Effect of soluble corn fiber supplementation for 1 year on bone metabolism in children, the MetA-bone trial: Rationale and design

Calcium intake is critical for adequate bone mineralization in adolescence, but it is usually inadequate in US adolescents. A strategy to maximize bone mineralization is to increase calcium absorption, which could be achieved by soluble corn fiber (SCF). There are no studies determining the long-term effects of SCF on bone mass in children.

OBJECTIVES

To determine the effect of one-year SCF supplementation compared to placebo on bone mass and bone biomarkers in children with low habitual calcium intake. We hypothesize that SCF supplementation will result in a higher bone mineral content and higher levels of bone formation and lower bone resorption biomarkers.

METHODS

240 healthy children (10-13 years), with usual low calcium intake, will be randomized to four experimental groups for 1 year: (1) SCF (12 g/d); (2) SCF (12 g/d) + 600 mg/d of calcium; (3) Placebo (maltodextrin); and (4) Placebo +600 mg/d of calcium. The supplements have been pre-mixed with a flavored powder beverage and participants will only need to dilute it in water and drink this twice per day. Bone will be measured using dual energy x-ray absorptiometry (DXA) at baseline, 6 and 12 months. Serum bone biomarkers will be measured at baseline and at 12 months.

CONCLUSIONS

If supplementing diets with SCF lead to higher bone mass during adolescence, this could help achieve the genetic potential for PBM and to start adult life with stronger bones. If successful, SCF can be incorporated into diets for promoting bone health in adolescents.

Investigation of carryover effect of prior fibre consumption on growth, serum and tissue metabolic markers in Ossabaw pigs fed a high-fat diet

Carryover effect of prior fibre consumption on metabolic markers was investigated. Treatments were arranged in 2 × 2 factorial with 2 fibre sources, 4% inulin or cellulose (Solka-Floc®) and fat levels (5 or 15%) for the low-fat diet (LFD) and high-fat diet (HFD) respectively. Pigs were fed the two fibre diets for the first 56d (nursery phase), and thereafter fed either the LFD or HFD containing no added fibre source from d56 to 140 (growing phase). Pigs on the HFD were heavier (p = .05) than those on LF (64.61 vs. 68.38 kg), regardless of prior fibre type consumed. Pigs that were fed cellulose during the nursery and later fed the HFD had the highest ADG (p < .05). Feeding the HFD resulted in higher back fat (BF) (13.41 and 18.18 ± 0.12 mm for LFD and HFD, respectively; p < .01). The HFD resulted in higher (p < .01) insulin (0.014 and 0.016 ± 0.001 mg/L for LF and HF respectively) and glucose (100.89 and 125.03 ± 4.39 mg/dl for LF and HF respectively) concentrations in the serum. Inulin increased (p ≤ .02) jejunal expression of SREBP-1c and CL-4, but reduced (p < .05) TNFɑ and IL-6 expression in the ileum. Alpha-diversity was significantly different (p < .05) between the inulin and cellulose fed pigs at the end of the nursery and finishing phases. Therefore, inulin feeding before a HFD may lead to reduction in ADG and inflammatory markers in the small intestine of pigs, and thus prevent future metabolic disorders.

Effects of dietary resistant starch content on metabolic status, milk composition, and microbial profiling in lactating sows and on offspring performance

In the present study, the effects of dietary resistant starch (RS) content on serum metabolite and hormone concentrations, milk composition, and faecal microbial profiling in lactating sows, as well as on offspring performance was investigated. Sixteen sows were randomly allotted at breeding to two treatments containing low- and high-RS contents from normal and high-amylose corn varieties, respectively, and each treatment had eight replicates (sows). Individual piglet body weight (BW) and litter size were recorded at birth and weaning. Milk samples were obtained on day 10 after farrowing for composition analysis. On day 2 before weaning, blood and faecal samples were collected to determine serum metabolite and hormone concentrations and faecal microbial populations, respectively. Litter size at birth and weaning were not influenced (p > 0.05) by the sow dietary treatments. Although feeding the RS-rich diet to sows reduced (p = 0.004) offspring birth BW, there was no difference in piglet BW at weaning (p > 0.05). High-RS diet increased (p < 0.05) serum triacylglycerol and nonesterified fatty acid concentrations and milk total solid content, and tended (p = 0.09) to increase milk fat content in lactating sows. Feeding the RS-rich diet to sows increased (p < 0.01) faecal bacterial population diversity. These results indicate that high-RS diets induce fatty acid mobilization and a greater intestinal bacterial richness in lactating sows, as well as a greater nutrient density in maternal milk, without affecting offspring performance at weaning.

