Identification of four families of peptidoglycan lytic transglycosylases

The lytic transglycosylases are a class of autolysins which cleave the bacterial cell wall heteropolymer peptidoglycan (murein) to facilitate its biosynthesis and turnover. A search of the National Center for Biotechnology Information (NCBI) databases using the primary sequences of the six characterized lytic transglycosylases of Escherichia coli, a membrane-bound form of the enzyme from Pseudomonas aeruginosa, and the endolysins of lambda bacteriophage permitted the identification of a total of 127 known and hypothetical enzymes from a wide variety of bacteria and bacteriophage. These amino acid sequences have been arranged into four families based on alignments, and consensus motifs have been identified. Family 1 represents a superfamily comprising 86 sequences which are subdivided into five (1A--1E) subfamilies.

Identification of residues involved in catalytic activity of the inverting glycosyl transferase WbbE from Salmonella enterica serovar borreze

Synthesis of the O:54 O antigen of Salmonella enterica is initiated by the nonprocessive glycosyl transferase WbbE, assigned to family 2 of the glycosyl transferase enzymes (GT2). GT2 enzymes possess a characteristic N-terminal domain, domain A. Based on structural data from the GT2 representative SpsA (S. J. Charnock and G. J. Davies, Biochemistry 38:6380-6385, 1999), this domain is responsible for nucleotide binding. It possesses two invariant Asp residues, the first forming a hydrogen bond to uracil and the second coordinating a Mn(2+) ion. Site-directed replacement of Asp41 (D41A) of WbbE, the analogue of the first Asp residue of SpsA, revealed that this is not required for activity. WbbE possesses three Asp residues near the position analogous to the second conserved residue. Whereas D95A reduced WbbE activity, activity in D93A and D96A mutants was abrogated, suggesting that either D93 or D96 may coordinate the Mn(2+) ion. Our studies also identified a C-terminal region of sequence conservation in 22 GT2 members, including WbbE. SpsA was not among these. This region is characterized by an ED(Y) motif. The Glu and Asp residues of this motif were individually replaced in WbbE. E180D in WbbE had greatly reduced activity, and an E180Q replacement completely abrogated activity; however, D181E had no effect. E180 is predicted to reside on a turn. Combined with the alignment of the motif with potential catalytic residues in the GT2 enzymes ExoM and SpsA, we speculate that E180 is the catalytic residue of WbbE. Sequence and predicted structural divergence in the catalytic region of GT2 members suggests that this is not a homogeneous family.

Assay for lytic transglycosylases: a family of peptidoglycan lyases

An assay has been developed to monitor the activity of the lytic transglycosylases which does not involve the use of radiolabel. Samples of lytic transglycosylase were incubated with isolated and purified insoluble peptidoglycan as substrate for varying lengths of time. Residual insoluble material was removed by ultracentrifugation in a microfuge and the solubilized components were treated with sodium borohydride prior to acid hydrolysis. The optimal conditions for this acid hydrolysis were established to be incubation at 96 degrees C for 1 h in 6 M HCl, in vacuo. The hydrolyzed samples were subjected to amino acid/sugar analysis by cation-exchange chromatography on a Beckman System Gold amino acid analyzer. To effect a clear resolution of muramic acid from serine and glutamic acid, the equilibration buffer was modified to be composed of 33 mM sodium citrate, pH 3.12. The product of the lyase reaction of the lytic transglycosylases are 1,6-anhydromuramyl residues, which are not reduced by the sodium borohydride treatment. On the other hand, the muramyl residues arising at the reducing ends of peptidoglycan after treatment with muramidases (hydrolyases) are reduced to muramitol residues, which elute from the amino acid analyzer prior to aspartic acid. This assay thus distinguishes the activity of the two enzymes and was applied to determine the initial activities of increasing concentrations of a soluble derivative of lytic transglycosylase B from the opportunistic pathogen Pseudomonas aeruginosa.

