Methyl-alpha-maltoside and 5-thiomaltose: analogs transported by the Escherichia coli maltose transport system

Specific Imaging of Bacterial Infection Using 6″-18F-Fluoromaltotriose: A Second-Generation PET Tracer Targeting the Maltodextrin Transporter in Bacteria

6″-¹⁸F-fluoromaltotriose is a PET tracer that can potentially be used to image and localize most bacterial infections, much like ¹⁸F-FDG has been used to image and localize most cancers. However, unlike ¹⁸F-FDG, 6″-¹⁸F-fluoromaltotriose is not taken up by inflammatory lesions and appears to be specific to bacterial infections by targeting the maltodextrin transporter that is expressed in gram-positive and gram-negative strains of bacteria. Methods: 6″-¹⁸F-fluoromaltotriose was synthesized with high radiochemical purity and evaluated in several clinically relevant bacterial strains in cultures and in living mice. Results: 6″-¹⁸F-fluoromaltotriose was taken up in both gram-positive and gram-negative bacterial strains. 6″-¹⁸F-fluoromaltotriose was also able to detect Pseudomonas aeruginosa in a clinically relevant mouse model of wound infection. The utility of 6″-¹⁸F-fluoromaltotriose to help monitor antibiotic therapies was also evaluated in rats. Conclusion: 6″-¹⁸F-fluoromaltotriose is a promising new tracer that has significant diagnostic utility, with the potential to change the clinical management of patients with infectious diseases of bacterial origin.

A Molecular Genetic Basis Explaining Altered Bacterial Behavior in Space

Bacteria behave differently in space, as indicated by reports of reduced lag phase, higher final cell counts, enhanced biofilm formation, increased virulence, and reduced susceptibility to antibiotics. These phenomena are theorized, at least in part, to result from reduced mass transport in the local extracellular environment, where movement of molecules consumed and excreted by the cell is limited to diffusion in the absence of gravity-dependent convection. However, to date neither empirical nor computational approaches have been able to provide sufficient evidence to confirm this explanation. Molecular genetic analysis findings, conducted as part of a recent spaceflight investigation, support the proposed model. This investigation indicated an overexpression of genes associated with starvation, the search for alternative energy sources, increased metabolism, enhanced acetate production, and other systematic responses to acidity-all of which can be associated with reduced extracellular mass transport.

Proteomics analysis of a long-term survival strain of Escherichia coli K-12 exhibiting a growth advantage in stationary-phase (GASP) phenotype

The aim of this work was the functional and proteomic analysis of a mutant, W3110 Bgl(+) /10, isolated from a batch culture of an Escherichia coli K-12 strain maintained at room temperature without addition of nutrients for 10 years. When the mutant was evaluated in competition experiments in co-culture with the wild-type, it exhibited the growth advantage in stationary phase (GASP) phenotype. Proteomes of the GASP mutant and its parental strain were compared by using a 2DE coupled with MS approach. Several differentially expressed proteins were detected and many of them were successful identified by mass spectrometry. Identified expression-changing proteins were grouped into three functional categories: metabolism, protein synthesis, chaperone and stress responsive proteins. Among them, the prevalence was ascribable to the "metabolism" group (72%) for the GASP mutant, and to "chaperones and stress responsive proteins" group for the parental strain (48%).

Synthesis of [¹⁸F]-labelled maltose derivatives as PET tracers for imaging bacterial infection

PURPOSE

To develop novel positron emission tomography (PET) agents for visualization and therapy monitoring of bacterial infections.

