Klebsiella pneumoniae ST147: and then there were three carbapenemases

Gram-negative bacteria containing three different carbapenemases are extremely rare. Klebsiella pneumoniae (N22-925) with KPC-2, NDM-1, and OXA-48 was obtained from a Canadian patient with recent hospitalization in Romania. Short and long read whole genome sequencing showed that the bla_KPC-2 was situated on a 214 kb IncFIB(K)/IncFII(K) plasmid, the bla_NDM-1 on a 104 kb IncFIB (pQil)/IncFII(K) plasmid, and the bla_OXA-48 on a 64 kb IncL plasmid. These plasmids were conjugated to Escherichia coli J53. N22-925 belonged to a unique ST147 cluster that is likely endemic in Romania. This case emphasizes the need for rapid carbapenemase screening in patients from endemic regions. We described the first complete genome sequence of a K. pneumoniae isolate with three different carbapenemases, providing a reference for future studies on this rarely reported occurrence.

The role of whole genome sequencing in antimicrobial susceptibility testing of bacteria: report from the EUCAST Subcommittee

Whole genome sequencing (WGS) offers the potential to predict antimicrobial susceptibility from a single assay. The European Committee on Antimicrobial Susceptibility Testing established a subcommittee to review the current development status of WGS for bacterial antimicrobial susceptibility testing (AST). The published evidence for using WGS as a tool to infer antimicrobial susceptibility accurately is currently either poor or non-existent and the evidence / knowledge base requires significant expansion. The primary comparators for assessing genotypic-phenotypic concordance from WGS data should be changed to epidemiological cut-off values in order to improve differentiation of wild-type from non-wild-type isolates (harbouring an acquired resistance). Clinical breakpoints should be a secondary comparator. This assessment will reveal whether genetic predictions could also be used to guide clinical decision making. Internationally agreed principles and quality control (QC) metrics will facilitate early harmonization of analytical approaches and interpretive criteria for WGS-based predictive AST. Only data sets that pass agreed QC metrics should be used in AST predictions. Minimum performance standards should exist and comparative accuracies across different WGS laboratories and processes should be measured. To facilitate comparisons, a single public database of all known resistance loci should be established, regularly updated and strictly curated using minimum standards for the inclusion of resistance loci. For most bacterial species the major limitations to widespread adoption for WGS-based AST in clinical laboratories remain the current high-cost and limited speed of inferring antimicrobial susceptibility from WGS data as well as the dependency on previous culture because analysis directly on specimens remains challenging. For most bacterial species there is currently insufficient evidence to support the use of WGS-inferred AST to guide clinical decision making. WGS-AST should be a funding priority if it is to become a rival to phenotypic AST. This report will be updated as the available evidence increases.

Management of a hospital outbreak of extensively drug-resistant Acinetobacter baumannii using a multimodal intervention including daily chlorhexidine baths

BACKGROUND

Extensively drug-resistant Acinetobacter baumannii (XDR-Ab) is an increasingly important cause of healthcare-associated infection. Uncertainties remain concerning optimal control measures for healthcare-associated outbreaks.

AIM

To describe the epidemiology and control of an XDR-Ab outbreak that involved multiple units of a large hospital from March 2012 to January 2014.

METHODS

Case-finding included screening of rectum, groin, throat, nose, wounds, iatrogenic portals of entry, and catheterized sites. Antimicrobial susceptibility was evaluated by disc diffusion and E-test. Resistance genes were detected by polymerase chain reaction. Clonality was assessed by pulsed-field gel electrophoresis. Charts of cases were reviewed to identify risk factors for invasive infection. Control measures included isolation and cohorting of cases, hand hygiene reinforcement, environmental decontamination, and source control with daily baths using wipes pre-impregnated with chlorhexidine gluconate.

FINDINGS

A single clonal strain of XDR-Ab colonized or infected 29 patients. Five patients died of XDR-Ab bacteraemia. Transmission occurred primarily on two wards. Colonization was detected at all anatomical screening sites; only 57% (16/28) of cases were rectal carriers. Advanced malignancy was a risk factor for bacteraemia (relative risk: 5.8; 95% confidence interval: 1.2-27.0). Transmission ended following implementation of the multimodal control strategy. No additional nosocomial cases occurred during the following 20 months.

