Investigations of the intermolecular forces between RDX and polyethylene by force-distance spectroscopy and molecular dynamics simulations.

The development of novel nanoenergetic materials with enhanced bulk properties requires an understanding of the intermolecular interactions occurring between molecular components. We investigate the surface interactions between 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) and polyethylene (PE) crystals on the basis of combined use of molecular dynamics (MD) simulations and force-distance spectroscopy, in conjunction with Lifshitz macroscopic theory of van der Waals forces between continuous materials. The binding energy in the RDX-PE system depends both on the degree of PE crystallinity and on the RDX crystal face. Our MD simulations yield binding energies of approximately 132 and 120 mJ/m(2) for 100% amorphous and 100% crystalline PE on RDX (210), respectively. The average value is about 36% greater than our experimental value of 81 ± 15 mJ/m(2) for PE (∼48% amorphous) on RDX (210). By comparison, Liftshitz theory predicts a value of about 79 mJ/m(2) for PE interacting with RDX. Our MD simulations also predict larger binding energies for both amorphous and crystalline PE on RDX (210) compared to the RDX (001) surface. Analysis of the interaction potential indicates that about 60% of the binding energy in the PE-RDX system is due to attractive interactions between HPE-ORDX and CPE-NRDX pairs of atoms. Further, amorphous PE shows a much longer interaction distance than crystalline PE with the (210) and (001) RDX surfaces due to the possibility of larger polymer elongations in the case of amorphous PE as strain is applied. Also, we report estimates of the binding energies of energetic materials RDX and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) with PE, propylene, polystyrene, and several fluorine-containing polymers using Lifshitz theory and compare these with reported MD calculations.

Clinical spectrum of gram-positive infections in lung transplantation.

PURPOSE: Gram-positive (GP) organisms are among the most common cause of infections in early postsurgical and immunocompromised populations. Patients recovering from lung transplantation (LT) are particularly susceptible owing to the physiologic stress imposed by surgery and induction with intense immunosuppression. Sites, types, and timing of GP infections following LT are not well documented. This report describes the clinical spectrum of GP infections and their effects on surgical airway complications (SAC) and bronchiolitis obliterans syndrome (BOS) following LT. METHODS AND MATERIALS: Data were collected from 202 patients undergoing 208 LT procedures at a single institution between November 1990 and November 2005. Data were retrospectively analyzed according to timing, location, and causative pathogen. RESULTS: In the median follow-up period of 2.7 years (range, 0-13.6 years), 137 GP infections were confirmed in 72 patients. Sites of infection included respiratory tract (42%), blood (27%), skin, wound and catheter (21%), and other (10%). GP pathogens identified were Staphylococcus species (77%), Enterococcus species (12%), Streptococcus species (6%), Pneumococcus (4%), and Eubacterium lentum (1%). The likelihood of SAC and BOS was increased in lung allograft recipients with GP pneumonia as compared with those without (hazard ratio 2.1; 95% confidence interval=1.5-3.1). CONCLUSIONS: GP organisms were responsible for infections in 40% of lung allograft recipients and most commonly isolated from the respiratory tract and blood stream. Staphylococcal species were most frequently identified, 42% of which were methicillin-resistant Staphylococcus aureus (MRSA). Given the strong association of respiratory tract infections with the development of SAC and BOS, empiric antimicrobial strategies after LT should include agents directed against GP organisms, especially MRSA.

Clostridium difficile colitis in lung transplantation.

