Stacking efficiency of diselenadiazolyl pi-dimers. Consequences for electronic structure and transport properties

The preparation and crystal structure of 5-cyanofuran-2-[1,2,3,5-diselenadiazolyl], [RCN(2)Se(2)] (R = 5-cyanofuran), is reported. Crystal data for C(6)H(2)ON(3)Se(2): monoclinic, space group P2(1), a = 7.1121(7), b= 20.541(2), c =20.923(2) A, beta = 99.785(1) degrees, Z = 16. The crystal structure consists of diselenadiazolyl pi-dimer stacks running parallel to the x direction; the asymmetric unit consists of four pi-dimer units. The dimers are aligned into snakelike ribbons along the y direction, with consecutive dimers linked by head-to-tail CN-Se contacts. Each pi-dimer stack is bordered by two out-of-register stacks, but most interstack Se-Se contacts lie outside the van der Waals separation. Along the pi-dimer stacks, the intradimer Se-Se distances range from 3.183(10) to 3.294(1) A, and the interdimer Se-Se distances range from 3.826(1) to 3.945(1) A. Like other pi-dimer stacked diselenadiazolyls, [C(6)H(2)ON(3)Se(2)](2) is diamagnetic over the temperature range 4-380K. Variable temperature single-crystal conductivity measurements reveal a room-temperature conductivity near 10(-5) S cm(-1) and provide a calculated band gap of 0.72 eV. The structural results and transport properties are interpreted in the light of Extended Hückel band structure calculations.

Coinfection with Campylobacter species: an epidemiological problem?

AIMS

To determine the frequency of coinfection with multiple strains in sporadic cases of human Campylobacter infection.

METHOD AND RESULTS

During 1999 10 single colonies of Campylobacter were cultured from each of 53 positive faecal samples. Five isolates were taken from nonselective agar after passive filtration of faecal suspensions and five isolates were taken from selective agar plates. All isolates were sero- and phage typed and their antibiotic resistance determined. Pulsed-field gel electrophoresis and flagellin gene typing were performed on selected isolates. One patient was infected with Camp. coli, the remainder with strains of Camp. jejuni. The majority of patients was infected with a single strain of Campylobacter, but from each of four samples, 7.5%, two strains of Camp. jejuni, confirmed by molecular typing, were identified.

CONCLUSION

Coinfection occurs in sporadic cases of campylobacteriosis.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study has implications in outbreak investigation when distinct strains have been isolated from epidemiologically related patients and/or the suspected source or vehicle.

Strong supramolecular-based magnetic exchange in pi-stacked radicals. Structure and magnetism of a hydrogen-bonded verdazyl radical:hydroquinone molecular solid

The X-ray crystal structure and magnetic properties of a molecular crystal consisting of 1,5-dimethyl-3-(2-pyridyl)-6-oxoverdazyl radical and hydroquinone (pyvd:hq) are presented. The structure contains a two-dimensional network of hydrogen bonds involving the hydroquinones and the pyridine ring of the pyvd radical. The radicals adopt an unusual head-over-tail (antiparallel) pi-stacked array perpendicular to the hydrogen-bonded planes. The variable-temperature magnetic susceptibility data can be modeled using a one-dimensional antiferromagnetic chain model, with J = -58 cm(-1). The strength of the magnetic coupling is very unusual because there are no close intermolecular radical-radical contacts to provide conventional pathways for magnetic interactions. A pathway for coupling is proposed involving the mediation of magnetic exchange interactions between radical centers by the pyridine rings. Density functional calculations on the pyvd radical, as well as aggregates thereof based on the X-ray structure, have been employed in attempts to understand the possible mechanisms by which the strong magnetic interactions are achieved.

