Influence of shrub willow buffers strategically integrated in an Illinois corn-soybean field on soil health and microbial community composition

Soil serves many important ecological functions and is an integral part of our existence as a society. However, concerns for soil health are growing globally, in part due to the negative impacts of agricultural management on soil resources. The production of perennial bioenergy crops on marginal land in row-crop production systems is one solution that could improve land-use efficiency and address the sustainability of cropland management. Because the relationship between crop management and the environment is complex, more research is needed to evaluate the potential benefits perennial bioenergy crop production has on soil health, as well as other ecosystem services. In this study, shrub willow buffers were strategically integrated into a corn-soybean cropping system with the main objective of reducing nitrate-N leaching from grain crop production while producing biomass for bioenergy. Two buffer systems (defined by landscape positions) were included for comparison, one on marginal land with exposure to nitrate-N leaching from upslope grain (southern plots) and one on fertile soils with less nitrate-N leaching potential (northern plots). Evaluation of soil (chemistry, bulk density, microbial community) and shrub willow vegetation properties (fine roots, leaf litter decomposition, and nutrient uptake dynamics), showed that landscape position plays an important role in (1) the dynamics of soil chemical properties, (2) shrub willow's influence and productivity, and (3) the provision of additional ecosystem services such as reductions in nitrous oxide emissions and nitrate-N leaching. In addition, the combination of crop type and landscape position (N-grain, N-willow, S-grain, and S-willow) influenced the species composition of the soil microbial community, resulting in unique and identifiable communities. These results highlight the potential application of shrub willow buffers for ecosystem service provision and support of ecosystem processes; however, understanding the relationship between the microbial community, crop type, and landscape is important for understanding the sustainability of the design.

An Integrated Landscape Designed for Commodity and Bioenergy Crops for a Tile-Drained Agricultural Watershed

Locating bioenergy crops on strategically selected subfield areas of marginal interest for commodity agriculture can increase environmental sustainability. Location and choice of bioenergy crops should improve environmental benefits with minimal disruption of current food production systems. We identified subfield soils of a tile-drained agricultural watershed as marginal if they had areas of low crop productivity index (CPI), were susceptible to nitrate-nitrogen (NO-N) leaching, or were susceptible to at least two other forms of environmental degradation (marginal areas). In the test watershed (Indian Creek watershed, IL) with annual precipitation of 852 mm, 3% of soils were CPI areas and 22% were marginal areas. The Soil and Water Assessment Tool was used to forecast the impact of growing switchgrass ( L.), willow ( spp.), and big bluestem ( Vitman) in these subfield areas on annual grain yields, NO-N and sediment exports, and water yield. Simulated conversion of CPI areas from current land use to bioenergy crops had no significant ( 0.05) impact on grain production and reduced NO-N and sediment exports by 5.0 to 6.0% and 3.0%, respectively. Conversion of marginal areas from current land use to switchgrass forecasted the production of 34,000 t of biomass and reductions in NO-N (26.0%) and sediment (33.0%) exports. Alternatively, conversion of marginal areas from current land use to willow forecasted similar reductions as switchgrass for sediment but significantly ( 0.01) lower reductions in annual NO-N export (18.0 vs. 26.0%).

Meeting world's most stringent Hg criterion: a pilot-study for the treatment of oil refinery wastewater using an ultrafiltration membrane process

A membrane ultrafiltration (UF) technology was tested using an oil refinery's end-of-pipe effluent to demonstrate the proof of concept, i.e. can the Great Lakes Initiative criterion of less than 1.3 ppt be consistently met at the pilot-scale, and to provide the data necessary for preliminary full-scale process design. This study presents the successful pilot test conducted with continuous but varying feed conditions over a protracted period. The UF membrane process consistently provided a constant permeate quality at all tested operating conditions, virtually independent of the feed water characteristics and the feed Hg concentration (0.5-22.7 ppt). The treatment target of less than 1.3 ppt of Hg was met and exceeded for all tested conditions during the pilot study. Turbidity measurements were <0.5 NTU (with a MDL of 0.5 NTU) 85% of the time and <0.16 NTU 95% of the time when analyzed on-line. The TMP values were below the specification of (negative) 7-12 psi at all tested conditions during the pilot-study. Weekly maintenance cleans and monthly clean in place (CIP) events were very effective in consistently restoring the membrane permeability during the pilot-study.