Evidence for Acquisition in Nature of a Chromosomal 2,4-Dichlorophenoxyacetic Acid/(alpha)-Ketoglutarate Dioxygenase Gene by Different Burkholderia spp

We characterized the gene required to initiate the degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) by the soil bacterium Burkholderia sp. strain TFD6, which hybridized to the tfdA gene of the canonical 2,4-D catabolic plasmid pJP4 under low-stringency conditions. Cleavage of the ether bond of 2,4-D by cell extracts of TFD6 proceeded by an (alpha)-ketoglutarate-dependent reaction, characteristic of TfdA (F. Fukumori and R. P. Hausinger, J. Bacteriol. 175:2083-2086, 1993). The TFD6 tfdA gene was identified in a recombinant plasmid which complemented a tfdA transposon mutant of TFD6 created by chromosomal insertion of Tn5. The plasmid also expressed TfdA activity in Escherichia coli DH5(alpha), as evidenced by enzyme assays with cell extracts. Sequence analysis of the tfdA gene and flanking regions from strain TFD6 showed 99.5% similarity to a tfdA gene cloned from the chromosome of a different Burkholderia species (strain RASC) isolated from a widely separated geographical area. This chromosomal gene has 77.2% sequence identity to tfdA from plasmid pJP4 (Y. Suwa, W. E. Holben, and L. J. Forney, abstr. Q-403, in Abstracts of the 94th General Meeting of the American Society for Microbiology 1994.). The tfdA homologs cloned from strains TFD6 and RASC are the first chromosomally encoded 2,4-D catabolic genes to be reported. The occurrence of highly similar tfdA genes in different bacterial species suggests that this chromosomal gene can be horizontally transferred.

Molecular analysis of bacterial community based on 16S rDNA and functional genes in activated sludge enriched with 2,4-dichlorophenoxyacetic acid (2,4-D) under different cultural conditions

Differential emergence and diversity of bacterial communities from activated sludge in response to varied cultural conditions using 2,4-dichlorophenoxyacetic acid (2,4-D) were investigated by coupling molecular analyses based on 16S rDNA with functional genes. We employed three different cultural conditions: (1) a culture sequentially fed a high concentration (300 mg/L) of 2,4-D (HS); (2) a culture continuously fed a low concentration (10 mg/L) of 2,4-D (LC); and (3) a serial batch culture in which 1% (v/v) of culture was transferred to a fresh medium containing a high concentration (300 mg/L) of 2,4-D (HB). The HS and LC bioreactors were operated for 3 months and HB was repeatedly transferred for 1 month. The 2,4-D was stably degraded under all the cultural conditions tested. PCR amplification and cloning-based analysis of functional genes using community DNAs from the cultures revealed five different oxygenase genes that may be involved in the initial step of 2,4-D degradation. All five gene-types were present in HS, while one of the five genes, type V (tftA) was not detected in LC. Quantitative PCR analysis showed that in HS, Ralstonia eutropha JMP 134 type-tfdA4 (type I) was the most abundant in copy number (2.0 +/- 0.1 x 10(7) copies/microg DNA) followed by RASC type-tfdA (type II) (1.8 +/- 1.0 x 10(6) copies/microg DNA), putative cadA-like gene (type IV) (2.6 +/- 0.8 x 10(5) copies/microg DNA), cadA gene (type III) (1.3 +/- 1.0 x 10(4) copies/microg DNA), and tftA gene (type V) (3.5 +/- 1.1 x 10(3) copies/microg DNA). Similar results were obtained in LC. In contrast, HB contained only type I and type III genes, and the type I gene was five orders of magnitude greater in copy number than the type III gene. Denaturing gel gradient electrophoresis (DGGE) analysis of PCR, amplified 16S rDNA fragments of bacterial communities in the three different cultures showed low similarity coefficient values (< or =0.35) when compared to the original activated sludge, suggesting that 2,4-D amendment caused a drastic change in the bacterial community. Particularly, HB showed only six bands (16-18 bands in the other cultures) and very low similarity coefficient values when compared to the other communities (0.10 to HS, 0.17 to LC, and 0.0 to original sludge). These results indicated that serial batch culturing (HB) resulted in a phylogenetically limited number of 2,4-D degrading bacteria carrying limited catabolic genes whereas more diverse 2,4-D degraders and catabolic genes were present in HS and LC. Therefore, the approach used for monitoring should be taken into account when one evaluates the population dynamics of contaminant-degrading bacteria at bioremediation sites.