CONCLUSION

Our study highlights the need to screen multiple anatomic sites to diagnose carriage and identifies risk factors for XDR-Ab bacteraemia. A multimodal intervention that included daily chlorhexidine baths for cases was rapidly followed by the termination of the outbreak. Hospitals should consider similar interventions when managing future XDR-Ab outbreaks.

Prevalence of community-associated methicillin-resistant Staphylococcus aureus colonization in men who have sex with men

Summary

Outbreaks of skin and soft tissue infections mediated by community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) are being reported with increasing frequency among men who have sex with men (MSM). However, the potential role of asymptomatic colonization with this organism in perpetuating these infections is unclear. The purpose of this cross-sectional study was to determine the prevalence of colonization with CA-MRSA among a cohort of 500 MSM recruited from two inner city clinics in Toronto, Canada. Following the provision of informed consent, subjects completed a questionnaire capturing demographic and clinical variables, which may be associated with MRSA colonization. A nasal swab for MRSA was collected from each subject, and instructions were provided regarding the self-collection of a rectal swab. Cultured MRSA underwent pulsed-field gel electrophoresis and virulence testing for Panton-Valentine leukocidin gene expression. The prevalence of CA-MRSA colonization was 1.6% (95% CI: 0.5–2.6%).

Characterization of Clostridium difficile strains isolated from patients in Ontario, Canada, from 2004 to 2006

Clostridium difficile is the bacterium most commonly surmised to cause antimicrobial- and hospital-associated diarrhea in developed countries worldwide, and such infections are thought to be increasing in frequency and severity. A laboratory-based study was carried out to characterize C. difficile strains isolated from persons in Ontario, Canada, during 2004 to 2006 according to toxin type (enterotoxin A, cytotoxin B, and binary toxin [CDT]), tcdC gene characterization, ribotyping, pulsed-field gel electrophoresis, and toxinotyping. Clostridium difficile was isolated from 1,080/1,152 (94%) samples from 21 diagnostic laboratories. Isolates with toxin profiles A(+) B(+) CDT(-), A(+) B(+) CDT(+), A(-) B(+) CDT(-), and A(-) B(+) CDT(+) accounted for 63%, 34%, 2.4%, and 0.6% of isolates, respectively. Alterations in tcdC were detected in six different ribotypes, including ribotype 027. A total of 39 different ribotypes were identified, with ribotype 027/North American pulsotype 1 (NAP1), an internationally recognized outbreak strain associated with severe disease, being the second most common ribotype (19% of isolates). Transient resistance to metronidazole was identified in 19 (1.8%) isolates. While a large number of ribotypes were found, a few predominated across the province. The high prevalence and wide distribution of ribotype 027/NAP1 are disconcerting in view of the severity of disease associated with it.

Genetic variation in the Desert Springsnail (Tryonia porrecta): implications for reproductive mode and dispersal

Allozymes and mitochondrial cytochrome c oxidase subunit I (mtCOI) sequences were analysed to determine whether populations of the western North American gastropod Tryonia porrecta (from California, Nevada, Utah, and northwest Mexico) are strongly differentiated in accordance with traditional interpretation of regional fauna as ancient relicts inhabiting isolated fragments of late Tertiary palaeodrainages. These data were also used to assess whether this species, for which males have not been recorded, is a rare example of a molluscan parthenogen. Both data sets strongly supported monophyly of T. porrecta populations. Five of the nine sampled populations consisted of a single monoallelic allozyme genotype while the others contained two to 10 distinct genotypes. Allozymic data for genetically diverse Utah populations provided evidence of clonal and sexual reproduction. mtCOI haplotypes of T. porrecta formed two subgroups which differed by 1.99-2.60%. The common haplotype was found in seven populations with rare haplotypes observed in single populations. Based on these results and an available mtCOI molecular clock for related hydrobiid snails, T. porrecta is interpreted as a primarily parthenogenetic species that undergoes occasional sexual reproduction and has accumulated substantial diversity following its mid-Pliocene to mid-Pleistocene origin. Our results also suggest that the distribution of present-day populations of these gill-breathing snails did not result from fragmentation of an ancient, well-integrated drainage but instead reflects overland colonization of habitats which only recently became available following desiccation of late Quaternary pluvial lakes.