Achieving the Great Lakes Initiative mercury limits in oil refinery effluent

To meet the stringent Great Lakes Initiative (GLI) wastewater discharge mercury (Hg) limit of 1.3 ppt (ng/L), mercury removal technologies need to be identified and investigated. The goals of this study were to (1) identify and assess available wastewater treatment technologies for mercury removal from an oil refinery wastewater; and (2) conduct bench-scale tests to provide comparable, transparent, and uniform results to assess their performance at low mercury concentrations. The study found that many tested technologies were able to achieve the GLI mercury target concentration at the bench-scale, albeit with different efficiencies and engineering implications. These results demonstrate that at this scale there is no fundamental physical or chemical barrier to achieving < 1.3 ng Hg/L in the tested wastewater. The study also found that some technologies were effective on particulate mercury whereas others were effective on dissolved mercury. One emerging treatment technology was found to be effective on both particulate and dissolved mercury. Three mercury-removal technologies--ultrafiltration (particulate mercury), adsorption (dissolved mercury), and an emerging reactive filtration technology (particulate and dissolved mercury)--are recommended for further study. This research offers treatment alternatives for different forms of mercury in an oil refinery wastewater, which might be applicable to other types of mercury-containing wastewater.

Achieving very low mercury levels in refinery wastewater by membrane filtration

Microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) membranes were evaluated for their ability to achieve the world's most stringent Hg discharge criterion (<1.3ng/L) in an oil refinery's wastewater. The membrane processes were operated at three different pressures to demonstrate the potential for each membrane technology to achieve the targeted effluent mercury concentrations. The presence of mercury in the particulate form in the refinery wastewater makes the use of MF and UF membrane technologies more attractive in achieving very low mercury levels in the treated wastewater. Both NF and RO were also able to meet the target mercury concentration at lower operating pressures (20.7bar). However, higher operating pressures (≥34.5bar) had a significant effect on NF and RO flux and fouling rates, as well as on permeate quality. SEM images of the membranes showed that pore blockage and narrowing were the dominant fouling mechanisms for the MF membrane while surface coverage was the dominant fouling mechanism for the other membranes. The correlation between mercury concentration and particle size distribution was also investigated to understand mercury removal mechanisms by membrane filtration. The mean particle diameter decreased with filtration from 1.1±0.0μm to 0.74±0.2μm after UF.

Improving data quality on low level mercury wastewater analysis

In order to compare treatability test results evaluating low-level mercury (Hg) removal from oil refinery wastewater, improvements in Hg analytical methods were conducted at two US EPA certified analytical labs. The revisions in the analytical protocols improved Hg recoveries and hence enabled more reliable data interpretation and comparison for the specific wastewater tested. Nevertheless, significant differences between results from the two laboratories were identified in a split-sample experiment.

Comparative evaluation of As, Se and V removal technologies for the treatment of oil refinery wastewater

In this study, a broad range of readily deployable metal removal technologies were tested on a US refinery's wastewater to determine vanadium, arsenic and selenium removal performance. The bench-scale treatability studies were designed and performed so that test conditions could be as uniform as possible given the different mechanisms of action and engineering applications of each technology. The experimental data show that both ferric precipitation and reactive filtration were able to remove As, Se and V more efficiently from the wastewater than other tested technologies. Additionally, granular ferric hydroxide (GFH) adsorption was also effective in both V and As removal. Although the thiol-SAMMS adsorbent was developed for mercury removal, it also demonstrated appreciable selenium removal. None of the tested membrane filtration technologies showed any significant metals removal. This was attributed to the dissolved form of the metals as well as the wastewater's fouling characteristics.

A novel framework to classify marginal land for sustainable biomass feedstock production

To achieve food and energy security, sustainable bioenergy has become an important goal for many countries. The use of marginal lands to produce energy crops is one strategy for achieving this goal, but what is marginal land? Current definitions generally focus on a single criterion, primarily agroeconomic profitability. Herein, we present a framework that incorporates multiple criteria including profitability of current land use, soil health indicators (erosion, flooding, drainage, or high slopes), and environmental degradation resulting from contamination of surface water or groundwater resources. We tested this framework for classifying marginal land in the state of Nebraska and estimated the potential for using marginal land to produce biofuel crops. Our results indicate that approximately 1.6 million ha, or 4 million acres, of land (approximately 8% of total land area) could be classified as marginal on the basis of at least two criteria. Second-generation lignocellulosic bioenergy crops such as switchgrass ( Panicum virgatum L.), miscanthus (Miscanthus giganteus), native prairie grasses, and short-rotation woody crops could be grown on this land in redesigned landscapes that meet energy and environmental needs, without significant impacts on food or feed production. Calculating tradeoffs between the economics of redesigned landscapes and current practices at the field scale is the next step for determining functional designs for integrating biofuel feedstock production into current land management practices.