Characterization of function and activity of domains A, B and C of xylanase C from Fibrobacter succinogenes S85

Xylanase C from the ruminant bacterium Fibrobacter succinogenes is comprised of two catalytic domains, A and B, and a third domain, C, of unknown function. The DNA coding for domains A and B of xylanase C were separately cloned and expressed in Escherichia coli as fusion proteins with glutathione-S:-transferase. The fusion proteins were isolated by affinity chromatography on glutathione-Sepharose 4B, cleaved with thrombin and the released xylanase C catalytic domains A and B were purified to apparent homogeneity by anion-exchange chromatography on Mono Q. Electrospray mass spectrometry provided a molecular mass of 27 818 Da (expected, 27 820 Da) for domain B. The pH and temperature optima for activity of domain B on oat spelt xylan were 5.0 and 52 degrees C, respectively. A kinetic analysis of the activity of the catalytic domain A on oat spelt xylan, birch wood xylan and xylooligomers at pH 6.5 and 37 degrees C provided data significantly different to those obtained previously with a protease-derived form of the enzyme [Zhu et al. (1994) J. Bacteriol. 176, 3885-3894]. The isolated domain A was more active on barley-glucan than the protease-derived form and its affinity for birch wood xylan was enhanced resulting in greater overall catalytic efficiency as reflected by k(cat)/K:(M) values. Likewise, significant differences in the Michaelis-Menten parameters K:(M), k(cat) and k(cat)/K:(M) were obtained with domain B compared with values previously reported with this domain attached to domain C. In general, the presence of domain C appeared to decrease the overall efficiency of domain B 7- and 36-fold with birch wood xylan and xylopentaose as substrates, respectively, as reflected by values of k(cat)/K:(M). The removal of domain C also affected the mode of action of domain B such that it more closely resembled that of catalytic domain A. However, no change in either pH and temperature optima or stability were found with domain B compared with the combined domains B and C. The function of domain C remains unknown, but hydrophobic cluster analysis indicated that it may belong to a class of dockerin domains involved in the protein-protein interactions of cellulolytic and xylanolytic complexes.

The role of tryptophan residues in substrate binding to catalytic domains A and B of xylanase C from Fibrobacter succinogenes S85

Oxidation of the isolated catalytic domain B of xylanase C (XynC-B) from Fibrobacter succinogenes with N-bromosuccinimide (NBS) resulted in the modification of five of the seven Trp residues present in the enzyme. Hydrolytic activity of the enzyme was rapidly lost upon initiation of oxidation as a molar ratio of about two NBS molecules per molar equivalent of protein was sufficient to cause 50% inhibition of enzyme activity, and the addition of five molar equivalents of NBS resulted in less than 10% activity. Pre-incubation of XynC-B with the competitive inhibitor D-xylose resulted in the apparent protection of two Trp residues from oxidation. Xylose protection of the enzyme also resulted in a maintenance of activity, with 60% activity still evident after addition of 8-9 molar equivalents of NBS. This protection from inactivation was enhanced by the inclusion of xylohexaose in reaction mixtures. Under these conditions, however, a further Trp residue was protected from NBS oxidation. The three protected Trp residues were identified as Trp135, Trp161 and Trp202 by differential labelling and peptide mapping of NBS-oxidized preparations of the xylanase employing a combination of electrospray mass spectroscopic analysis and N-terminal sequencing. By analogy to the known structures of the family 11 xylanases, the fully conserved Trp202 residue is located on the only alpha-helix present in the enzymes, at the interface between it and the back of the beta-sheet which forms the active site cleft. Trp135 represents a highly conserved aromatic residue in family 11, but it is replaced with Thr in domain A of F. succinogenes xylanase C. To investigate the role of Trp135 in conferring the different activity profile of domain B relative to domain A, the Trp135Thr and Trp135Ala derivatives of domain B were prepared by site-directed mutagenesis. However, the kinetic parameters of the two domain B derivatives were not significantly different compared to the wild-type enzyme as reflected by K(M) and k(cat) values and product distribution profiles. Similar results were obtained with the Trp161Ala derivative of domain B, indicating that these two residues do not directly participate in the binding of substrate but likely form the foundation for binding subsite 2.

