycin were unable to rescue larvae. The most effective antibiotic for the prevention or treatment of disease was ceftazidime. Pharmacokinetic properties of a single dose of these antibiotics in G. mellonella larvae were also determined, and it was demonstrated that this model is useful for approximating the antibiotic response in humans. The G. mellonella model was used to screen a panel of novel antimicrobials for activity towards B. thailandensis and Burkholderia pseudomallei, and three novel compounds with antibiotic activity were identified. These results support the hypothesis that G. mellonella can be used to screen antimicrobial efficacy. This is the first study to determine the pharmacokinetic parameters of clinically relevant antibiotics in this model system.

Protection against avian necrotic enteritis after immunisation with NetB genetic or formaldehyde toxoids

Highlights • NetB from Clostridium perfringens is the major virulence factor in avian necrotic enteritis.• Vaccination with a NetB genetic or formaldehyde toxoid protects chicken in an in vivo disease model.• NetB toxoids could form the bases of an efficient vaccine against necrotic enteritis.

The conserved translocase Tim17 prevents mitochondrial DNA loss

Maintenance of an intact mitochondrial genome is essential for oxidative phosphorylation in all eukaryotes. Depletion of mitochondrial genome copy number can have severe pathological consequences due to loss of respiratory capacity. In Saccharomyces cerevisiae, several bifunctional metabolic enzymes have been shown to be required for mitochondrial DNA (mtDNA) maintenance. For example, Ilv5 is required for branched chain amino acid biosynthesis and mtDNA stability. We have identified OXA1 and TIM17 as novel multicopy suppressors of mtDNA instability in ilv5 cells. In addition, overexpression of TIM17, but not OXA1, prevents the complete loss of mtDNA in cells lacking the TFAM homologue Abf2. Introduction of the disease-associated A3243G mutant mtDNA into human NT2 teratocarcinoma cells frequently causes mtDNA loss. Yet when human TIM17A is overexpressed in NT2 cybrids carrying A3243G mtDNA, the proportion of cybrid clones maintaining mtDNA increases significantly. TIM17A overexpression results in long-term mtDNA stabilization, since NT2 cybrids overexpressing TIM17A maintain mtDNA at levels similar to controls for several months. Tim17 is a conserved suppressor of mtDNA instability and is the first factor to be identified that can prevent mtDNA loss in a human cellular model of mitochondrial disease.

Clostridium perfringens epsilon toxin H149A mutant as a platform for receptor binding studies.

Clostridium perfringens epsilon toxin (Etx) is a pore‐forming toxin responsible for a severe and rapidly fatal enterotoxemia of ruminants. The toxin is classified as a category B bioterrorism agent by the U.S. Government Centres for Disease Control and Prevention (CDC), making work with recombinant toxin difficult. To reduce the hazard posed by work with recombinant Etx, we have used a variant of Etx that contains a H149A mutation (Etx‐H149A), previously reported to have reduced, but not abolished, toxicity. The three‐dimensional structure of H149A prototoxin shows that the H149A mutation in domain III does not affect organisation of the putative receptor binding loops in domain I of the toxin. Surface exposed tyrosine residues in domain I of Etx‐H149A (Y16, Y20, Y29, Y30, Y36 and Y196) were mutated to alanine and mutants Y30A and Y196A showed significantly reduced binding to MDCK.2 cells relative to Etx‐H149A that correlated with their reduced cytotoxic activity. Thus, our study confirms the role of surface exposed tyrosine residues in domain I of Etx in binding to MDCK cells and the suitability of Etx‐H149A for further receptor binding studies. In contrast, binding of all of the tyrosine mutants to ACHN cells was similar to that of Etx‐H149A, suggesting that Etx can recognise different cell surface receptors. In support of this, the crystal structure of Etx‐H149A identified a glycan (β‐octyl‐glucoside) binding site in domain III of Etx‐H149A, which may be a second receptor binding site. These findings have important implications for developing strategies designed to neutralise toxin activity.

Clostridium perfringens epsilon toxin mutant Y30A-Y196A as a recombinant vaccine candidate against enterotoxemia

Highlights • Etx mutant Y30A-Y196A showed markedly reduced cytotoxicity towards MDCK.2 cells.• Y30A-Y196A is inactive in mice after intraperitoneal administration.• Y30A-Y196A is able to induce a specific antibody response in rabbits.• Y30A-Y196A polyclonal antibody is able to induce protective immunity in vitro.• Y30A-Y196A could form the basis of a recombinant vaccine against enterotoxemia.