An attempt towards simultaneous biobased solvent based extraction of proteins and enzymatic saccharification of cellulosic materials from distiller's grains and solubles

Distiller's grains and solubles (DGS) is the major co-product of corn dry mill ethanol production, and is composed of 30% protein and 30-40% polysaccharides. We report a strategy for simultaneous extraction of protein with food-grade biobased solvents (ethyl lactate, d-limonene, and distilled methyl esters) and enzymatic saccharification of glucan in DGS. This approach would produce a high-value animal feed while simultaneously producing additional sugars for ethanol production. Preliminary experiments on protein extraction resulted in recovery of 15-45% of the protein, with hydrophobic biobased solvents obtaining the best results. The integrated hydrolysis and extraction experiments showed that biobased solvent addition did not inhibit hydrolysis of the cellulose. However, only 25-33% of the total protein was extracted from DGS, and the extracted protein largely resided in the aqueous phase, not the solvent phase. We hypothesize that the hydrophobic solvent could not access the proteins surrounded by the aqueous phase inside the fibrous structure of DGS due to poor mass transfer. Further process improvements are needed to overcome this obstacle.

Biofuels, land, and water: a systems approach to sustainability

There is a strong societal need to evaluate and understand the sustainability of biofuels, especially because of the significant increases in production mandated by many countries, including the United States. Sustainability will be a strong factor in the regulatory environment and investments in biofuels. Biomass feedstock production is an important contributor to environmental, social, and economic impacts from biofuels. This study presents a systems approach where the agricultural, energy, and environmental sectors are considered as components of a single system, and environmental liabilities are used as recoverable resources for biomass feedstock production. We focus on efficient use of land and water resources. We conducted a spatial analysis evaluating marginal land and degraded water resources to improve feedstock productivity with concomitant environmental restoration for the state of Nebraska. Results indicate that utilizing marginal land resources such as riparian and roadway buffer strips, brownfield sites, and marginal agricultural land could produce enough feedstocks to meet a maximum of 22% of the energy requirements of the state compared to the current supply of 2%. Degraded water resources such as nitrate-contaminated groundwater and wastewater were evaluated as sources of nutrients and water to improve feedstock productivity. Spatial overlap between degraded water and marginal land resources was found to be as high as 96% and could maintain sustainable feedstock production on marginal lands. Other benefits of implementing this strategy include feedstock intensification to decrease biomass transportation costs, restoration of contaminated water resources, and mitigation of greenhouse gas emissions.

In vitro activity of telithromycin against Spanish Streptococcus pneumoniae isolates with characterized macrolide resistance mechanisms

The susceptibilities to telithromycin of 203 Streptococcus pneumoniae isolates prospectively collected during 1999 and 2000 from 14 different geographical areas in Spain were tested and compared with those to erythromycin A, clindamycin, quinupristin-dalfopristin, penicillin G, cefotaxime, and levofloxacin. Telithromycin was active against 98.9% of isolates (MICs, < or =0.5 microg/ml), with MICs at which 90% of isolates are inhibited being 0.06 microg/ml, irrespective of the resistance genotype. The corresponding values for erythromycin were 61.0% (MICs, < or =0.25 microg/ml) and >64 microg/ml. The erm(B) gene (macrolide-lincosamide-streptogramin B resistance phenotype) was detected in 36.4% (n = 74) of the isolates, which corresponded to 93.6% of erythromycin-intermediate and -resistant isolates, whereas the mef(A) gene (M phenotype [resistance to erythromycin and susceptibility to clindamycin and spiramycin without blunting]) was present in only 2.4% (n = 5) of the isolates. One of the latter isolates also carried erm(B). Interestingly, in one isolate for which the erythromycin MIC was 2 microg/ml, none of these resistance genes could be detected. Erythromycin MICs for S. pneumoniae erm(B)-positive isolates were higher (range, 0.5 to >64 microg/ml) than those for erm(B)- and mef(A)-negative isolates (range, 0.008 to 2 microg/ml). The corresponding values for telithromycin were lower for both groups, with ranges of 0.004 to 1 and 0.002 to 0.06 microg/ml, respectively. The erythromycin MIC was high for a large number of erm(B)-positive isolates, but the telithromycin MIC was low for these isolates. These results indicate the potential usefulness of telithromycin for the treatment of infections caused by erythromycin-susceptible and -resistant S. pneumoniae isolates when macrolides are indicated.