The role of the nonconserved residues at position 167 of class A beta-lactamases in susceptibility to mechanism-based inhibitors

Differences in specificities between the class A beta-lactamases for both substrate and inhibitors are known. The role of the nonconserved amino acid residue at position 167 of the class A enzyme, which forms a cis bond with the catalytically essential Glu-166 residue, in both the hydrolysis of beta-lactam substrates and inactivation by mechanism-based inhibitors, was investigated. Site-directed mutagenesis was performed on the penPC gene encoding the Bacillus cereus 569/H beta-lactamase I to replace thr-167 with the corresponding Staphylococcus aureus PC1 residue Ile. Kinetic data obtained from the purified Thr-167-Ile B. cereus 569/H beta-lactamase was compared to that obtained from the wild-type B. cereus and S. aureus enzymes and indicated that the replacement had little effect on the Michaelis parameters for the hydrolysis of S- and A-type penicillins. However, the Thr-167-Ile enzymes became more S. aureus PC1-like in its response to the mechanism-based inhibitors clavulanic acid and 6-beta-(trifluoromethane sulfonyl)amidopenicillanic acid sulfone. A model for the role of this nonconserved residue at position 167 in the mechanism of inactivation by mechanism-based inhibitors is proposed.

The role of O-acetylation in the metabolism of peptidoglycan in Providencia stuartii

The gentamicin 2'-N-acetyltransferase [EC 2.3.1.59; AAC(2')-Ia] of Providencia stuartii was shown to contribute to the O-acetylation of peptidoglycan and mutants that either under- or overexpress the aac(2')-Ia gene was characterized phenotypically to possess either lower or higher levels of peptidoglycan O-acetylation, respectively, compared to the wild-type. These mutants were subjected to scanning electron microscopy. P. stuartii PR100, with 42-44% peptidoglycan O-acetylation compared to 54% for the wild-type, appeared as irregular rods. In direct contrast, strains PR50.LM3 and PR51, with increased levels of peptidoglycan O-acetylation (63 and 65%, respectively), appeared as coccobacilli or chain formers, respectively. Zymogram analysis of the autolysins produced by another member of the closely related Proteeae group of bacteria, Proteus mirabilis, indicated the presence of three classes of enzymes: one that acts preferentially on native, O-acetylated peptidoglycan, a second that hydrolyses non-O-acetylated peptidoglycan, and a third that is not distinguished by the two forms of substrate. On the basis of the apparent morphological changes directly related to levels of O-acetylation combined with the presence of different classes of autolysins, a model is proposed that invokes the role of this modification in the control of autolysins for the maintenance of the structure of the peptidoglycan sacculus.

The role of the non-conserved residue at position 104 of class A beta-lactamases in susceptibility to mechanism-based inhibitors

The role of the non-conserved amino acid residue at position 104 of the class A beta-lactamases, which comprises a highly conserved sequence of amino acids at the active sites of these enzymes, in both the hydrolysis of beta-lactam substrates and inactivation by mechanism-based inhibitors was investigated. Site-directed mutagenesis was performed on the penPC gene encoding the Bacillus cereus 569/H beta-lactamase I to replace Asp104 with the corresponding Staphylococcus aureus PC1 residue Ala104. Kinetic data obtained with the purified Asp104Ala B. cereus 569/H beta-lactamase I was compared to that obtained from the wild-type B. cereus and S. aureus enzymes. Replacement of amino acid residue 104 had little effect on the Michaelis parameters for the hydrolysis of both S- and A-type penicillins. Relative to wild-type enzyme, the Asp104Ala beta-lactamase I had 2-fold higher Km values for benzylpenicillin and methicillin, but negligible difference in Km for ampicillin and oxacillin. However, kcat values were also slightly increased resulting in little change in catalytic efficiency, kcat/Km. In contrast, the Asp104Ala beta-lactamase I became more like the S. aureus enzyme in its response to the mechanism-based inhibitors clavulanic acid and 6-beta-(trifluoromethane sulfonyl)amido-penicillanic acid sulfone with respect to both response to the inhibitors and subsequent enzymatic properties. Based on the known three-dimensional structures of the Bacillus licheniformis 749/C, Escherichia coli TEM and S. aureus PC1 beta-lactamases, a model for the role of the non-conserved residue at position 104 in the process of inactivation by mechanism-based inhibitors is proposed.