PROCEDURES

It is known that maltose and maltodextrins are energy sources for bacteria. Hence, (18)F-labelled maltose derivatives could be a valuable tool for imaging bacterial infections. We have developed methods to synthesize 4-O-(α-D-glucopyranosyl)-6-deoxy-6-[(18)F]fluoro-D-glucopyranoside (6-[(18)F]fluoromaltose) and 4-O-(α-D-glucopyranosyl)-1-deoxy-1-[(18)F]fluoro-D-glucopyranoside (1-[(18)F]fluoromaltose) as bacterial infection PET imaging agents. 6-[(18)F]fluoromaltose was prepared from precursor 1,2,3-tri-O-acetyl-4-O-(2',3',-di-O-acetyl-4',6'-benzylidene-α-D-glucopyranosyl)-6-deoxy-6-nosyl-D-glucopranoside (5). The synthesis involved the radio-fluorination of 5 followed by acidic and basic hydrolysis to give 6-[(18)F]fluoromaltose. In an analogous procedure, 1-[(18)F]fluoromaltose was synthesized from 2,3, 6-tri-O-acetyl-4-O-(2',3',4',6-tetra-O-acetyl-α-D-glucopyranosyl)-1-deoxy-1-O-triflyl-D-glucopranoside (9). Stability of 6-[(18)F]fluoromaltose in phosphate-buffered saline (PBS) and human and mouse serum at 37 °C was determined. Escherichia coli uptake of 6-[(18)F]fluoromaltose was examined.

RESULTS

A reliable synthesis of 1- and 6-[(18)F]fluoromaltose has been accomplished with 4-6 and 5-8% radiochemical yields, respectively (decay-corrected with 95 % radiochemical purity). 6-[(18)F]fluoromaltose was sufficiently stable over the time span needed for PET studies (∼96% intact compound after 1-h and ∼65% after 2-h incubation in serum). Bacterial uptake experiments indicated that E. coli transports 6-[(18)F]fluoromaltose. Competition assays showed that the uptake of 6-[(18)F]fluoromaltose was completely blocked by co-incubation with 1 mM of the natural substrate maltose.

CONCLUSION

We have successfully synthesized 1- and 6-[(18)F]fluoromaltose via direct fluorination of appropriate protected maltose precursors. Bacterial uptake experiments in E. coli and stability studies suggest a possible application of 6-[(18)F]fluoromaltose as a new PET imaging agent for visualization and monitoring of bacterial infections.

The maltodextrin system of Escherichia coli: metabolism and transport

The maltose/maltodextrin regulon of Escherichia coli consists of 10 genes which encode a binding protein-dependent ABC transporter and four enzymes acting on maltodextrins. All mal genes are controlled by MalT, a transcriptional activator that is exclusively activated by maltotriose. By the action of amylomaltase, we prepared uniformly labeled [(14)C]maltodextrins from maltose up to maltoheptaose with identical specific radioactivities with respect to their glucosyl residues, which made it possible to quantitatively follow the rate of transport for each maltodextrin. Isogenic malQ mutants lacking maltodextrin phosphorylase (MalP) or maltodextrin glucosidase (MalZ) or both were constructed. The resulting in vivo pattern of maltodextrin metabolism was determined by analyzing accumulated [(14)C]maltodextrins. MalP(-) MalZ(+) strains degraded all dextrins to maltose, whereas MalP(+) MalZ(-) strains degraded them to maltotriose. The labeled dextrins were used to measure the rate of transport in the absence of cytoplasmic metabolism. Irrespective of the length of the dextrin, the rates of transport at a submicromolar concentration were similar for the maltodextrins when the rate was calculated per glucosyl residue, suggesting a novel mode for substrate translocation. Strains lacking MalQ and maltose transacetylase were tested for their ability to accumulate maltose. At 1.8 nM external maltose, the ratio of internal to external maltose concentration under equilibrium conditions reached 10(6) to 1 but declined at higher external maltose concentrations. The maximal internal level of maltose at increasing external maltose concentrations was around 100 mM. A strain lacking malQ, malP, and malZ as well as glycogen synthesis and in which maltodextrins are not chemically altered could be induced by external maltose as well as by all other maltodextrins, demonstrating the role of transport per se for induction.

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.