Structure-based design and in-parallel synthesis of inhibitors of AmpC beta-lactamase

BACKGROUND

Group I beta-lactamases are a major cause of antibiotic resistance to beta-lactams such as penicillins and cephalosporins. These enzymes are only modestly affected by classic beta-lactam-based inhibitors, such as clavulanic acid. Conversely, small arylboronic acids inhibit these enzymes at sub-micromolar concentrations. Structural studies suggest these inhibitors bind to a well-defined cleft in the group I beta-lactamase AmpC; this cleft binds the ubiquitous R1 side chain of beta-lactams. Intriguingly, much of this cleft is left unoccupied by the small arylboronic acids.

RESULTS

To investigate if larger boronic acids might take advantage of this cleft, structure-guided in-parallel synthesis was used to explore new inhibitors of AmpC. Twenty-eight derivatives of the lead compound, 3-aminophenylboronic acid, led to an inhibitor with 80-fold better binding (2; K(i) 83 nM). Molecular docking suggested orientations for this compound in the R1 cleft. Based on the docking results, 12 derivatives of 2 were synthesized, leading to inhibitors with K(i) values of 60 nM and with improved solubility. Several of these inhibitors reversed the resistance of nosocomial Gram-positive bacteria, though they showed little activity against Gram-negative bacteria. The X-ray crystal structure of compound 2 in complex with AmpC was subsequently determined to 2.1 A resolution. The placement of the proximal two-thirds of the inhibitor in the experimental structure corresponds with the docked structure, but a bond rotation leads to a distinctly different placement of the distal part of the inhibitor. In the experimental structure, the inhibitor interacts with conserved residues in the R1 cleft whose role in recognition has not been previously explored.

CONCLUSIONS

Combining structure-based design with in-parallel synthesis allowed for the rapid exploration of inhibitor functionality in the R1 cleft of AmpC. The resulting inhibitors differ considerably from beta-lactams but nevertheless inhibit the enzyme well. The crystal structure of 2 (K(i) 83 nM) in complex with AmpC may guide exploration of a highly conserved, largely unexplored cleft, providing a template for further design against AmpC beta-lactamase.

Concentration-dependent selection of small phenotypic differences in TEM beta-lactamase-mediated antibiotic resistance

In this paper, the first robust experimental evidence of in vitro and in vivo concentration-dependent selection of low-level antibiotic-resistant genetic variants is described. The work is based on the study of an asymmetric competition assay with pairs of isogenic Escherichia coli strains, differing only (apart from a neutral chromosomal marker) in a single amino acid replacement in a plasmid-mediated TEM-1 beta-lactamase enzyme, which results in the new TEM-12 beta-lactamase. The mixture was challenged by different antibiotic concentrations, both in vitro and in the animal model, and the selective process of the variant population was carefully monitored. A mathematical model was constructed to test the hypothesis that measured growth and killing rates of the individual TEM variants at different antibiotic concentrations could be used to predict quantitatively the strength of selection for TEM-12 observed in competition experiments at these different concentrations.

Selection of naturally occurring extended-spectrum TEM beta-lactamase variants by fluctuating beta-lactam pressure

Despite the large number of in vitro mutations that increase resistance to extended-spectrum cephalosporins in TEM-type beta-lactamases, only a small number occur in naturally occurring enzymes. In nature, and particularly in the hospital, bacteria that contain beta-lactamases encounter simultaneous or consecutive selective pressure with different beta-lactam molecules. All variants obtained by submitting an Escherichia coli strain that contains a bla(TEM-1) gene to fluctuating challenge with both ceftazidime and amoxicillin contained only mutations previously detected in naturally occurring beta-lactamases. Nevertheless, some variants obtained by ceftazidime challenge alone contained mutations never detected in naturally occurring TEM beta-lactamases, suggesting that extended-spectrum TEM variants in hospital isolates result from fluctuating selective pressure with several beta-lactams rather than selection with a single antibiotic.