Crystallization and preliminary X-ray diffraction studies of the FimC-FimH chaperone-adhesin complex from Escherichia coli

A complex of the periplasmic chaperone FimC and the mannose-binding adhesin FimH from the Escherichia coli type 1 pilus system has been crystallized from ammonium sulfate solution using the hanging-drop vapour-diffusion method. The crystals diffract to a minimum Bragg spacing of 2.7 A and belong to the space group P4(1)2(1)2 or P4(3)2(1)2 with cell dimensions a = b = 97.7, c = 215.9 A at room temperature. Data to 3.0 A have been collected from a single-crystal frozen to T = 100 K.

Multiplex PCR for the detection of tetracycline resistant genes

Specific primer pairs were selected for the PCR amplification of 14 tetracycline resistant genes commonly found in Gram positive and Gram negative organisms. Combinations of primer pairs were used in multiplex PCR reactions to detect specific groups of tet genes as follows; Group I tet (B), tet (C), tet (D); Group II tet (A), tet (E), tet (G); Group III tet (K), tet (L), tet (M), tet (O), tet (S); Group IV tetA (P), tet (Q), tet (X). To test the multiplex PCR, Groups I and II were used on 25 clinical isolates of Salmonella enterica serovar Typhimurium DT104. Group III primers were used to investigate 19 clinical isolates of methicillin-resistant Staphylococcus aureus. Multiplex PCR should result in significant savings in terms of labour and cost in analysis of a large number of strains when compared with using an individual PCR for targeting each gene. It may also be a useful method to differentiate the types of tetracycline resistance when used as an additional marker for the purpose of outbreak investigation and surveillance.

The evolution of methicillin-resistant Staphylococcus aureus in Canadian hospitals: 5 years of national surveillance

BACKGROUND

To better understand the epidemiology of methicillin-resistant Staphylococcus aureus (MRSA) in Canadian hospitals, surveillance has been conducted in sentinel hospitals across the country since 1995. We report the results of the first 5 years of the program.

METHODS

For each newly identified inpatient with MRSA, medical records were reviewed for demographic and clinical data. Isolates were subjected to susceptibility testing and molecular typing by pulsed-field gel electrophoresis.

RESULTS

A total of 4507 patients infected or colonized with MRSA were identified between January 1995 and December 1999. The rate of MRSA increased each year from a mean of 0.95 per 100 S. aureus isolates in 1995 to 5.97 per 100 isolates in 1999 (0.46 per 1000 admissions in 1995 to 4.12 per 1000 admissions in 1999) (p < 0.05). Most of the increase in MRSA occurred in Ontario, Quebec and the western provinces. Of the 3009 cases for which the site of MRSA acquisition could be determined, 86% were acquired in a hospital, 8% were acquired in a long-term care facility and 6% were acquired in the community. A total of 1603 patients (36%) were infected with MRSA. The most common sites of infection were skin or soft tissue (25% of MRSA infections), pulmonary tissues (24%) and surgical sites (23%); 13% of the patients were bacteremic. An epidemiologic link with a previously identified MRSA patient was suspected in 53% of the cases. Molecular typing indicated that most (81%) of the isolates could be classified as related to 1 of the 4 Canadian epidemic strains of MRSA.

INTERPRETATION

There has been a significant increase in the rate of isolating MRSA in many Canadian hospitals, related to the transmission of a relatively small number of MRSA strains.

A herpes simplex virus type 1 gamma34.5 second-site suppressor mutant that exhibits enhanced growth in cultured glioblastoma cells is severely attenuated in animals

We describe here the neurovirulence properties of a herpes simplex virus type 1 gamma34.5 second-site suppressor mutant. gamma34.5 mutants are nonneurovirulent in animals and fail to grow in a variety of cultured cells due to a block at the level of protein synthesis. Extragenic suppressors with restored capacity to replicate in cells that normally do not support the growth of the parental gamma34.5 deletion mutant have been isolated. Although the suppressor virus reacquires the ability to grow in nonpermissive cultured cells, it remains severely attenuated in mice and is indistinguishable from the mutant gamma34.5 parent virus at the doses investigated. Repairing the gamma34.5 mutation in the suppressor mutant restores neurovirulence to wild-type levels. These studies illustrate that (i) the protein synthesis and neurovirulence defects observed in gamma34.5 mutant viruses can be genetically separated by an extragenic mutation at another site in the viral chromosome; (ii) the extragenic suppressor mutation does not affect neurovirulence; and (iii) the attenuated gamma34.5 mutant, which replicates poorly in many cell types, can be modified by genetic selection to generate a nonpathogenic variant that regains the ability to grow robustly in a nonpermissive glioblastoma cell line. As this gamma34.5 second-site suppressor variant is attenuated and replicates vigorously in neoplastic cells, it may have potential as a replication-competent, viral antitumor agent.