Substrate specificity of the Escherichia coli maltodextrin transport system and its component proteins

Maltooligosaccharides up to maltoheptaose are transported by the maltodextrin transport system of Escherichia coli. The overall substrate specificity of the transport system was investigated by using 15 maltodextrin analogues with various modifications at the reducing end of the oligosaccharides as competing substrates. The binding interaction of the analogues with maltoporin in the outer membrane and the periplasmic maltose-binding protein, the two protein components of the transport system with known specificity for maltodextrins, was also investigated. All analogues containing several alpha, 1----4-glucosyl linkages were bound with high affinity by maltoporin and maltose-binding protein, regardless of O-methyl, O-nitrophenyl, beta-glucosyl or beta-fructosyl substitutions at the reducing end of the dextrins. Introduction of a negative charge or lack of a ring structure at the reducing end were also ineffective in abolishing binding by these two proteins. These results suggest that the structure of the reducing glucose is not important in the binding specificity of maltoporin or maltose-binding protein. However, the high affinity of these proteins for analogues was not in itself sufficient for recognition by the transport system overall. Maltohexaitol, 4-nitrophenyl alpha-maltotetraoside and 4-beta-D-maltopentaosyl-D-glucopyranose were bound with the same affinity as comparable maltodextrins by both maltoporin and maltose-binding protein but were poorly recognized by the transport system. These results suggest that another, yet uninvestigated component of the transport system has a more restricted specificity towards changes at the reducing end of the maltodextrin molecule.

Galactose- and maltose-stimulated lipoamide dehydrogenase activities related to the binding-protein-dependent transport of galactose and maltose in toluenized cells of Escherichia coli

The binding protein-dependent transport of galactose and maltose occurs at a reduced but significant rate in Escherichia coli cells which have undergone a mild toluenization. Dihydrolipoate and 3-acetyl-NAD produce a severalfold stimulation of these transports in the toluenized cells. In parallel to the stimulation of galactose and maltose transport by dihydrolipoate and 3-acetyl-NAD, there is a stimulation by galactose and maltose of lipoamide dehydrogenase activities which seem to be related to the binding-protein-dependent transport of these sugars. The lipoamide dehydrogenase component of the pyruvate and 2-oxoglutarate dehydrogenase complexes (the lpd gene product) is not involved in this stimulation. These results are discussed in relation to our recent studies showing a possible involvement of lipoic acid and of the 2-oxoacid dehydrogenases in the binding-protein-dependent transports.

Formation and excretion of acetylmaltose after accumulation of maltose in Escherichia coli

malB(+)malQ strains accumulate maltose via the maltose-binding-protein-dependent transport system but are unable to metabolize it. Nevertheless, some of the maltose is modified after entering the cell. This newly formed compound exhibited a higher R(f) value than did maltose upon thin-layer and paper chromatography with the usual sugar-separating solvents. Treatment of this compound with acid and alkali reformed maltose. The identity of this compound with acetylmaltose was derived from mass spectrometry. Nuclear magnetic resonance spectra of the compound confirmed the presence of the acetyl group but did not allow its precise location on the maltose moiety. However, linkage to the 1-position of maltose could be excluded. Analysis of the mass spectra indicated that the nonreducing end of maltose was acetylated. Other substrates of the maltose transport system, such as maltotetraose, maltopentaose, and maltohexaose, were also modified after accumulation into the cell. Several products were formed; the heterogeneity of these products was probably caused by different degrees of acetylation. The enzymatic activity responsible for maltose and maltodextrin acetylation is unknown. However, it is clear that the lacA-dependent thiogalactoside transacetylase was not necessary for the acetylation of maltose. Strains that accumulate maltose via a bypass of the normal malB-dependent transport system also acetylated maltose even in the absence of any malB gene products. Thus, the acetylating activity was not connected to the malB system. Acetylmaltose as well as acetylated maltodextrins was excreted into the medium. Acetylmaltose is not a substrate of the maltose transport system. Thus, maltose acetylation may be an effective detoxification mechanism.