[National multicenter study of the in vitro activity of moxifloxacin against respiratory tract pathogens. Spanish Study Group on Moxifloxacin]

The activity of moxifloxacin, a novel 8-methoxyquinolone, was evaluated against 1,218 respiratory pathogens isolated in nine Spanish hospitals and was compared with ciprofloxacin, sparfloxacin, amoxicillin, amoxicillin-clavulanic acid, cefuroxime, erythromycin and clarithromycin. Moxifloxacin exhibited an excellent in vitro activity against most tested isolates with MIC90 values of 0.25 mg/l for Streptococcus pneumoniae and viridans group streptococci; 0.12 mg/l for Streptococcus pyogenes; 0.25 mg/l for Streptococcus agalactiae; 0.06 and 4 mg/l for methicillin-susceptible and -resistant Staphylococcus aureus, respectively; 0.06 mg/l for Haemophilus influenzae and 0.12 mg/l for Moraxella catarrhalis. Moxifloxacin susceptibility rates were not affected by penicillin resistance in S. pneumoniae and S. viridans, by the betalactamase production in H. influenzae and M. catarrhalis or by macrolide resistance. Moxifloxacin was twice as active as sparfloxacin and four to sixteen times more active than ciprofloxacin against Gram-positive isolates. Sparfloxacin and ciprofloxacin were slightly more active than moxifloxacin when tested against H. influenzae and M. catarrhalis isolates. The microbiological data obtained confirm that moxifloxacin is a promising antimicrobial agent for treating respiratory tract infections.

Selective removal of plutonium 238 from a canal sediment using a carbonate-chelant soil washing technology (ACT*DE*CON)

The Mound laboratory site in Miamisburg, OH, a former plutonium processing facility, contains approximately 40000 yd(3) (30,580 m3) of plutonium- and thorium-contaminated soils and sediments at levels that require remediation. Existing applicable remediation technologies are unsatisfactory, because they are expensive and do not provide volume reduction. ACT*DE*CON is a chemical soil leaching technology for the treatment of soils that utilizes contaminant dissolution via dilute selective solutions to remove radionuclides. In bench-scale tests, process parameters were developed for the optimal treatment of the Miami Erie Canal soil at the Mound site, combining the maximum plutonium removal with an acceptable amount of soil dissolution and minimizing the costs of reagents. Parameters evaluated included soil to extractant mass ratio, temperature, rinse solution composition, kinetics, and the application of several dewatering aids. Plutonium removal rates of >95% were achieved, and the residual plutonium in the treated soil proved to be very immobile-confirming that the process had removed the most accessible species of the radionuclide. Currently being tested at Mound is an engineering scale-up that includes an attrition scrubber, a counter-current extractor, and a reverse osmosis system. Economic evaluations based on bench-scale results put the treatment cost at US$278/yd(3) (US$364/m3), compared to US$350/yd(3) (US$458/m3) for the 'box-and-bury' baseline alternative treatment system.

Selection of very small differences in bacterial evolution

As the Science of Biology is constantly changing due to new discoveries and advanced techniques it is essential that a systematic study of the environmental causes of natural selection on microorganisms be conducted. Very small phenotypic differences among individuals within bacterial populations arise as a result of spontaneous genetic variation, but the evolutionary importance of these small changes is frequently considered to be non-significant. Recent in vitro experiments indicate that efficient selection of these very small differences may take place in environmental compartments where a particular intensity of the selective agent is exerted. Model studies based on competition between bacterial populations only differing in one or two amino acid changes of a detoxifying antibiotic enzyme (e.g. beta-lactamase) have shown that at a narrow range of antibiotic concentrations the variant population is strongly selected over the original type, despite the extremely low phenotypic differences in antibiotic susceptibility. These selective concentrations are expected to occur in precise environmental compartments (selective compartments). Due to the high frequency of structured habitats in natural environments, the intensity of selective agents is commonly exerted along certain gradients. Each one of the points forming these gradients (or intersection among gradients) may have a particular selective ability for a specific genetic variant. Considering the environment as a composition of an extremely high number of specific selective compartments may help to understand the existence of high levels of genetic variability in natural bacterial populations. This may be one of the clues towards the unraveling of bacterial evolution.