PURPOSE: Clostridium difficile colitis (CDC) is the most common nosocomial infection of the gastrointestinal tract in patients with recent antibiotic use or hospitalization. Lung transplant recipients receive aggressive antimicrobial therapy postoperatively for treatment and prophylaxis of respiratory infections. This report describes the epidemiology of CDC in lung recipients from a single center and explores possible associations with bronchiolitis obliterans syndrome (BOS), a surrogate marker of chronic rejection. METHODS: Patients were divided into those with confirmed disease (CDC+) and those without disease (CDC-) based on positive C. difficile toxin assay. Because of a bimodal distribution in the time to develop CDC, the early postoperative CDC+ group was analyzed separately from the late postoperative CDC+ cohort with respect to BOS development. RESULTS: Between 1990 and 2005, 202 consecutive patients underwent 208 lung transplantation procedures. Of these, 15 lung recipients developed 23 episodes of CDC with a median follow-up period of 2.7 years (range, 0-13.6). All patients with confirmed disease had at least 1 of the following 3 risk factors: recent antibiotic use, recent hospitalization, or augmentation of steroid dosage. Of the early CDC+ patients, 100% developed BOS, but only 52% of the late CDC+ patients developed BOS, either preceding or following infection. CONCLUSION: CDC developed in 7.4% of lung transplant patients with identified risk factors, yielding a cumulative incidence of 14.7%. The statistical association of BOS development in early CDC+ patients suggests a relationship between early infections and future chronic lung rejection.

A role for the tet(O) plasmid in maintaining Campylobacter plasticity.

Genomic sequencing projects are beginning to reveal regions of extensive DNA homology between bacterial genera. Public fears of the spread of genetically modified organisms into the food chain and the increasing prevalence of multi-drug resistant disease in humans highlight the implications of horizontal gene transfer. The striking DNA sequence similarity between the two uniquely identified tetracycline resistant (Tc(R)) Campylobacter plasmids, pCC31 and pTet, suggests their conserved acquisition and maintenance within Campylobacter [Batchelor, R.A., Pearson, B.M., Friis, L.M., Guerry, P., Wells, J.M. 2004. Nucleotide sequences and comparison of two large conjugative plasmids from different Campylobacter species. Microbiology 150, 3507-3517]. It is thus likely that these and other conjugative plasmids are highly prevalent and broadly distributed across several continents. Microarray technology is now enabling fast and extensive genomic comparisons to be made and allows us to investigate intra- and inter-genetic conservation and variability. This study details the development of a microarray specific for genes from Campylobacter plasmids pCC31, pTet and pVir and its application to the analysis of Campylobacter plasmid gene presence and preservation throughout environmental and clinical isolates. Application of the iterative algorithm GENCOM (freely available at ) is used as a rapid and effective way of comparing the content and conservation of plasmids in bacteria and provides details of the Campylobacter flexible gene pool and its contribution to genomic plasticity.

Multilaboratory comparison of proficiencies in susceptibility testing of Helicobacter pylori and correlation between agar dilution and E test methods.

Susceptibility testing was performed at seven Canadian microbiology laboratories and the Helicobacter Reference Laboratory, Halifax, Nova Scotia, Canada, to assess susceptibility testing proficiency and the reproducibility of the results for clarithromycin and metronidazole and to compare the Epsilometer test (E test) method to the agar dilution reference method. Control strain Helicobacter pylori ATCC 43504 (American Type Culture Collection) and 13 clinical isolates (plus duplicates of four of these strains including ATCC 43504) were tested blindly. The National Committee for Clinical Laboratory Standards (NCCLS) guidelines for agar dilution testing were followed, and the same suspension of organisms was used for agar dilution and E test. Antimicrobials and E test strips were provided to the investigators. Methods were provided on a website (www.Helicobactercanada.org). Each center reported MICs within the stated range for strain ATCC 43504. Compared to the average MICs, interlaboratory agreements within 2 log(2) dilutions were 90% (range, 69 to 100%) for clarithromycin by agar dilution, with seven very major errors [VMEs], and 85% (range, 65 to 100%) by E test, with three VMEs. Interlaboratory agreements within 2 log(2) dilutions were 83% (range, 50 to 100%) for metronidazole by agar dilution, with six VMEs and eight major errors (MEs), and 75% (range, 50 to 94%) by E test, with four VMEs and four MEs. At lower and higher concentrations of antibiotic, E test MICs were slightly different from agar dilution MICs, but these differences did not result in errors. When a standardized protocol based on NCCLS guidelines was used, most participants in this study correctly identified clarithromycin- and metronidazole-susceptible and -resistant strains of H. pylori 93% of the time by either the agar dilution or E test method, and the numbers of errors were relatively equivalent by both methods.