PCR-RFLP analysis of the coagulase gene of Staphylococcus aureus: application to the differentiation of epidemic and sporadic methicillin-resistant strains

Preventing cross-infection with epidemic strains of methicillin-resistant Staphylococcus aureus (MRSA) requires effective control measures. These call for simple, rapid, discriminatory and reproducible methods for typing this pathogen. In this study 140 isolates/strains from 105 hospitals in England and Wales, representing 72 diverse phage types, were analysed by bacteriophage typing and PCR coagulase (coa) gene restriction fragment length polymorphism (RFLP). Isolates gave a coa gene PCR product that was either 660 base pairs (bp), 603 bp or 547 pb in size. The PCR products were digested with Alu I and Cfo I, and the fragments separated by gel electrophoresis. Eight coa gene RFLP patterns, numbered 1 to 8, were observed. Pattern 3 was most common (N = 25 isolates), followed by patterns 2 and 5 (18 isolates each), pattern 1 (14 isolates), pattern 4 (11 isolates), pattern 7 (10 isolates), pattern 8 (eight isolates) and pattern 6 (six isolates). Isolates of the same phage type often gave different coa gene RFLP patterns, and the patterns within the epidemic types EMRSA-03, EMRSA-15 and EMRSA-16 were heterogeneous. Thus, representatives of EMRSA-03 were subtyped to coa RFLP patterns 1 and 2, those of EMRSA-05 to coa RFLP patterns 1, 2, 7 and 8, and those for EMRSA-16 to coa RFLP patterns 2, 3, 4, 5 and 6. The range of patterns within single phage types of S. aureus could help to discriminate between isolates/strains, and in a hierarchical approach coa gene RFLP could occupy an intermediate position between phage typing and pulsed-field gel electrophoresis (PFGE).

Use of fluorescent amplified fragment length polymorphism (fAELP) to characterise methicillin-resistant Staphylococcus aureus

The new PCR-based genotyping technique, fluorescent amplified fragment length polymorphism (fAFLP), was compared for discriminatory power and reproducibility with standard phenotypic methods, a coagulase gene (coa) restriction fragment length polymorphism (RFLP) method and pulsed-field gel electrophoresis (PFGE), in typing 34 isolates and four reference strains of methicillin-resistant Staphylococcus aureus (MRSA). The fAFLP showed from 40 to 75 fragments, 50 to 450 base pairs (bp) in size. Based on replicate studies, the isolates were judged indistinguishable when their fAFLP pattern was >93.7% similar. Only two of the isolates were indistinguishable by this criterion. Thirty-one MRSA fell into four major fAFLP groups (1, 2, 3 and 4) at the level of >79.9% similarity. Three other isolates and an EMRSA-16 strain fell outside these major groups. Within both fAFLP groups 1 and 2, two subgroups, A and B, could be identified at approximately 82.0% similarity. While most isolates within group 1 could also be separated by their phenotypic and coagulase gene (coa) RFLP pattern, all the isolates within fAFLP groups 2A and 2B were identical on the basis of these characters. The MRSA within fAFLP groups 3 and 4 were heterogeneous by their phenotypic characteristics and coa gene RFLP patterns. fAFLP was reproducible and distinguished between MRSA isolates that appeared identical by other methods. It is likely to contribute to the epidemiological analysis of outbreaks of MRSA infection.

Phages for methicillin-resistant Staphylococcus aureus: an international trial

An internationally agreed and validated set of phages is used worldwide for the typing of strains of Staphylococcus aureus of human origin. However, because of the sometimes reduced susceptibility of methicillin-resistant strains (MRSA) to these phages, some of the national typing centres use locally isolated and characterized sets of experimental phages. In this trial, 42 such phages were distributed to 6 centres and tested against 744 isolates of MRSA with the intention of defining a phage set to augment the international set. The use of these experimental phages increased the percentage typability from 75% with the international set to 93% and the number of identifiable lytic patterns from 192 to 424. A subset of 10 experimental phages was selected. When this subset was compared with the experimental panel, the typability rate was 91% and 370 distinct patterns were obtained. This subset of phages has been distributed for international trial.