Analysis of environmental Escherichia coli isolates for virulence genes using the TaqMan PCR system

AIMS

To assess the presence of virulence genes in environmental and foodborne Escherichia coli isolates using the TaqMan PCR system.

METHODS AND RESULTS

Three TaqMan pathogen detection kits called O157:H7, StxI and StxII were used to investigate the presence of virulence genes in Escherichia coli isolates. All 54 foodborne E. coli O157:H7 isolates showed expected results using these kits. Ninety (15%) of 604 environmental isolates gave positive amplification with an O157:H7-specific kit. TaqMan PCR amplification products from these 90 isolates were analysed by agarose gel electrophoresis, and 90% (81 of 90) of the environmental samples contained the expected PCR product. Sixty-six of these 90 were chosen for serotyping tests and only 35% (23 of 66) showed agglutination with both anti-O157 and anti-H7 antibodies. Further ribotyping of 16 sero-positive isolates in an automated Riboprinter did not identify these to be O157:H7. Multiplex PCR with primers for eaeA, stxI and stxII genes was used to confirm the TaqMan results in 10 selected environmental isolates.

CONCLUSIONS

All three TaqMan pathogen detection kits were useful for virulence gene analysis of prescreened foodborne O157:H7 isolates, while the O157:H7-specific kit may not be suitable for virulence gene analysis of environmental E. coli isolates, because of high false positive identification.

SIGNIFICANCE AND IMPACT OF THE STUDY

The ability to rapidly identify the presence of pathogenic E. coli in food or environmental samples is essential to avert outbreaks. These results are of importance to microbiologists seeking to use TaqMan PCR to rapidly identify pathogenic E. coli in environmental samples. Furthermore, serotyping may not be a reliable method for identification of O157:H7 strains.

Functional and structural adaptations of bacterial communities growing on particulate substrates under stringent nutrient limitation

Biomass recycle reactors (BRRs) were used as a model system to study the functional and structural adaptations of mixed bacterial communities in response to the imposition of increasingly severe nutrient limitation. BRRs were fed synthetic media containing either spinach homogenate or autoclaved yeast cells to simulate the complex mixtures of particulate carbon sources that are often present in nature. In the BRRs fed spinach homogenate, the biomass (measured as particulate protein) exhibited a physiological response similar to previous studies as detected by 40-80% reductions in respiratory potential and by relatively stable catabolic ectoenzyme activities. Concomitant adaptations in bacterial community structure were detected by PCR-DGGE and RT-PCR-DGGE of 16S rDNA and 16S rRNA fragments, respectively. The microbial community structure was dynamic even after the biomass had reached a quasi-steady state with respect to physiological measurements. In the BRRs fed yeast cells, respiratory potentials increased 2- to 5-fold during the initial portion of the BRR run and alpha-glucosidase and beta-glucosidase activities increased 2- to 4-fold. Substantial bacterial community shifts were also detected in both the rDNA and rRNA profiles, indicating that this community was also structurally dynamic. These experiments suggest that phylogenetically different bacteria sustained the functional activities in these ecosystems in response to increasingly stringent nutrient limitation.