Diversity of silver resistance genes in IncH incompatibility group plasmids.

Silver compounds are used as antimicrobial agents in medicine and bacteria that develop resistance to silver cations (Ag(+)) pose problems similar to those of antibiotic-resistant bacteria. The first set of Ag(+) resistance genes (sil) was from plasmid pMG101, now assigned to the IncHI incompatibility group. Questions of whether sil genes are unique to pMG101 or are more widely found, and whether they are associated with a specific incompatibility group or occur in many plasmid groups and on bacterial chromosomes were addressed. sil genes were identified in five IncH plasmids, but not in plasmids of the IncP incompatibility group. Three sil genes (silP, silR and silE) from these plasmids were PCR-amplified, cloned, sequenced and compared to those of pMG101. Differences of 0-50 nt per kb of sequence were found. Predicted gene products were 0-6% different in amino acid sequence, but the differences did not alter residues thought to be involved in protein function (see supplementary data at http://mic.sgmjournals.org or http://www.uic.edu/depts/mcmi/individual/gupta/index.htm). For representative IncH plasmid R476b and pMG101 the effects of Ag(+) exposure on resistance levels were measured by growth. The inducibility of silC, silR and silE gene expression after Ag(+) exposure was studied by reverse transcriptase (RT)-PCR. Silver resistance increased after Ag(+) exposure for strains carrying plasmid R476b. silC and silE expression from R476b was inducible after Ag(+) exposure and was constitutive and high from pMG101. The mRNA levels for the regulatory gene silR was constitutive for both pMG101 and R476b. Close homologues for silABC(ORF96)RS from pMG101 are clustered on the chromosomes of Escherichia coli strains K-12 and O157:H7, without contiguous silP and silE homologues. Insertion deletions of the E. coli K-12 chromosomal homologues for silA and silP gave Ag(+) hypersensitivity for growth. The silA homologue knockout was complemented back to wild-type resistance by the same gene cloned on a plasmid. Homologues of sil genes have also been identified on other enterobacterial genomes.

Cellular location and temperature-dependent assembly of IncHI1 plasmid R27-encoded TrhC-associated conjugative transfer protein complexes.

Conjugal transfer of IncHI plasmid DNA between Gram-negative bacteria is temperature sensitive, as mating is optimal between 22 degrees C and 30 degrees C but is inhibited at 37 degrees C. R27, isolated from Salmonella enterica serovar Typhi, is an IncHI1 plasmid of 180 kbp that has been sequenced completely. The gene encoding green fluorescent protein (GFP) was inserted into R27 in frame with trhC. TrhC is a mating pair formation (Mpf) protein that is essential for plasmid transfer and H-pilus production. Fluorescence microscopy allowed visualization of the TrhC-GFP fusion protein, and Escherichia coli cells were examined for the subcellular localization and temperature-dependent production of TrhC-GFP. At 27 degrees C, TrhC-GFP was found at the periphery of cells as discrete foci, indicating an association of TrhC within protein complexes in the bacterial cell membrane, whereas at 37 degrees C, little fluorescence was detected. These foci probably represent the intracellular position of protein complexes involved in conjugative transfer, as the formation of foci was dependent upon the presence of other Mpf proteins. During temperature shift experiments from 37 degrees C to 27 degrees C, a long lag period was required for generation of GFP foci. Conversely, during short shifts from 27 degrees C to 37 degrees C, the GFP foci remained stable. These results suggest that the expression of transfer genes in the Tra2 region of R27 is temperature dependent. Subcellular localization of TrhC was verified by cellular fractionation. Expression patterns of TrhC-GFP were confirmed with immunoblot analysis and reverse transcriptase-polymerase chain reaction (RT-PCR). These results allow us to propose mechanisms to explain the temperature-sensitive transfer of R27.

Helicobacter pylori from asymptomatic hosts expressing heptoglycan but lacking Lewis O-chains: Lewis blood-group O-chains may play a role in Helicobacter pylori induced pathology.