Molecular typing of Staphylococcus aureus based on PCR restriction fragment length polymorphism and DNA sequence analysis of the coagulase gene

A typing procedure for Staphylococcus aureus was developed based on improved PCR amplification of the coagulase gene and restriction fragment length polymorphism (RFLP) analysis of the product. All coagulase-positive staphylococci produced a single PCR amplification product of either 875, 660, 603, or 547 bp. Those strains of epidemic methicillin-resistant S. aureus 16 (EMRSA-16) studied all gave a product of 547 bp. PCR products were digested with AluI and CfoI, and the fragments were separated by gel electrophoresis. Ten distinct RFLP patterns were found among 85 isolates of methicillin-resistant S. aureus (MRSA) and 10 propagating strains (PS) of methicillin-sensitive S. aureus (MSSA) examined. RFLP patterns 1, 2, and 3 were specific to strains of EMRSA-3, -15, and -16, respectively. By contrast, RFLP patterns 4 and 5 were seen with a heterogeneous collection of strains, together with drug-resistant forms of S. aureus isolated in Europe and four propagating strains used for the international phage set. RFLP pattern 6 was given by the Airedale isolate and PS 95. RFLP pattern 7 encompassed EMRSA-2 (isolate 331), PS 94, and PS 96. An isolate from Germany gave RFLP pattern 8. Eight strains of MSSA gave patterns similar to those of methicillin-resistant strains (RFLP patterns 3, 4, 5, 6, and 7), but two, PS 42E and PS 71, gave unique RFLP patterns 9 and 10, respectively. The coagulase gene PCR products for 24 isolates of MRSA and two isolates of MSSA were sequenced for both strands. The sequences were aligned, and evolutionary lineages were inferred based on pairwise distances between isolates.

Comparison and application of ribosome spacer DNA amplicon polymorphisms and pulsed-field gel electrophoresis for differentiation of methicillin-resistant Staphylococcus aureus strains

Analysis of sequences in the fragments of the 16S-23S rRNA intergenic spacer region by the ribosome spacer PCR (RS-PCR) can differentiate strains of methicillin-resistant Staphylococcus aureus (MRSA). We compared this technique with pulsed-field gel electrophoresis (PFGE) for typing MRSA strains and its application during an investigation of an outbreak. A total of 180 isolates of MRSA collected from various hospital laboratories within the United Kingdom and elsewhere were typed by PFGE and RS-PCR. PFGE identified 17 different types among the 180 strains examined, and RS-PCR generated 13 different types. PFGE could detect minor genetic variations among the isolates and could identify the variants which were not discriminated by RS-PCR. Four unique strain types detected by PFGE were not detected by RS-PCR. When applied to typing the outbreak-related strains from the vascular surgery unit at the General Infirmary at Leeds, the results of RS-PCR were identical to those of PFGE. Our results have shown that RS-PCR is a rapid, inexpensive technique that is highly reproducible and almost as discriminatory as PFGE for typing MRSA isolates and should be useful in the local investigation of MRSA outbreaks.

Epidemiological typing of coagulase-negative staphylococci from nosocomial infections

Biotyping, antibiograms, bacteriophage typing, plasmid profile analysis and SDS-PAGE protein profiles were used to determine the relatedness of 44 Staphylococcus epidermidis and four S. haemolyticus isolates from 14 patients. A selection of these were further characterised by ribotyping. Biotyping classified the isolates into three major groups but was considered a poor strain marker. Although antibiograms classified the S. epidermidis isolates into 20 groups, some changes in the susceptibility patterns of related isolates from a single patient were demonstrated. Bacteriophage typing was the least discriminatory of the methods used. SDS-PAGE gave highly related patterns for the majority of S. epidermidis isolates. Plasmid profile analysis and ribotyping, with a minimum of two restriction endonucleases, were the most discriminatory methods for typing S. epidermidis. Nonetheless, some isolates from the same patient - probably representing a single strain - varied in plasmid profile indicating plasmid instability. One of six related isolates from a single patient lacked two bands from the ribotyping pattern of the other isolates. Although no single method proved entirely satisfactory on all occasions, the combination of typing methods was sufficient to provide evidence of the relatedness of S. epidermidis isolates from individual patients.