Effect of mercury and Gpi-2 genotype on standard metabolic rate of eastern mosquitofish (Gambusia holbrooki)

Previous studies demonstrated differential mortality among mosquitofish of different Gpi-2 genotypes during acute mercury and arsenate exposures. Mercury-exposed mosquitofish also had Gpi-2 genotype-specific differences in glycolytic and Krebs cycle metabolite pools. The mortality and metabolite data suggested that mosquitofish bearing specific Gpi-2 genotypes might differ in metabolic efficiency, with less efficient Gpi-2 genotypes having higher standard metabolic rates (SMRs) and shorter times to death during acute mercury exposure. Effect of Gpi-2 genotype on SMR was assessed with a factorial arrangement of six Gpi-2 genotypes and two exposure sequences (Control - Control; Control - 100 microg/L Hg). The SMRs were estimated by measuring oxygen consumption using an indirect, closed-circuit, computer-controlled respirometer. A 48-h exposure to 100 microg/L of mercury resulted in a 16.7% elevation of SMR above control levels (p = 0.001). The Gpi-2 genotype and the number of heterozygous loci per individual had no significant effect on SMR in mercury-exposed mosquitofish. The experimental results do not support the hypothesis that Gpi-2 genotype-specific differences in glycolytic and Krebs cycle metabolite pools and mortality in mosquitofish exposed to mercury are associated with differences in SMR.

Molecular genetic markers provide no evidence for reproductive isolation among retreat building phenotypes of the net-spinning caddisfly Macrostemum carolina

Larvae of the stream-dwelling, filter-feeding caddisfly Macrostemum carolina construct silken catchnets within protective retreats. In the Savannah River, M. carolina individuals make three different retreats, each with a distinct water entrance hole: (i) at the end of a silken tube; (ii) with a approximately 180 degrees silken backstop; and (iii) flush with the top of the retreat. To resolve whether these different retreats represent alternative behavioural phenotypes within a single panmictic population or fixed phenotypes within three genetically distinct populations or species, we compared the allele frequencies at three polymorphic nuclear loci (allozyme electrophoresis for Gpi, Mpi and Pgm) and the mitochondrial DNA (mtDNA) haplotype frequencies among individuals displaying the three retreat morphs. We also calculated pairwise exact tests of population differentiation using the allozyme and mtDNA allele frequencies. No significant genetic differentiation was detected among caddisflies exhibiting the different retreat morphs. Therefore, these morphs apparently represent a single panmictic population in the Savannah River. Consequently, additional study is required to assess whether this retreat polymorphism is a phenotypically plastic trait under conditional control, or is mediated by alternative alleles at a Mendelian gene or genes (or a combination of the two).

Inhibition of PKR activation by the proline-rich RNA binding domain of the herpes simplex virus type 1 Us11 protein

Upon activation by double-stranded RNA in virus-infected cells, the cellular PKR kinase phosphorylates the translation initiation factor eukaryotic initiation factor 2 (eIF2) and thereby inhibits protein synthesis. The gamma 34.5 and Us11 gene products encoded by herpes simplex virus type 1 (HSV-1) are dedicated to preventing the accumulation of phosphorylated eIF2. While the gamma 34.5 gene specifies a regulatory subunit for protein phosphatase 1 alpha, the Us11 gene encodes an RNA binding protein that also prevents PKR activation. gamma 34.5 mutants fail to grow on a variety of human cells as phosphorylated eIF2 accumulates and protein synthesis ceases prior to the completion of the viral life cycle. We demonstrate that expression of a 68-amino-acid fragment of Us11 containing a novel proline-rich basic RNA binding domain allows for sustained protein synthesis and enhanced growth of gamma 34.5 mutants. Furthermore, this fragment is sufficient to inhibit activation of the cellular PKR kinase in a cell-free system, suggesting that the intrinsic activities of this small fragment, notably RNA binding and ribosome association, may be required to prevent PKR activation.