Helicobacter pylori is a widespread Gram-negative bacterium responsible for the onset of various gastric pathologies and cancers in humans. A familiar trait of H. pylori is the production of cell-surface lipopolysaccharides (LPSs; O-chain --> core --> lipid A) with O-chain structures analogous to some mammalian histo-blood-group antigens, those being the Lewis determinants (Lea, Leb, Lex, sialyl Lex, Ley) and blood groups A and linear B. Some of these LPS antigens have been implicated as autoimmune, adhesion, and colonization components of H. pylori pathogenic mechanisms. This article describes the chemical structures of LPSs from H. pylori isolated from subjects with no overt signs of disease. Experimental data from chemical- and spectroscopic-based studies unanimously showed that these H. pylori manufactured extended heptoglycans composed of 2- and 3-linked D-glycero-alpha-D-manno-heptopyranose units and did not express any blood-group O-antigen chains. The fact that another H. pylori isolate with a similar LPS structure was shown to be capable of colonizing mice indicates that H. pylori histo-blood-group structures are not an absolute prerequisite for colonization in the murine model also. The absence of O-chains with histo-blood groups may cause H. pylori to become inept in exciting an immune response. Additionally, the presence of elongated heptoglycans may impede exposure of disease-causing outer-membrane antigens. These factors may render such H. pylori incapable of creating exogenous contacts essential for pathogenesis of severe gastroduodenal diseases and suggest that histo-blood groups in the LPS may indeed play a role in inducing a more severe H. pylori pathology.

Lewis antigen expression by Helicobacter pylori.

Although Helicobacter pylori express Lewis antigens as a component in the lipopolysaccharide, their role in the infectious process is not well understood. Lewis antigen expression with growth phase was investigated, as well as the distribution of Lewis antigens among isolates from asymptomatic and symptomatic individuals. Lewis antigens are expressed by H. pylori in a growth phase-dependent manner, with the greatest expression occurring in the logarithmic phase of growth. As growth proceeds, an increasing amount of Lewis antigens are shed into the culture supernatant. Lewis antigen expression among H. pylori isolates from asymptomatic individuals is characterized by an absence of type I Lewis antigens, a decrease in Le(x) expression, and an increase in nontypeable H. pylori, as compared with that among H. pylori isolates from symptomatic patients. The data support a role for Lewis antigens in the pathogenesis associated with symptomatic H. pylori infection in colonized individuals.

Localization of the ribosomal protection protein Tet(O) on the ribosome and the mechanism of tetracycline resistance.

Tet(O) belongs to a class of ribosomal protection proteins that mediate tetracycline resistance. It is a G protein that shows significant sequence similarity to elongation factor EF-G. Here we present a cryo-electron microscopic reconstruction, at 16 A resolution, of its complex with the E. coli 70S ribosome. Tet(O) was bound in the presence of a noncleavable GTP analog to programmed ribosomal complexes carrying fMet-tRNA in the P site. Tet(O) is directly visible as a mass close to the A-site region, similar in shape and binding position to EF-G. However, there are important differences. One of them is the different location of the tip of domain IV, which in the Tet(O) case, does not overlap with the ribosomal A site but is directly adjacent to the primary tetracycline binding site. Our findings give insights into the mechanism of tetracycline resistance.

Antibiotic susceptibility of attached and free-floating Helicobacter pylori.

Helicobacter pylori are found attached to mucous cells of the human stomach or under the mucous layer. Models mimicking the in vivo situation may be more suitable for H. pylori MIC determinations than traditional agar dilution methods. Megraud et al. (Antimicrobial Agents and Chemotherapy 1991, 35, 869-72) developed a model for measuring the susceptibility of attached and free-floating H. pylori. We have modified this model so that free-floating and attached H. pylori are treated in a more similar manner, before and after incubation with antibiotic, and performed additional controls to ensure H. pylori and tissue culture cells are not detrimentally affected and maintain their viability during the course of the experiment. We found only 10% of plate-grown H. pylori were competent for attachment to HEp-2 cells; however, all progeny of attached bacteria remained adherent. Killing curves were performed using 0, 0.001, 0.01, 0.1 and 1 mg/L amoxycillin, and 0, 0.0025, 0.0075 and 0.01 mg/L clarithromycin. H. pylori divided at concentrations

Characterization of a Plasmodium falciparum erythrocyte-binding protein paralogous to EBA-175.