A phantom for quantitative ultrasound of trabecular bone

The propagation mechanisms of ultrasound in trabecular bone are poorly understood and have been the subject of extended debate; also, the reproducibility of ultrasonic measurements on bone in vivo using commercial ultrasound heel-scanning devices is such that the interpretation of the obtained data is difficult. In this paper we describe recent developments in the production of a bone-mimicking material which is well suited to the task of routine monitoring of commercial ultrasound bone scanners. The material, based on a standard epoxy resin is fabricated to a pre-determined porosity value by the inclusion of a marrow-mimicking material thereby introducing a known and controlled mean pore size. Measurements of the velocity and attenuation of the material have been performed over a range of porosity values from 10% to 80% in the frequency range 500-900 kHz; also, broadband ultrasonic attenuation (BUA) values have been obtained from commercial equipment. The material displays velocities in the range 1844-3118 m s(-1) and attenuation ranging from 7.0 to 17.7 dB cm(-1) at 500 kHz.

Gram-negative bacteria produce membrane vesicles which are capable of killing other bacteria

Naturally produced membrane vesicles (MVs), isolated from 15 strains of gram-negative bacteria (Citrobacter, Enterobacter, Escherichia, Klebsiella, Morganella, Proteus, Salmonella, and Shigella strains), lysed many gram-positive (including Mycobacterium) and gram-negative cultures. Peptidoglycan zymograms suggested that MVs contained peptidoglycan hydrolases, and electron microscopy revealed that the murein sacculi were digested, confirming a previous modus operandi (J. L. Kadurugamuwa and T. J. Beveridge, J. Bacteriol. 174:2767-2774, 1996). MV-sensitive bacteria possessed A1alpha, A4alpha, A1gamma, A2alpha, and A4gamma peptidoglycan chemotypes, whereas A3alpha, A3beta, A3gamma, A4beta, B1alpha, and B1beta chemotypes were not affected. Pseudomonas aeruginosa PAO1 vesicles possessed the most lytic activity.

Restoration of catalytic activity beyond wild-type level in glucoamylase from Aspergillus awamori by oxidation of the Glu400-->Cys catalytic-base mutant to cysteinesulfinic acid

Glucoamylase catalyzes the hydrolysis of glucosidic bonds with inversion of the anomeric configuration. Site-directed mutagenesis and three-dimensional structure determination of the glucoamylase from Aspergillus awamori previously identified Glu179 and Glu400 as the general acid and base catalyst, respectively. The average distance between the two carboxyl groups was measured to be 9.2 A, which is typical for inverting glycosyl hydrolases. In the present study, this distance was increased by replacing the catalytic base Glu400 with cysteine which was then oxidized to cysteinesulfinic acid. Initially, this oxidation occurred during attempts to carboxyalkylate the Cys400 residue with iodoacetic acid, 3-iodopropionic acid, or 4-bromobutyric acid. However, endoproteinase Lys-C digestion of modified glucoamylase followed by high-pressure liquid chromatography in combination with matrix-assisted laser desorption ionization/time-of-flight mass spectrometry on purified peptide fragments demonstrated that all enzyme derivatives contained the cysteinesulfinic acid oxidation product of Cys400. Subsequently, it was demonstrated that treatment of Glu400-->Cys glucoamylase with potassium iodide in the presence of bromine resulted in complete conversion to the cysteinesulfinic acid product. As expected, the catalytic base mutant Glu400-->Cys glucoamylase had very low activity, i.e., 0.2% compared to wild-type. The oxidation of Cys400 to cysteinesulfinic acid, however, restored activity (kcat) on alpha-1,4-linked substrates to levels up to 160% of the wild-type glucoamylase which corresponded to approximately a 700-fold increase in the kcat of the Glu400-->Cys mutant glucoamylase. Whereas Glu400-->Cys glucoamylase was much less thermostable and more sensitive to guanidinium chloride than the wild-type enzyme, the oxidation to cysteinesulfinic acid was accompanied by partial recovery of the stability.