Ribotyping of Staphylococcus aureus: an assessment using well-defined strains

Ribotyping, with homologous or heterologous (Escherichia coli) r-RNA, of the propagating strains for phages of the international set for strains of Staphylococcus aureus of human origin was undertaken to determine the discrimination of this typing method. Ribotyping could distinguish between strains of different phage groups, but could not distinguish between seven phage group III strains of different phage type. Ribotying may be a useful adjunct to phage typing in S. aureus but is unlikely to replace it as the primary method of epidemiological typing.

Some properties of coagulase-negative staphylococci isolated from cases of ovine mastitis

Of 41 coagulase-negative staphylococcal isolates from cases of ovine mastitis, 80% were speciated by the 'API-Staph SYSTEM' and 90% by a combination of biochemical tests. Staphylococcus simulans and Staph. xylosus were the two most prevalent species.

Characterization of a strain of methicillin-resistant Staphylococcus aureus (EMRSA-15) by conventional and molecular methods

A strain of methicillin-resistant Staphylococcus aureus (MRSA), EMRSA-15, was isolated in both the Midlands and south-east of England. This strain could be distinguished from another, very similar strain, found in the north of England, by both conventional and molecular typing. Conventional typing allowed distinction between the Midlands and southern variants of EMRSA-15, while molecular typing (pulse-field gel electrophoresis) allowed recognition of local variants in the south. In this investigation conventional and molecular typing methods were complementary.

Crystal structures of two cyclic pseudopentapeptides containing psi[CH2S] and psi[CH2SO] backbone surrogates

The solid state conformations of cyclo [Gly-Pro psi[CH2S] Gly-D-Phe-Pro] and cyclo [Gly-Pro psi[CH2-(S)-SO]Gly-D-Phe-Pro] have been characterized by X-ray diffraction analysis. Crystals of the sulfide trihydrate are orthorhombic, P2(1)2(1)2(1), with a = 10.156(3) A, b = 11.704(3) A, c = 21.913(4) A, and Z = 4. Crystals of the sulfoxide are monoclinic, P2(1) with a = 10.662(1) A, b = 8.552(3) A, c = 12.947(2) A, beta = 94.28(2), and Z = 2. Unlike their all-amide parent, which adopts an all-trans backbone conformation and a type II beta-turn encompassing Gly-Pro-Gly-D-Phe, both of these peptides contain a cis Gly1-Pro2 bond and form a novel turn structure, i.e., a type II' beta-turn consisting of Gly-D-Phe-Pro-Gly. The turn structure in each of these peptides is stabilized by an intramolecular H bond between the carbonyl oxygen of Gly1 and the amide proton of D-Phe4. In the cyclic sulfoxide, the sulfinyl group is not involved in H bonding despite its strong potential as a hydrogen-bond acceptor. The crystal structure made it possible to establish the absolute configuration of the sulfinyl group in this peptide. The two crystal structures also helped identify a type II' beta-turn in the DMSO-d6 solution conformers of these peptides.

Distribution of mec regulator genes in methicillin-resistant Staphylococcus clinical strains

The distributions of the mec regulator genes mecI and mecR1, which were identified on the chromosome of mecA-carrying Staphylococcus aureus N315, in methicillin-resistant staphylococci isolated in Japan and various countries were studied. Screening by dot blot hybridization by using polymerase chain reaction (PCR)-amplified probes revealed that at least the 5'-end region of the mecR1 gene was present in all strains tested, whereas about 40% of the strains were negative for the mecI gene. The data suggested that these regulator genes were the original components of the additional mec region DNA of methicillin-resistant S. aureus as well as methicillin-resistant, coagulase-negative staphylococci of seven staphylococcal species (S. epidermidis, S. haemolyticus, S. hominis, S. sciuri, S. capitis, S. caprae, and S. warneri). The mecI gene, which presumably codes for the repressor protein of the mecA gene, was found to harbor a point mutation in all six mecI-positive methicillin-resistant S. aureus strains, and their basal level of mecA gene transcription was elevated compared with that of strain N315, which harbors a presumably intact counterpart of the mecI gene. The data suggested that the mecI gene encodes for a strong repressor function on mecA gene transcription and is deleted or mutated in clinical methicillin-resistant S. aureus strains with high levels of resistance to methicillin.