A member of a Plasmodium receptor family for erythrocyte invasion was identified on chromosome 13 from the Plasmodium falciparum genome sequence of the Sanger Centre (Cambridge, U.K.). The protein (named BAEBL) has homology to EBA-175, a P. falciparum receptor that binds specifically to sialic acid and the peptide backbone of glycophorin A on erythrocytes. Both EBA-175 and BAEBL localize to the micronemes, organelles at the invasive ends of the parasites that contain other members of the family. Like EBA-175, the erythrocyte receptor for BAEBL is destroyed by neuraminidase and trypsin, indicating that the erythrocyte receptor is a sialoglycoprotein. Its specificity, however, differs from that of EBA-175 in that BAEBL can bind to erythrocytes that lack glycophorin A, the receptor for EBA-175. It has reduced binding to erythrocytes with the Gerbich mutation found in another erythrocyte, sialoglycoprotein (glycophorin C/D). The interest in BAEBL's reduced binding to Gerbich erythrocytes derives from the high frequency of the Gerbich phenotype in some regions of Papua New Guinea where P. falciparum is hyperendemic.

Quinolone and macrolide resistance in Campylobacter jejuni and C. coli: resistance mechanisms and trends in human isolates.

The incidence of human Campylobacter jejuni and C. coli infections has increased markedly in many parts of the world in the last decade as has the number of quinolone-resistant and, to a lesser extent, macrolide-resistant Campylobacter strains causing infections. We review macrolide and quinolone resistance in Campylobacter and track resistance trends in human clinical isolates in relation to use of these agents in food animals. Susceptibility data suggest that erythromycin and other macrolides should remain the drugs of choice in most regions, with systematic surveillance and control measures maintained, but fluoroquinolones may now be of limited use in the empiric treatment of Campylobacter infections in many regions.

Spontaneous mutations that confer antibiotic resistance in Helicobacter pylori.

In this study, we systematically examined in vitro frequencies and spectra of the spontaneous mutations in Helicobacter pylori that confer resistance to clarithromycin (Cla(r)), metronidazole (Mtz(r)), amoxicillin (Amx(r)), ciprofloxacin (Cip(r)), and rifampin (Rif(r)). The mutation rate of Rif(r) or Cip(r) determined in a fluctuation assay is 1 x 10(-8) to 2 x 10(-8) per cell per division. In contrast, the mutation rates of Cla(r), Mtz(r), and Amx(r) are much lower (<10(-9)). However, Mtz(r) mutants could be readily selected in vitro by using the serial passage method, suggesting that the mutagenic effect and selective effect of a sublethal dose of metronidazole contribute to the rapid development of Mtz(r). Analysis of spontaneous Rif(r), Cla(r), and Cip(r) mutants confirmed previous results indicating that mutations within the rpoB gene, the 23S rRNA gene, and the gyrA gene, respectively, are responsible; also, several new mutant alleles were identified. Mtz(r) mutants resulted most frequently, but not always, from mutations in the rdxA gene. DNA fragments containing each mutant allele could readily transform susceptible H. pylori strains to resistance, confirming that each mutant allele is responsible for the resistance phenotype.

Glutathione is a target in tellurite toxicity and is protected by tellurite resistance determinants in Escherichia coli.