Enzymatic properties of the cysteinesulfinic acid derivative of the catalytic-base mutant Glu400-->Cys of glucoamylase from Aspergillus awamori

The pKa of the catalytic base was lowered and its distance to the general acid catalyst, Glu179, was increased in the glucoamylase from Aspergillus awamori by replacing the catalytic base Glu400 with cysteine followed by oxidation to cysteinesulfinic acid [Fierobe, H.-P., Mirgorodskaya, E., McGuire, K. A., Roepstorff, P., Svensson, B. and Clarke, A. J. (1998) Biochemistry 37, 3743-3752. 1H NMR spectroscopy demonstrated that the oxidized mutant Glu400-->Cys-SO2H glucoamylase, like the wild-type, catalyzed hydrolysis with inversion of the anomeric configuration of the product. Relative to the catalytic base mutant Glu400-->Cys, the Cys400-SO2H glucoamylase had 700 times higher kcat toward maltose, while K(m) was unchanged. Compared to wild-type glucoamylase, the Cys400-SO2H derivative had kcat values of 150-190% and 85-320% on malto- and isomaltooligosaccharides, respectively, while K(m) values were similar to those of wild-type with the two disaccharides and 3.5-5.5- and 1.8-2.5-fold higher for the longer malto- and isomaltooligosaccharides substrates, respectively. The pH-activity dependence at saturating concentration of maltose indicated that the pKa of the catalytic base Cys400-SO2H was about 0.5 pH unit lower than that of wild-type Glu400. The Ki of Cys400-SO2H glucoamylase for the pseudotetrasaccharide and potent inhibitor acarbose increased more than 10(4)-fold, but Ki values of the mono- and disaccharide analogues 1-deoxynojirimycin and beta-O-methylacarviosinide were unchanged, suggesting perturbation at binding subsites beyond the catalytic center. A distinct property of Cys400-SO2H glucoamylase was the catalysis of the condensation of beta-D-glucopyranosyl fluoride and subsequent hydrolysis of the product to beta-glucose, under conditions where this was not detected for the wild-type enzyme.

Isolation, purification, and characterization of the major autolysin from Pseudomonas aeruginosa

The major (26 kDa) autolysin from Pseudomonas aeruginosa was purified to apparent homogeneity by a combination of preparative electrophoresis, ion-exchange, and dye-ligand chromatographies. This purification was facilitated by the development of a spot-assay that involved the spotting and subsequent incubation of autolysin samples on polyacrylamide gels containing peptidoglycan. The pl of the 26-kDa autolysin was determined to be between 3.5 and 4 and disulfide bonds within the enzyme were essential for activity. The autolysin catalyzed the release of reducing sugars from the peptidoglycans of Pseudomonas aeruginosa and Escherichia coli indicating it to be a beta-glycosidase. It was ineffective at hydrolysing the peptidoglycan from Gram-positive bacteria and the O-acetylated peptidoglycans from either Proteus mirabilis or Staphylococcus aureus. The N-terminal sequence of the purified autolysin was determined to be His-Glu-Pro-Pro-Gly. The 26-kDa autolysin together with a 29-kDa autolysin was determined to be secreted into the medium by a mechanism that involves the production and release of surface membrane vesicles during normal growth, but the enzymes were not found free and active in culture broth supernatants.

Identification of the catalytic nucleophile in the cellulase from Schizophyllum commune and assignment of the enzyme to Family 5, subtype 5 of the glycosidases

Differential chemical modification of the cellulase from Schizophyllum commune with [N-methyl-3H]1-ethyl-3(4-azonia-4,4-dimethylpentyl)-carbodiimide in the presence and absence of substrate identified an active site glutamate residue within the peptide: Leu-Gln-Ala-Ala-Thr-Glu-Trp-Leu-(Lys). This Glu residue is proposed to participate in binding of substrate as amino acid sequence homology studies combined with mechanism-based inhibition of the cellulase with 4',5'-epoxypentyl-beta-D-cellobioside identified a neighboring Glu residue, which conforms to the Glu-X-Gly motif of Family 5 glycosidases, as the catalytic nucleophile. These data allow the assignment of the S. commune cellulase to Family 5, subtype 5 of the glycosidases.