Phylogenetic relationships within the aquatic snail genus Tryonia: implications for biogeography of the North American Southwest

We examined the phylogenetic relationships among 23 species of the North American aquatic snail genus Tryonia (Hydrobiidae), 10 additional representatives of the subfamily Cochliopinae, and two outgroups. Maximum parsimony analysis of a 601-base-pair sequence from the mitochondrial COI gene did not support monophyly of the genus nor its subgenus Paupertryonia. A clade composed of the type species of Tryonia and 16 congeners was strongly supported by the COI data and congruent with recently discovered variation in female genitalic morphology. This "true Tryonia" clade included two large western subclades having a sister-group relationship. The phylogenetic structure of one of these subclades is congruent with vicariant events associated with late Neogene history of the lower Colorado River drainage. The other subclade mirrors development of the modern Rio Grande rift and inception of modern topography in the southwestern Great Basin during the late Neogene. Both subclades are represented in the composite Tryonia fauna of the Amargosa River basin, whose assembly is attributed to the complex geological history of the Death Valley region.

Comparison of molecular methods for typing Vibrio parahaemolyticus

An outbreak of Vibrio parahaemolyticus gastroenteritis on Canada's west coast in 1997 emphasized the need to develop molecular methods for differentiation and typing of these organisms. Isolates were analyzed by enterobacterial repetitive intergenic consensus sequence (ERIC) PCR, detection of restriction fragment length polymorphisms (RFLP) in rRNA genes (ribotyping), pulsed-field gel electrophoresis (PFGE), and RFLP analysis of the genetic locus encoding the polar flagellum (Fla locus RFLP analysis). ERIC PCR and ribotyping were the most informative typing methods, especially when used together, while Fla locus RFLP analysis was the least discriminatory. PFGE exhibited good discrimination but suffered from a high incidence of DNA degradation. ERIC PCR and ribotyping will be useful for the evaluation of genetic and epidemiological relationships among V. parahaemolyticus strains.

Characterization and proposed nomenclature of epidemic strains of methicillin-resistant Staphylococcus aureus in Canada

We hope that standardized nomenclature for identifying epidemic MRSA strains prevalent in Canadian hospitals will be helpful to physicians and infection control practitioners attempting to understand and control the spread of the organism in health-care facilities. It is anticipated that as MRSA continues to evolve in Canadian health-care facilities other strains may be recognized as "epidemic"; as these strains become better characterized they may be added to those designated above. Laboratory physicians and infection control personnel are invited to submit strains that may warrant characterization and designation as a Canadian epidemic strain to the Laboratory Centre for Disease Control, Health Canada, Winnipeg, Manitoba.

A herpesvirus ribosome-associated, RNA-binding protein confers a growth advantage upon mutants deficient in a GADD34-related function

The herpes simplex virus type 1 gamma34.5 gene product and the cellular GADD34 protein both contain similar domains that can regulate the activity of eukaryotic initiation factor 2 (eIF2), a critical translation initiation factor. Viral mutants that lack the GADD34-related function grow poorly on a variety of malignant human cells, as activation of the cellular PKR kinase leads to the accumulation of inactive, phosphorylated eIF2 at late times postinfection. Termination of translation prior to the completion of the viral reproductive cycle leads to impaired growth. Extragenic suppressors that regain the ability to synthesize proteins efficiently in the absence of the viral GADD34-related function have been isolated. These suppressor alleles are dominant in trans and affect the steady-state accumulation of several viral mRNA species. We demonstrate that deregulated expression of Us11, a virus-encoded RNA-binding, ribosome-associated protein is necessary and sufficient to confer a growth advantage upon viral mutants that lack a GADD34-related function. Ectopic expression of Us11 reduces the accumulation of the activated cellular PKR kinase and allows for sustained protein synthesis. Thus, an RNA-binding, ribosome-associated protein (Us11) and a GADD34-related protein (gamma34.5) both function in a signal pathway that regulates translation by modulating eIF2 phosphorylation.