Evaluation of methods for typing coagulase-negative staphylococci

One hundred and forty-two coagulase-negative staphylococci (CNS) isolated from dialysate effluent or skin of patients receiving continuous ambulatory peritoneal dialysis (CAPD) were typed by extended antibiogram (16 antibiotics) and biotype (26 reactions). These isolates were then typed by supplementary methods to determine the most suitable typing method for an epidemiological study of antibiotic resistance. These included phage typing, reverse phage typing, plasmid typing, whole-cell protein typing by SDS-PAGE with analysis by densitometry, and immunoblotting. The percentage of isolates typed successfully by the supplementary methods were: phage typing 20%, reverse phage typing 0%, plasmid typing 66%, SDS-PAGE 100%, immunoblotting 100%. The discrimination of each method was: phage typing 20%, plasmid typing 37%, SDS-PAGE 69%, immunoblotting 57%. Reproducibility was 88% for phage typing and 97% for plasmid typing. The reproducibility of the whole-cell protein typing was 83% if the same extracts were used but only 43% when separate protein extracts were analysed on separate occasions. However, strain relatedness was highly reproducible. The determination of an antibiogram-biotype profile was not a sufficiently accurate typing method for an epidemiological study of antibiotic resistance. Whole-cell protein typing by SDS-PAGE or immunoblotting was technically demanding but was the most effective of the supplementary methods for detecting erroneous discrimination and false matching produced by antibiogram-biotype combinations.

Beta-lactamase-negative, methicillin-resistant Staphylococcus aureus in a newborn nursery: report of an outbreak and laboratory investigations

An outbreak of skin infection caused by a beta-lactamase-negative strain of methicillin-resistant Staphylococcus aureus (MRSA) occurred during a five-week period in a newborn nursery. Twelve babies, two mothers and two members of staff were involved. One baby had a diagnosis of staphylococcal scalded skin syndrome and two others required treatment with antibiotics. The infecting strain produced exfoliative toxin A. It was thought that it had been introduced from a different maternity unit by a nasal carrier. Laboratory investigations tended to support this hypothesis.

Complex typing of methicillin-resistant Staphylococcus aureus (MRSA)

To discriminate between methicillin-resistant Staphylococcus aureus from 5 nosocomial outbreaks and from sporadic nosocomial infections, the efficacy of a complex typing scheme by phage typing, biochemical typing, resistance phenotype, plasmid profiles, plasmid patterns and attribution of resistance determinants to the chromosome was studied. In addition to the International Basic Set and experimental phages 88-93, 10 experimental phages from the Public Health Laboratory Service, Colindale, London, were used for phage-typing. The 10 experimental phages from PHLS in particular, in combination with plasmid profiles and plasmid patterns, were of special discriminative value.

Evaluation of electrophoretic methods for typing methicillin-resistant Staphylococcus aureus

Three electrophoretic methods of typing methicillin-resistant Staphylococcus aureus (MRSA) strains--plasmid profiles (PP), whole-cell protein profiles (WCPP) and immunoblotting profiles (IP)--were evaluated and compared with phage typing. The results obtained with isolates from 12 outbreaks were compared both within the outbreaks, to determine the consistency of results, and between outbreaks. There was generally good agreement between the typing methods but in only six outbreaks did all four methods indicate the same relationship between isolates. WCPP comprised more than 50 bands; when differences occurred, they were seen in only a few bands. In contrast, IP comprised only one or two major bands and the differences were much easier to interpret. The PPs of many of the isolates were similar; many isolates contained a plasmid of mol. wt (18-25) x 10(6). In several outbreaks both WCPP and IP showed minor differences between isolates that were not apparent with phage typing. When comparisons were made between the 12 index strains and an isolate representing the London epidemic MRSA strain, phage typing and WCPP were the most discriminatory methods; both gave nine distinct patterns, whereas there were eight IPs and only six PPs amongst the 13 strains. It was concluded that both WCPP and IP could provide valuable epidemiological data on MRSA and that IP was the easiest of the three methods to interpret.