Bacterial genetic loci implicated in the Pseudomonas putida GR12-2R3--canola mutualism: identification of an exudate-inducible sugar transporter

Pseudomonas putida GR12-2R3 promotes the emergence and growth of diverse plant species. Analyses of TnphoA insertion mutations are revealing bacterial characteristics pertinent to the plant-microbe interaction. Pseudomonas putida PG269 is a TnphoA insertion derivative of GR12-2R3 that expresses canola seed exudate-inducible alkaline phosphatase (PhoA) activity. It promoted the growth of canola roots, as well as strain GR12-2R3, and outgrew its parent when they were cocultured in the presence of canola roots or in liquid seed exudate medium. (In contrast, mutant PG126 failed to promote canola root growth and was outgrown by its parent strain.) The PhoA activity of strain PG269 was induced by glucosamine and other sugars; glucosamine inhibited the growth of strain GR12-2R3 and stimulated the growth of strain PG269. Strain PG269 contained two TnphoA insertions: seiA1::TnphoA and seiB1::TnphoA. Strain PG312, which contained only insertion seiA1::TnphoA, shared all aspects of the PG269 phenotype, except the ability to outcompete strain GR12-2R3 during coculture. Insertion seiA1::TnphoA interrupted an open reading frame related in sequence to members of the MalF family of sugar transporter subunits. The PhoA-inducing fraction of canola seed exudate was hydrophilic, low in molecular weight, and heat stable. It cochromatographed with basic amino acids and amino sugars, and was inactivated by strains GR12-2R3 and PG269. Gene seiA may encode a subunit of an ABC transporter with broad specificity for glucose and related sugars whose expression can be induced by exudate sugars.

Characterization of gentamicin 2'-N-acetyltransferase from Providencia stuartii: its use of peptidoglycan metabolites for acetylation of both aminoglycosides and peptidoglycan

The relationship between the acetylation of peptidoglycan and that of aminoglycosides in Providencia stuartii has been investigated both in vivo and in vitro. Adaptation of the assay for peptidoglycan N-->O-acetyltransferase permitted an investigation of the use of peptidoglycan as a source of acetate for the N acetylation of aminoglycosides by gentamicin N-acetyltransferase [EC 2.3.1.59; AAC(2')]. The peptidoglycan from cells of P. stuartii PR50 was prelabelled with 3H by growth in the presence of N-[acetyl-3H]glucosamine. Under these conditions, [3H]acetate was confirmed to be transferred to the C-6 position of peptidoglycan-bound N-acetylmuramyl residues. Isolated cells were subsequently incubated in the presence of various concentrations of gentamicin and tobramycin (0 to 5x MIC). Analysis of various cellular fractions from isolated cells and spent culture medium by the aminoglycoside-binding phosphocellulose paper assay revealed increasing levels of radioactivity associated with the filters used for whole-cell sonicates of cells treated with gentamicin up to 2 x MIC. Beyond this concentration, a decrease in radioactivity was observed, consistent with the onset of cell lysis. Similar results were obtained with tobramycin, but the increasing trend was less obvious. The transfer of radiolabel to either aminoglycoside was not observed with P. stuartii PR100, a strain that is devoid of AAC(2')-Ia. A high-performance anion-exchange chromatography-based method was established to further characterize the AAC(2')-Ia-catalyzed acetylation of aminoglycosides. The high-performance liquid chromatography (HPLC)-based method resolved a tobramycin preparation into two peaks, both of which were collected and confirmed by 1H nuclear magnetic resonance to be the antibiotic. Authentic standards of 2'-N-acetyltobramycin were prepared and were well separated from the parent antibiotic when subjected to the HPLC analysis. By applying this technique, the transfer of radiolabelled acetate from the cell wall polymer peptidoglycan to tobramycin was confirmed. In addition, isolated and purified AAC(2')-Ia was shown to catalyze in vitro the transfer of acetate from acetyl-coenzyme A, soluble fragments of peptidoglycan, and N-acetylglucosamine to tobramycin. These data further support the proposal that AAC(2')-Ia from P. stuartii may have a physiological role in its secondary metabolism and that its activity on aminoglycosides is simply fortuitous.