Bank voles in linear habitats show restricted gene flow as revealed by mitochondrial DNA (mtDNA)

Genetic structure of bank vole populations in linear river bank habitat in southeast Norway was determined from analyses of DNA sequences for the mitochondrial D-loop. Animals were sampled at sites separated by 1 km, along two forested river banks separated by approximately 100 m of open water. Twenty-six distinct haplotypes were found among 120 voles. The voles showed significant deviation from panmixis on both sides of the river. Animals from the same site or from sites 1 km apart were more likely to share haplotypes than animals 2 km apart or more. Common haplotypes were widespread on both river banks, and had a wider distribution than relatively rare haplotypes. Some rare haplotypes were found on both banks, but most were restricted to a single bank. The results suggest that short-term gene flow may be restricted for female bank voles in linear habitats. Female territorial behaviour may vary with habitat geometry. In the linear habitat described here, females defend only two territorial borders and may effectively limit female dispersal. Results were compared to a previous study of bank voles from this region in a two-dimensional habitat. Gene flow in the linear habitat was much more restricted than gene flow in the two-dimensional habitat. Probable mechanisms underlying this difference are discussed.

Role of glycoprotein PE2 in formation and maturation of the Sindbis virus spike

Sindbis virus envelope assembly is a multistep process resulting in the maturation of a rigid, highly ordered T=4 icosahedral protein lattice containing 80 spikes composed of trimers of E1-E2 heterodimers. Intramolecular disulfide bonds within E1 stabilize E1-E1 associations required for envelope formation and maintenance of the envelope's structural integrity. The structural integrity of the envelope protein lattice is resistant to reduction by dithiothreitol (DTT), indicating that E1 disulfides which stabilize structural domains become inaccessible to DTT at some point during virus maturation. The development of E1 resistance to DTT occurs prior to the completion of E1 folding and is temporally correlated with spike assembly in the endoplasmic reticulum. From these data we have predicted that in the final stages of spike assembly, E1 intramolecular disulfides, which stabilize the structural integrity of the envelope protein lattice, are buried within the spike and become inaccessible to the reductive activity of DTT. The spike is formed prior to the completion of E1 folding, and we have suggested that PE2 (the precursor to E2) may play a critical role in E1 folding after PE2-E1 oligomer formation has occurred. In this study we have investigated the role of PE2 in E1 folding, oligomer formation, and development of E1 resistance to both protease digestion and reduction by DTT by using a Sindbis virus replicon (SINrep/E1) which allows for the expression of E1 in the presence of truncated PE2. Through pulse-chase analysis of both Sindbis virus- and SINrep/E1-infected cells, we have determined that the folding of E1 into a trypsin-resistant conformation and into its most compact and stable form is not dependent upon association of E1 with PE2. However, E1 association with PE2 is required for oligomer formation, the export of E1 from the endoplasmic reticulum, and E1 acquisition of resistance to DTT.

Molecular systematics and evolution of reproductive traits of North American freshwater unionacean mussels (Mollusca: Bivalvia) as inferred from 16S rRNA gene sequences

North American freshwater unionacean bivalves are a diverse group of nearly 300 species. Unionaceans exhibit an array of conchological, anatomical, life history, and reproductive characteristics that have figured prominently in proposed classification schemes. Recently, two very different classifications of North American unionaceans have been proposed. Depending on the classification system utilized, a very different evolutionary trajectory of anatomical and reproductive features is obtained. The lack of a robust, well corroborated phylogeny of North American unionacean bivalves hinders the progress of evolutionary and ecological studies involving these species. Here we present a mitochondrial DNA (mtDNA) based phylogeny for North American unionacean mussels and compare it to previously proposed classifications. In addition, we present a 'total evidence' phylogeny which incorporates both the mtDNA sequence data and available morphological data. The molecular and total evidence phylogenies agree largely with the conclusions of a previous study based largely on immunoelectrophoretic data. North American unionaceans can be divided into two families: the Unionidae, which is comprised of most of the species and the Margaritiferidae. Within the Uniondae are two subfamilies, the Anodontinae and Ambleminae. The resultant phylogeny was used to examine the evolution of several key anatomical features including the number of gills (demibranchs) used by females to brood developing embryos, incubation length (bradytictic vs tachytictic), larval (glochidial) tooth structures, and shell texture. Both molecular and total evidence phylogenies indicate several of the aforementioned characters evolved independently or were subsequently lost or gained in several lineages.