Tellurite (TeO3(2-)) is highly toxic to most microorganisms. The mechanisms of toxicity or resistance are poorly understood. It has been shown that tellurite rapidly depletes the reduced thiol content within wild-type Escherichia coli. We have shown that the presence of plasmid-borne tellurite-resistance determinants protects against general thiol oxidation by tellurite. In the present study we observe that the tellurite-dependent depletion of cellular thiols in mutants of the glutathione and thioredoxin thiol:redox system was less than in wild-type cells. To identify the type of low-molecular-weight thiol compounds affected by tellurite exposure, the thiol-containing molecules were analyzed by reverse phase HPLC as their monobromobimane derivatives. Results indicated that reduced glutathione is a major initial target of tellurite reactivity within the cell. Other thiol species are also targeted by tellurite, including reduced coenzyme A. The presence of the tellurite resistance determinants kilA and ter protect against the loss of reduced glutathione by as much as 60% over a 2 h exposure. This protection of glutathione oxidation is likely key to the resistance mechanism of these determinants. Additionally, the thiol oxidation response curves were compared between selenite and tellurite. The loss of thiol compounds within the cell recovered from selenite but not to tellurite.

The role of cysteine residues in tellurite resistance mediated by the TehAB determinant.

TehATehB is a tellurite (TeO(2-)(3)) resistance determinant found on the Escherichia coli chromosome. Normally silent, it specifies a minimal inhibitory concentration (MIC) of 2 microg K(2)TeO(3)/ml unless upregulated or present on a multicopy plasmid which results in an MIC of 128 microg/ml. Both TehA and TehB have three cysteine residues. Oligonucleotide site-directed mutagenesis was carried out to systematically replace all six cysteine residues by alaninies. The results showed that cysteine residues in both TehA and TehB play a role in tellurite resistance: A single cysteine change had no effect, however increasing combinations of two or three cysteine substitutions demonstrated strong phenotypic effects with minimal inhibitory concentrations ranging from 16-64 microg K(2)TeO(3)/ml. A cysteine-free mutant in which all six cysteine residues were replaced by alanines maintained a MIC of 16 microg/ml. Further investigations on the role of cysteines in resistance were studied using thiol reactive reagents on the soluble subunit TehB. These studies confirmed that TehB is a dimer and undergoes a conformational change with tellurite and S-adenosyl-l-methionine binding. Studies using native and SDS denaturing PAGE under reducing and oxidizing conditions suggested that a cysteine in TehB is involved in binding tellurite.

Escherichia coli TehB requires S-adenosylmethionine as a cofactor to mediate tellurite resistance.

The Escherichia coli chromosomal determinant for tellurite resistance consists of two genes (tehA and tehB) which, when expressed on a multicopy plasmid, confer resistance to K(2)TeO(3) at 128 microg/ml, compared to the MIC of 2 microg/ml for the wild type. TehB is a cytoplasmic protein which possesses three conserved motifs (I, II, and III) found in S-adenosyl-L-methionine (SAM)-dependent non-nucleic acid methyltransferases. Replacement of the conserved aspartate residue in motif I by asparagine or alanine, or of the conserved phenylalanine in motif II by tyrosine or alanine, decreased resistance to background levels. Our results are consistent with motifs I and II in TehB being involved in SAM binding. Additionally, conformational changes in TehB are observed upon binding of both tellurite and SAM. The hydrodynamic radius of TehB measured by dynamic light scattering showed a approximately 20% decrease upon binding of both tellurite and SAM. These data suggest that TehB utilizes a methyltransferase activity in the detoxification of tellurite.

Synthesis of mono- and di-fucosylated type I Lewis blood group antigens by Helicobacter pylori.

The identification of Helicobacter pylori isolates that expresses exclusively type I Lewis antigens is necessary to determine the biosynthetic pathway of these antigens. Fast-atom bombardment MS provides evidence that the H. pylori isolate UA1111 expresses predominantly Leb, with H type I and Lea in lesser amounts. Cloning and expression of the H. pylori fucosyltransferases (FucTs) allow comparisons with previously identified H. pylori enzymes and determination of the enzyme specificities. Although all FucTs, one alpha(1,2) FucT and two alpha(1,3/4) FucTs, appear to be functional in this isolate, their activities are lower and enzyme specificities are different to other H. pylori FucTs previously characterized. Studies of the cloned enzyme activities and mutational analysis indicate that Lea acts as the substrate for the synthesis of Leb. This is different from the human Leb biosynthetic pathway, but analogous to the biosynthetic pathway utilized by H. pylori for the production of Ley.