Variable selection in high-dimensional multivariate binary data with application to the analysis of microbial community DNA fingerprints

In order to understand the relevance of microbial communities on crop productivity, the identification and characterization of the rhizosphere soil microbial community is necessary. Characteristic profiles of the microbial communities are obtained by denaturing gradient gel electrophoresis (DGGE) of polymerase chain reaction (PCR) amplified 16S rDNA from soil extracted DNA. These characteristic profiles, commonly called community DNA fingerprints, can be represented in the form of high-dimensional binary vectors. We address the problem of modeling and variable selection in high-dimensional multivariate binary data and present an application of our methodology in the context of a controlled agricultural experiment.

Aerobic biological treatment of a pharmaceutical wastewater: effect of temperature on cod removal and bacterial community development

The effect of temperature was studied on the efficiency of soluble COD removal and bacterial community development during the aerobic biological treatment of a pharmaceutical wastewater. Using wastewater and bacterial inoculum obtained from the full-scale facility treating this wastewater, batch laboratory cultures were operated at 5 degrees C intervals from 30 degrees C to 70 C. Following four culture transfers to allow for bacterial acclimation, residual soluble COD levels were measured and bacterial community fingerprints were obtained by denaturing gradient gel electrophoresis (DGGE) of polymerase chain reaction (PCR)-amplified 16S rRNA gene fragments. Soluble COD removal efficiency declined as temperature increased from 30 degrees C (62%) to 60 degrees C (38%). Biological treatment of this wastewater failed to occur at temperatures higher than 60 C. Gradual shifts in bacterial community structure were detected as temperature increased, including a concomitant reduction in the number of different bacterial populations. The impact of temperature on a two-stage biological treatment process was also compared. Better soluble COD removal was achieved when both reactors were operated at 30 degrees C compared to a system where the two stages were consecutively operated at 55 degrees C and 30 degrees C. These results indicate that operation of aerobic biological wastewater treatment reactors at elevated temperatures can have adverse effects on process performance.

Fate of the biological control agent Pseudomonas aureofaciens TX-1 after application to turfgrass

The fate and impact of Pseudomonas aureofaciens TX-1 following application as a biocontrol agent for fungi in turfgrass were studied. The organism was applied with a modified irrigation system by using a preparation containing 1 x 10(6) P. aureofaciens TX-1 CFU ml(-1) about 100 times between May and August. We examined the impact of this repeated introduction of P. aureofaciens TX-1 (which is known to produce the antimicrobial compound phenazine-1-carboxylic acid) on the indigenous microbial community of the turfgrass system and on establishment of introduced bacteria in the soil system. A PCR primer-DNA hybridization probe combination was developed to accurately monitor the fate of P. aureofaciens TX-1 following application in irrigation water. To assess the impact of frequent P. aureofaciens TX-1 applications on the indigenous bacterial community, turfgrass canopy, thatch, and rhizosphere samples were obtained during the growing season from control and treated plots and subjected to DNA extraction procedures and denaturing gradient gel electrophoresis (DGGE). PCR amplification and hybridization of extracted DNA with the P. aureofaciens TX-1-specific primer-probe combination revealed that P. aureofaciens TX-1 not only became established in the rhizosphere and thatch but also was capable of overwintering. Separation of PCR-amplified partial 16S rRNA genes by DGGE showed that the repeated application of P. aureofaciens TX-1 in irrigation water resulted in transient displacement of a leaf surface bacterial community member. There was no obvious alteration of any dominant members of the thatch and rhizosphere microbial communities.