Assembly of the Sindbis virus spike protein complex

The Sindbis virus glycoproteins E1 and E2 are organized into 80 trimers of heterodimers within the virus envelope. Using pulse-chase protocols and chemical crosslinkers, we have found that E1 and E2 precursor, PE2, rapidly assemble into heterodimers and then into trimers of heterodimers after translocation into the endoplasmic reticulum. E1 folds into its mature conformation within the endoplasmic reticulum via at least three intermediates differing in the configurations of their disulfide bonds. PE2 can pair with the second of these E1 folding intermediates. The remaining E1 folding steps, therefore, occur after E1-PE2 multimers begin to form. Quaternary interactions between E1 and PE2 may help guide the folding of E1. While no PE2 folding intermediates have yet been detected, we have found that PE2 transiently enters into large, noncovalent complexes or aggregates with other PE2 molecules and/or with unknown host factors prior to pairing with E1.

Characterization of Mengo virus neutralization epitopes. II. Infection of mice with an attenuated virus

A panel of five neutralizing monoclonal antibodies was generated from mice immunized with an attenuated strain of Mengo virus. Four of the antibodies were used to select mutants of Mengo virus which were able to escape neutralization by the selecting antibody, but it was not possible to select mutants which could escape neutralization by the fifth antibody. The capsid coding region of the RNA genome of each mutant was directly sequenced to identify the mutation(s) responsible for the neutralization escape phenotype. These results are compared to those of a previous study in which immunogenic determinants recognized by neutralizing antibodies generated against pentameric capsid subunits were located on the external surface of the Mengo virion. We have confirmed the existence of the previously identified immunogenic determinant in VP3 (site 2) as well as an immunodominant determinant in VP2 (site 1). Two previously uncharacterized determinants, located in surface loops of VP1 (sites 3 and 4A), were also identified. None of the mutations conferring the neutralization escape phenotype was found near the surface depressions on the virion which are believed to be the receptor binding sites.

Involvement of the molecular chaperone BiP in maturation of Sindbis virus envelope glycoproteins

Sindbis virus codes for two membrane glycoproteins, E1 and PE2, which assemble into heterodimers within the endoplasmic reticulum. We have examined the role of the molecular chaperone BiP (grp78) in the maturation of these two proteins. E1, which folds into its mature conformation via at least three intermediates differing in the configurations of their disulfide bonds, was found to interact strongly and transiently with BiP after synthesis. ATP depletion mediated by carbonyl cyanide m-chlorophenylhydrazone treatment results in the stabilization of complexes between BiP and E1. The depletion of intracellular ATP levels also greatly inhibits conversions between the E1 folding intermediates and results in the slow incorporation of E1 into disulfide-stabilized aggregates. These results suggest that the ATP-regulated binding and release of BiP have a role in modulating disulfide bond formation during E1 folding. In comparison with E1, very little PE2 is normally recovered in association with BiP. However, under conditions in which E1 folding is aberrant, increased amounts of PE2 become directly associated with BiP. The formation of these BiP-PE2 interactions occurs after E1 begins to misfold or fails to fold efficiently. We propose that nascent PE2 is stable prior to pairing with E1 for only a limited period of time, after which unpaired PE2 becomes recognized by BiP. This implies that the productive association of PE2 and E1 must occur within a restricted time frame and only after E1 has accomplished certain folding steps mediated by BiP binding and release. Kinetic studies which show that the pairing of E1 with PE2 is delayed after translocation support this conclusion.

Formation and rearrangement of disulfide bonds during maturation of the Sindbis virus E1 glycoprotein

The rigidly ordered icosahedral lattice of the Sindbis virus envelope is composed of a host-derived membrane bilayer in which the viral glycoproteins E1 and E2 reside. E1-E1 interactions stabilized by intramolecular disulfide bridges play a significant role in maintaining the envelope's structural integrity (R. P. Anthony and D. T. Brown, J. Virol. 65:1187-1194, 1991; R. P. Anthony, A. M. Paredes, and D. T. Brown, Virology 190:330-336, 1992). We have examined the acquisition of disulfide bridges within E1 during its maturation. Prior to exit from the endoplasmic reticulum, E1 folds via at least three intermediates, differing in the number and/or arrangement of their disulfides, into a single, compact form. This E1 species remains stable with respect to its disulfides until late in the secretory pathway, when E1 attains a metastable conformation. At this point, when appropriately triggered, intramolecular thiol-disulfide exchange reactions within E1 can occur, resulting in the generation of alternative E1 species. This metastable nature of mature E1 may have important implications for the mechanism of virus disassembly during the initial stages of the infection process (B. Abell and D. T. Brown, J. Virol. 67:5496-5501, 1993).