The influence of porosity and pore size on the ultrasonic properties of bone investigated using a phantom material

Ultrasonic propagation in bone has been investigated using the Leeds Ultrasonic Bone Phantom Material. Phantoms were produced with different porosities in the range of 45-83% and pore sizes of 1.3 and 0.6 mm. The phase velocity at 600 kHz was found to follow a second-order polynomial as a function of porosity. Phase velocity values between 1545 and 2211 m s-1 were measured and found to be largely independent of pore size for a given porosity. The slope of the phase velocity as a function of frequency (dispersion) decreases with increasing porosity. The values obtained from samples having different pore sizes were also similar. The attenuation coefficient and normalized broadband ultrasonic attenuation (nBUA) reached a maximum at about 50%. The normalized attenuation ranged from 6 to 25 dB cm-1 over the porosity range available and consistently showed higher values for the larger pore size. Similarly, the nBUA values were found to be between 14 and 53 dB MHz-1 cm-1, with the values for the larger pore size being roughly 10 dB MHz-1 cm-1 greater than those for the smaller pore size. These findings demonstrate that the Leeds phantom can be used to investigate the effect of structural changes in bone and to aid the understanding of quantitative ultrasound. The results support the assumption that the velocity in trabecular bone is not dependent on pore size but is influenced by the mechanical properties of the bone's constituents and the overall framework, whereas the attenuation and BUA are also influenced by structure.

The prevalence of gentamicin 2'-N-acetyltransferase in the Proteeae and its role in the O-acetylation of peptidoglycan

The prevalence of aac(2')-Ia, a gene coding for gentamicin 2'-N-acetyltransferase in Providencia stuartii, among species of the Proteeae was investigated to determine if it is a common resistance factor and whether the correlation observed in P. stuartii between its expression and the levels of peptidoglycan O-acetylation represents a general feature of bacteria producing this form of modified peptidoglycan. An evaluation of the MICs of gentamicin for each of the species of the Proteeae did not reveal any apparent relationship between resistance and the degree of O-acetylation of peptidoglycan. The entire aac(2')-Ia gene was used as a probe in Southern hybridization experiments against genomic DNA from each species of the Proteeae. A sequence with strong homology to aac(2')-Ia was present only in Proteus penneri while weak hybridization was also observed to the restriction digested DNA from Providencia rettgeri. Other bacteria that O-acetylate peptidoglycan were also screened with this probe and a homologous DNA sequence was only found in Neisseria subflava. These data suggest that AAC(2')-Ia may contribute to the O-acetylation of peptidoglycan in P. stuartii, but a more specific enzyme must also be produced for this function.

Value of intraoperative image intensifier prints in trauma surgery

We have studied the use of image intensification in a trauma theatre over a period of 6 months with particular reference to the acquisition of intraoperative image intensifier thermal prints instead of formal radiographs. The quality of the prints and the savings generated have been assessed. During the study period, 476 patients underwent orthopaedic trauma procedures. The image intensifier was used for intraoperative screening in 280 patients. In 278 of these a thermal print was obtained. This was used, instead of formal check radiographs, to plan further management in 210 patients (75%). In 68 patients, the printout was insufficient in its coverage of the operated area, and a check radiograph was also obtained. In no case did the clarity of the thermal image hinder accurate interpretation. We believe that thermal images are a useful substitute for formal postoperative radiographs in many trauma cases, and that, with notable exceptions, their use could decrease costs, reduce patient discomfort and radiation dose and spare overloaded radiology services.

A major autolysin of Pseudomonas aeruginosa: subcellular distribution, potential role in cell growth and division and secretion in surface membrane vesicles

A 26-kDa murein hydrolase is the major autolysin of Pseudomonas aeruginosa PAO1, and its expression can be correlated with the growth and division of cells in both batch and synchronously growing cultures. In batch cultures, it is detected primarily during the mid-exponential growth phase, and in synchronous cultures, it is detected primarily during the cell elongation and division phases. Immunogold labeling of thin sections of P. aeruginosa using antibodies raised against the 26-kDa autolysin revealed that it is associated mainly with the cell envelope and in particular within the periplasm. It is also tightly bound to the peptidoglycan layer, since murein sacculi, isolated by boiling 4% sodium dodecyl sulfate treatment, could also be immunogold labeled. Since division is due to cell constriction in this P. aeruginosa strain (septa are rarely seen), we cannot comment on the autolysin's contribution to septation, although constriction sites were always heavily labeled. Some labeling was also found in the cytoplasm, and this was thought to be due to the de novo synthesis of the enzyme before translocation to the periplasm. Interestingly, the autolysin was also found to be associated with natural membrane vesicles which blebbed from the surface during cell growth; the enzyme is therefore part of the complex makeup of these membrane packages of secreted materials (J. L. Kadurugamuwa and T. J. Beveridge, J. Bacteriol. 177:3998-4008, 1995). The expression of these membrane vesicles was correlated with the expression of B-band lipopolysaccharide.