Thermophilic aerobic treatment of a synthetic wastewater in a membrane-coupled bioreactor

Synthetic wastewater containing alpha-lactose and gelatin was treated in a thermophilic membrane-coupled bioreactor (MBR). Thermophilic (>45 degrees C) treatment represents a potentially advantageous process for high-temperature as well as high-strength industrial wastewaters susceptible to reactor autoheating. Thermophilic systems, however, generally support a nonflocculating biomass that resists conventional methods of cell separation from the treated wastewater. MBRs were applied to thermophilic treatment systems because bacterial cells can be retained regardless of cell aggregation. Thermophilic aerobic MBRs were successfully operated at high levels of biocatalyst and produced a better effluent quality than analogous thermophilic bioreactors without cell recycle. At a hydraulic residence time (HRT) of 13.1 h, the chemical oxygen demand (COD) of the membrane eluate improved from 760 mg l(-1) (without cell recycle) to 160 mg l(-1) (with cell recycle). Bacterial community shifts were detected by denaturing gradient gel electrophoresis (DGGE) of polymerase chain reaction (PCR) -amplified 16S rRNA gene fragments - 6 of 13 bands disappeared within 2 days of MBR operation. A concomitant 40-50% reduction in physiological indicators of cell reactivity (RNA:protein; ATP:protein) was also observed. The specific activity of beta-galactosidase and aminopeptidase, however, increased by 10-25%, indicating that there is a definite advantage to MBR operation at the highest biomass level possible. Nucleotide sequence analysis of 16S rDNA clones identified phylotypes from the low-G+C Gram-positive division and the beta- and gamma-subdivisions of Proteobacteria.

Phylogenetic analysis of bacterial communities in mesophilic and thermophilic bioreactors treating pharmaceutical wastewater

The phylogenetic diversity of the bacterial communities supported by a seven-stage, full-scale biological wastewater treatment plant was studied. These reactors were operated at both mesophilic (28 to 32 degrees C) and thermophilic (50 to 58 degrees C) temperatures. Community fingerprint analysis by denaturing gradient gel electrophoresis (DGGE) of the PCR-amplified V3 region of the 16S rRNA gene from the domain Bacteria revealed that these seven reactors supported three distinct microbial communities. A band-counting analysis of the PCR-DGGE results suggested that elevated reactor temperatures corresponded with reduced species richness. Cloning of nearly complete 16S rRNA genes also suggested a reduced species richness in the thermophilic reactors by comparing the number of clones with different nucleotide inserts versus the total number of clones screened. While these results imply that elevated temperature can reduce species richness, other factors also could have impacted the number of populations that were detected. Nearly complete 16S rDNA sequence analysis showed that the thermophilic reactors were dominated by members from the beta subdivision of the division Proteobacteria (beta-proteobacteria) in addition to anaerobic phylotypes from the low-G+C gram-positive and Synergistes divisions. The mesophilic reactors, however, included at least six bacterial divisions, including Cytophaga-Flavobacterium-Bacteroides, Synergistes, Planctomycetes, low-G+C gram-positives, Holophaga-Acidobacterium, and Proteobacteria (alpha-proteobacteria, beta-proteobacteria, gamma-proteobacteria and delta-proteobacteria subdivisions). The two PCR-based techniques detected the presence of similar bacterial populations but failed to coincide on the relative distribution of these phylotypes. This suggested that at least one of these methods is insufficiently quantitative to determine total community biodiversity-a function of both the total number of species present (richness) and their relative distribution (evenness).

Feeding inhibition and mortality in Reticulitermes flavipes (Isoptera: Rhinotermitidae) after exposure to imidacloprid-treated soils

Feeding inhibition and mortality of Reticulitermes flavipes (Kollar) exposed to sand, sandy loam, loam, and silty clay loam soils treated with several concentrations of imidacloprid were studied using bioassay techniques under laboratory conditions. Termite workers stopped feeding after exposure to treated soils. Differences in feeding reduction varied among the soil types. Based on the magnitude of the F-statistics, the effect of imidacloprid on the reduction of termite feeding was greatest in sand followed by sandy loam, loam, and silty clay loam soils. Soil properties such as organic matter content, silt and clay proportions, pH, and cation exchange capacity were suggested to affect the bioavailability of imidacloprid. Similar soil effects on mortality were observed in termites continuously exposed to treated soil for 21 d. In three of four soils tested, susceptibility to imidacloprid was not affected by the source of the termites tested.