Electrophoretic differentiation of two Halipegus (Trematoda: Hemiuridae) congeners in an amphibian population

The presence of Halipegus eccentricus, which previously had been absent from a study area, was confirmed when marked genetic differences were observed within what had been thought to be a homogenous population of Halipegus occidualis from the buccal region of the green frog, Rana clamitans. Starch gel electrophoresis was used to determine levels of genetic variation in H. occidualis and H. eccentricus. The H. occidualis population had 12% polymorphic loci and average individual heterozygosity of 0.05. Endemism of snail hosts, limited potential for parasite immigration, monoecious adults, and asexual reproduction occurring in the molluscan intermediate host may be factors that minimize the extent of genetic variability in the parasite population.

Linkage relationships of seven enzyme and two pigmentation loci in the snail Biomphalaria glabrata

Crossing experiments with inbred stocks of the snail (Biomphalaria glabrata) demonstrated that variants at two loci determining pigmentation and seven enzyme-determining loci exhibited normal Mendelian segregation ratios in F2 progeny. Among 39 pairwise comparisons for joint segregation, there was evidence of genetic linkage between a locus controlling mantle pigmentation (S) and 6-phosphogluconate dehydrogenase (Pgd) and confirmation of a previously described linkage between esterase-2 (Est-2) and catalase (Cat). Recombination fractions were estimated to be 17 +/- 4 for S-Pgd and 33 +/- 5 for Est-2-Cat. The remaining five loci--Acon-1, Pgm-1, Lap-1, Lap-2, and Pgd--assorted independently. This brings to 17 the number of loci examined for segregation and assortment in this medically important species. As Biomphalaria has a chromosome number n = 18, markers should soon be available for most or all of the linkage groups.

Molecular cloning and genetic analysis of the determinant for gamma-lysin, a two-component toxin of Staphylococcus aureus

The gamma-lysin determinant of Staphylococcus aureus strain Smith 5R has been cloned in phage lambda and plasmid vectors in Escherichia coli. Genetic evidence is presented which demonstrates that gamma-lysin requires the co-operative action of two polypeptides expressed by the closely linked hlgA and hlgB genes. Recombinants expressed haemolytic activity in agarose medium but not in agar, a known property of gamma-lysin. Haemolysis was inhibited by antiserum raised against the 32 kDa component of gamma-lysin, but not by anti-alpha-, anti-beta- or anti-delta-lysin serum. Subcloning and transposon Tn5 mutagenesis identified a 3.5 kb region which was necessary for gamma-lysin expression in E. coli. Two genes (hlgA and hlgB) were mapped and their polypeptide products identified. Non-haemolytic Tn5 mutants fell into two groups based upon complementation tests done between extracts of mutants in vitro and also between extracts of mutants and components of gamma-lysin purified from S. aureus culture supernates. Immunoblotting showed that some mutants in group A (defective in expression of hlgA) did not express a 32 kDa polypeptide which was synthesized by the parental haemolytic recombinant and by mutants in group B. Minicell analysis suggested that the products of the hlgB gene were proteins of 38 kDa and 36 kDa. The smaller molecule co-migrates with a protein in a fraction of the S. aureus culture supernate containing component B of gamma-lysin. The 38 kDa polypeptide is probably an unprocessed precursor. Southern hybridization demonstrated that the hlgA and hlgB genes are closely linked in the chromosome of several strains of S. aureus.

Genotype frequency differences in Halipegus occidualis-infected and uninfected Helisoma anceps

Allozyme frequencies in Helisoma anceps infected with the hemiurid trematode, Halipegus occidualis, were compared with those of uninfected H. anceps from a small, North Carolina farm pond. Of 6 loci found to be polymorphic, the frequencies of esterase-1 and leucine aminopeptidase were different in infected and uninfected snails. Genetic heterozygosity, as determined by starch gel electrophoresis, was greater in uninfected H. anceps relative to infected individuals. These observations combined with the high prevalence (up to 60%), complete castration in patent infections, and the absence of an encapsulation response in infected snails, suggest that factors conferring incompatibility may have been selected for in the H. anceps population within the pond.