The measurement of the velocity of ultrasound in fixed trabecular bone using broadband pulses and single-frequency tone bursts

The velocity of ultrasound in a series of 10 fixed os calces was measured using both short pulses and 750 kHz tonebursts. The values obtained from the pulse measurements differed from the toneburst values by up to 16% depending on the selection of the zero-crossing point used as a reference in the pulse measurements. It is demonstrated that the discrepancy between the values is itself a function of the frequency-dependent attenuation in the propagating medium and this is confirmed by theoretical modelling. The toneburst results are also compared with measurements using a cross-correlation technique, and these show a close agreement.

Ultrasonic measurement: an evaluation of three heel bone scanners compared with a bench-top system

Three commercial ultrasound bone scanners designed for os calcis measurements (Lunar Achilles, C.U.B.A. "Research" and UBA 575) were compared using the Leeds Ultrasonic Bone Phantoms. The porosity of the phantoms ranged from 50% to 83% with velocities between 1490 and 1621 m s-1 and broadband ultrasound attenuation (BUA) values in the range 46-115 dB MHz-1. The three devices tested were able to discriminate porosity differences of at least 3%, although the values obtained for the propagation parameters varied widely. Velocity differences of up to 38 m s-1 and BUA variations of up to 33 dB MHz-1 were found, although a relationship was identified between the velocity and BUA measurements. In some cases, the variation can be attributed to differences in the measurement technique adopted, although there also seem to be detailed differences in the definition of the parameters themselves. The variation between different devices from the same manufacturer (Lunar) was also studied. Measurements taken from five devices showed variation in velocity values of up to 25 m s-1 (SD 10.8 m s-1) and in BUA values of up to 11 dB MHz-1 (SD 4.3 dB MHz-1). We conclude that the variation both between manufacturers and between nominally identical machines may be of clinical significance. Both users and manufacturers need to consider urgently the introduction of quality standards and consensus definition of terms and techniques. The fact that all machines studied have been superseded commercially does not invalidate these conclusions, since many of the devices tested remain in clinical use and there is no evidence of fundamental change in manufacturers' procedures.

Contribution of gentamicin 2'-N-acetyltransferase to the O acetylation of peptidoglycan in Providencia stuartii

A collection of Providencia stuartii mutants which either underexpress or overexpress aac(2')-Ia, the chromosomal gene coding for gentamicin 2'-N-acetyltransferase (EC 2.3.1.59), have been characterized phenotypically as possessing either lower or higher levels of peptidoglycan O acetylation, respectively, than the wild type. These mutants were subjected to both negative-staining and thin-section electron microscopy. P. stuartii PR100, with 42% O acetylation of peptidoglycan compared with 52% O acetylation in the wild type, appeared as irregular rods. In direct contrast, P. stuartii strains PR50.LM3 and PR51, with increased levels of peptidoglycan O acetylation (65 and 63%, respectively), appeared as coccobacilli and chain formers, respectively. Membrane blebbing was also observed with the chain-forming strain PR51. Thin sectioning of this mutant indicated that it was capable of proper constriction and separation. P. stuartii PM1, when grown to mid-exponential phase, did not have altered peptidoglycan O-acetylation levels, and cellular morphology remained similar to that of wild-type strains. However, continued growth into stationary phase resulted in a 15% increase in peptidoglycan O acetylation concomitant with a change of some cells from a rod-shaped to a coccobacillus-shaped morphology. The fact that these apparent morphological changes were directly related to levels of O acetylation support the view that this modification plays a role in the maintenance of peptidoglycan structure, presumably through the control of autolytic activity.