Development of catechol 2,3-dioxygenase-specific primers for monitoring bioremediation by competitive quantitative PCR

Benzene, toluene, xylenes, phenol, naphthalene, and biphenyl are among a group of compounds that have at least one reported pathway for biodegradation involving catechol 2,3-dioxygenase enzymes. Thus, detection of the corresponding catechol 2,3-dioxygenase genes can serve as a basis for identifying and quantifying bacteria that have these catabolic abilities. Primers that can successfully amplify a 238-bp catechol 2,3-dioxygenase gene fragment from eight different bacteria are described. The identities of the amplicons were confirmed by hybridization with a 238-bp catechol 2,3-dioxygenase probe. The detection limit was 10(2) to 10(3) gene copies, which was lowered to 10(0) to 10(1) gene copies by hybridization. Using the dioxygenase-specific primers, an increase in catechol 2, 3-dioxygenase genes was detected in petroleum-amended soils. The dioxygenase genes were enumerated by competitive quantitative PCR with a 163-bp competitor that was amplified using the same primers. Target and competitor sequences had identical amplification kinetics. Potential PCR inhibitors that could coextract with DNA, nonamplifying DNA, soil factors (humics), and soil pollutants (toluene) did not impact enumeration. Therefore, this technique can be used to accurately and reproducibly quantify catechol 2, 3-dioxygenase genes in complex environments such as petroleum-contaminated soil. Direct, non-cultivation-based molecular techniques for detecting and enumerating microbial pollutant-biodegrading genes in environmental samples are powerful tools for monitoring bioremediation and developing field evidence in support of natural attenuation.

Parallel and divergent genotypic evolution in experimental populations of Ralstonia sp

Genetic rearrangements within a population of bacteria were analyzed to understand the degree of divergence occurring after experimental evolution. We used 18 replicate populations founded from Ralstonia sp. strain TFD41 that had been propagated for 1,000 generations with 2,4-dichlorophenoxyacetic acid (2,4-D) as the carbon source. Genetic divergence was examined by restriction fragment length polymorphism analysis of the incumbent plasmid that carries the 2,4-D catabolic genes and by amplification of random regions of the genome via PCR. In 18 evolved clones examined, we observed duplication within the plasmid, including the tfdA gene, which encodes a 2,4-D dioxygenase that catalyzes the first step in the 2,4-D catabolic pathway. In 71 of 72 evolved clones, a common 2.4-kb PCR product was lost when genomic fingerprints produced by PCR amplification using degenerate primers based on repetitive extragenic palindromic (REP) sequences (REP-PCR) were compared. The nucleotide sequence of the 2.4-kb PCR product has homology to the TRAP (tripartite ATP-independent periplasmic) solute transporter gene family. Hybridization of the 2. 4-kb REP-PCR product from the ancestor to genomic DNA from the evolved populations showed that the loss of the PCR product resulted from deletions in the genome. Deletions in the plasmid and presence and/or absence of other REP-PCR products were also found in these clones but at much lower frequencies. The common and uncommon genetic changes observed show that both parallel and divergent genotypic evolution occurred in replicate populations of this bacterium.

The cis-diol dehydrogenase cbaC gene of Tn5271 is required for growth on 3-chlorobenzoate but not 3,4-dichlorobenzoate

The nucleotide sequence of cbaC, the 1-carboxy-3-chloro-4,5-dihydroxycyclohexa-2,6-diene (cis-diol) dehydrogenase gene from the 3-chlorobenzoate (3-Cba) catabolic transposon Tn5271 was determined. The functional significance of the deduced open reading frame was evaluated by deletion of an internal BstEII restriction site in cbaC and by the creation of nested deletions using exonuclease III. Expression studies were carried out with Alcaligenes sp. strain BR6024, a chloramphenicol-resistant, tryptophan auxotroph derived from the wild-type isolate BR60. BR6024 hosts carrying complete cbaAB (3-Cba 3,4-(4,5)-dioxygenase and reductase) genes, with deletions of cbaC, metabolized 3Cba to the cis-4,5-diol metabolite. These mutants failed to grow on 3-Cba; however, they grew on 3,4-dichlorobenzoate, accumulating 5-chloroprotocatechuate transiently. These results indicated the cbaC dehydrogenase was not required for re-aromatization of the unstable 3,4-dCba cis-4,5-diol metabolite. Spontaneous elimination of HCl from this metabolite is proposed to generate 5-chloroprotocatechuate, which is a substrate for the protocatechuate metaring fission pathway in Alcaligenes sp. BR60. The relationship of the deduced amino acid sequence of cbaC with 15 other oxidoreductases and sequences of unknown function from bacteria, plants and animals revealed a conserved N-terminal GxxGxG dinucleotide-binding domain and a conserved region with a H(x11)KHVLxEKPxA consensus flanked by alpha-helical domains. o-Phthalate cis-diol dehydrogenase (Pseudomonas putida), glucose-fructose oxidoreductase (Zymomonas mobilis), myo-inositol-2-dehydrogenase (Bacillus subtilis) and D-galactose-1-dehydrogenase (Pseudomonas fluorescens) are related proteins. These dehydrogenases are unrelated to the Type I, II and III dehydrogenase superfamilies that include the cis-diol dehydrogenases involved in benzoate, toluene, biphenyl and naphthalene catabolism (Type II) and benzene catabolism (Type III).

The phylogenetic distribution of a transposable dioxygenase from the Niagara River watershed

Horizontal gene transfer in the Bacteria has been demonstrated to occur under natural conditions. The ecological impact of gene transfer events depends on the new genetic material being expressed in recipient organisms, and on natural selection processes operating on these recipients. The phylogenetic distribution of cbaAB genes for chlorobenzoate 3,4-(4,5)-dioxygenase, which are carried within Tn5271 on the IncP beta plasmid pBRC60, was investigated using isolates from freshwater microcosms and from the Niagara River watershed. The latter included isolates from surface water, groundwater and bioremediation reactor samples. The cbaAB genes have become integrated, through interspecific transfer, primarily into species of the beta Proteobacteria (44/48 isolates). Only four isolates, identified as Pseudomonas fluorescens (3/48) and Xanthomonas maltophilia (1/48), belonged to the gamma Proteobacteria, despite the observation that pBRC60 was capable of mobilizing these genes into a wide range of beta and gamma Proteobacteria in the laboratory. The natural host range correlated with the distribution of the meta-ring-fission pathway for metabolism of protocatechuates formed when the cbaAB genes were expressed (45/48 isolates). We proposed the hypothesis that natural selection has favoured recipients that successfully integrate the activity of the transferred dioxygenase with the conserved meta ring-fission pathway. The hypothesis was tested by transferring a plasmid construct containing the cbaAB genes into type strains representative of the beta and gamma Proteobacteria. The concept of applying mobile catabolic genes to probe the phylogenetic distribution of compatible degradative pathways is discussed.

The nucleotide sequence of the Tn5271 3-chlorobenzoate 3,4-dioxygenase genes (cbaAB) unites the class IA oxygenases in a single lineage

The nucleotide sequence of the 3-chlorobenzoate 3,4-dioxygenase genes, designated cbaAB, from the transposon Tn5271 was determined. The function of the two sequenced open reading frames was evaluated by mutagenesis and expression in vivo to show that the cbaA and cbaB genes code for dioxygenase and reductase proteins, respectively. Comparison of the deduced amino acid sequences of the cbaAB genes with sequences for other oxygenases revealed a clearly defined lineage among the class IA oxygenases that shows several unique features. This lineage includes phthalate 4,5-dioxygenase (pht23), and based on the available NH3-terminal sequence of component A, also includes 4-sulphobenzoate 3,4-dioxygenase. Vanillate demethylase, encoded by the vanAB genes and formally a monooxygenase enzyme catalysing an oxidative demethylation, is also included in this lineage. The terminal chlorobenzoate dioxygenase (CbaA) component is characterized by a conserved Rieske-type [2Fe-2S]R ligand centre. The reductase component (CbaB) contains a plant-type ferredoxin [2Fe-2S]Fd, FMN-isoalloxazine and NAD-ribose-binding domains and the orientation of these domains is conserved in all known class IA reductases. These results support the hypothesis that alternative fusions of the electron transfer modules of the reductases arose early in the divergence of oxygenase systems. The over-riding evolutionary constraint acting on the divergence of the class IA oxygenases would appear to be the requirement for a carboxyl group para to the site of oxygen insertion into the aromatic ring.

Catabolic transposons

The structure and function of transposable elements that code for catabolic pathways involved in the biodegradation of organic compounds are reviewed. Seven of these catabolic transposons have structural features that place them in the Class I (composite) or Class II (Tn3-family) bacterial elements. One is a conjugative transposon. Another three have been found to have properties of transposable elements but have not been characterized sufficiently to assign to a known class. Structural features of the toluene (Tn4651/Tn4653) and naphthalene (Tn4655) elements that illustrate the enormous potential for acquisition, deletion and rearrangement of DNA within catabolic transposons are discussed. The recently characterized chlorobenzoate (Tn5271) and chlorobenzene (Tn5280) catabolic transposons encode different aromatic ring dioxygenases, however they both illustrate the constraints that must be overcome when recipients of catabolic transposons assemble and regulate complete metabolic pathways for environmental pollutants. The structures of the chlorobenzoate catabolic transposon Tn5271 and the related haloacetate dehalogenase catabolic element of plasmid pUO1 are compared and a hypothesis for their formation is discussed. The structures and activities of catabolic transposons of unknown class coding for the catabolism of halogenated alkanoic acids (DEH) and chlorobiphenyl (Tn4371) are also reviewed.

Evidence for multiple adaptive peaks from populations of bacteria evolving in a structured habitat

Natural selection tends to promote the divergence of populations living in different environments. Even in identical environments, however, replicate populations may diverge if they find alternative adaptive solutions. We describe the evolution of 18 bacterial populations (Comamonas sp.) founded from a single progenitor genotype and propagated separately for 1000 generations in two distinct environments, one physically unstructured (mass-action liquid) and the other structured (agar surfaces). Phenotypic diversity, as reflected in colony morphology, was greater in the structured habitat than in the unstructured habitat. More importantly, the trajectories for mean fitness, as measured by competition against the common ancestor, were more divergent for populations in the structured habitat than those in the unstructured habitat. Structured environments may accelerate evolutionary diversification by promoting genetic polymorphisms within populations, thereby increasing the complexity of genetic constraints that allow divergence among replicate populations.

Cloning and expression of the transposable chlorobenzoate-3,4-dioxygenase genes of Alcaligenes sp. strain BR60

Growth on 3-chlorobenzoate was found to induce the enzymes of the protocatechuate meta ring fission pathway in Alcaligenes sp. strain BR60. The chlorobenzoate catabolic genes, designated cba, were localized to a 3.7-kb NotI-EcoRI fragment within the nonrepeated region of the composite transposon Tn5271. The cba genes were cloned onto two broad-host-range vectors and expressed in Escherichia coli and Alcaligenes sp. strain BR6024. In E. coli, expression of the cba genes with the IPTG (isopropyl-beta-D-thiogalactopyranoside)-inducible tac promoter of the IncQ vector pMMB66HE resulted in the production of protocatechuate and chlorodihydroxybenzoate metabolites of 3-chlorobenzoate. Expression of this construct in one orientation resulted in the formation of two polypeptides 51 and 42 kDa in size. This result was confirmed by subcloning into pGEM3Zf and then incorporating L-35S-methionine into newly synthesized proteins, using the thermally regulated T7 polymerase-promoter system. Introduction of the NotI-EcoRI fragment into Alcaligenes sp. strain BR6024 (Cba-P), using the IncP broad-host-range, mobilizable plasmid pBW13, restored the 3-chlorobenzoate-degradative phenotype and resulted in the accumulation of protocatechuate and chlorodihydroxybenzoate intermediates. The data indicate that a two-component dioxygenase specified by Tn5271 oxidizes 3-chlorobenzoate at the 3,4- or 4,5-positions. This activity extends the range of pathways for chloroaromatic compounds known to be functional in the environment. The new pathway avoids the toxicity attributed to the accumulation of chlorocatechol metabolites in bacteria degrading chlorobenzoates.