An extended-spectrum AmpC-type beta-lactamase obtained by in vitro antibiotic selection

A predictive approach was assayed to evaluate the possibility of mutant Amp-C beta-lactamase emergence with increased substrate spectrum (including new C-3' quaternary ammonium cephems). The ampC gene encoding the AmpC beta-lactamase from Enterobacter cloacae was cloned and expressed in an AmpC-defective strain of E. coli. After the AmpC containing strain was challenged with cefpirome, an ampC variant encoding an enzyme with increased resistance to cefpirome and cefepime was selected. In addition, this variant conferred increased resistance to penicillins and third generation cephalosporins. The complete nucleotide sequence of the gene was determined. The deduced peptide sequence showed a single change with respect to the wild-type gene: valine to glutamic acid at position 318 of the native protein (298 of the mature enzyme). The potential emergence and spread of this type of AmpC variants among pathogens should be considered.

Antibiotic-selective environments

The evolution and spread of antibiotic resistance depends on the antibiotic pressure exerted in the microbial environment. Selective effects occur in selective compartments, where particular antibiotic concentrations result in a differential growth rate of resistant bacterial variants. This may happen even at very low antibiotic concentrations able to select low-level-resistant bacteria. When more than one antibiotic is present in the environment, the multiple and fluctuating pressure produces the selection of bacterial variants that use multiple or multipurpose mechanisms or optimize a single mechanism of resistance to survive under the variable environmental conditions. Host factors such as immunity contribute to the selective process. Antibiotics themselves may promote bacterial diversity, either mediated by the random drift effect or triggering the increase of mutational events under bacterial stress. Analysis of selective environment-related antibiotic-host-bacteria interactions is essential to understanding the biology of antibiotic resistance.

A237T as a modulating mutation in naturally occurring extended-spectrum TEM-type beta-lactamases

A TEM-1 beta-lactamase derivative containing the single amino acid substitution A237T slightly increased (from 24 to 32 microg/ml) the cephalothin MIC for Escherichia coli RYC1000 but did not influence the activities of cefotaxime, ceftazidime, and aztreonam (MICs of 0.03, 0.12, and 0.06 microg/ml, respectively). Despite its apparent neutrality, addition of the A237T mutation to the pair of mutations characterizing TEM-10 (R164S and E240K) had a strong effect on substrate preference. Ceftazidime and aztreonam MICs decreased from 128 and 16 microg/ml to 16 and 2 microg/ml, respectively. In contrast, the cefotaxime MIC increased from 0.5 to 4 microg/ml. The acquisition of apparently neutral or even deleterious mutations results in a very effective mechanism of resistance to different beta-lactams that may be simultaneously or subsequently present in the environment. We propose here that the mutation in position 237 is an example of a modulating mutation and that consideration of this type of mutation may be important for understanding the evolution of beta-lactamases.

[Meropenem against bacteria carriers of wide spectrum TEM beta-lactamases: evolutive aspects]

The so-called wide spectrum beta-lactamases (WSBLs) are able to hydrolyze wide spectrum cephalosporins or monobactamics such as cefotaxime, ceftriaxone, ceftazidime, cefepime, cefpiroma or aztreonam. The natural wide spectrum beta-lactamases are mutational variants of TEM-1 consisting in the substitution of one of more amino acids within seven well defined positions in the molecule. Given the expected extremely low frequency for the simultaneous production of double or triple mutations, it is plausible that one of the mutational changes has been independently selected. A plurimutational remodelling of the TEM-beta-lactamase molecule is successively produced with the consequent appearance of highly effective ESBLs. Mutagenesis techniques allow clean molecular variants to be produced and allow the mutational effects under homogeneous conditions of bacterial strain, the plasmid implicated or the genic promotor to be studied. Meropenem remains active versus all the wide spectrum beta-lactamases referred in the 2be group of Bush, Jacoby and Medeiros as well as the new beta-lactamases produced in vitro by directed mutagenesis.

Selective compartments for resistant microorganisms in antibiotic gradients

The development of bacterial resistance to antibiotics is one of the best documented examples of contemporary biological evolution. Variability in the mechanisms of resistance depends on the diversity of genotypes in the huge bacterial populations, and also on the diversity of selective pressures that are produced along the antibiotic concentration gradients formed in the highly compartmentalized human body during therapy. These antibiotic gradients can be conceived as comprising selective compartments, each one of them defined as the concentration able to select a particular genetic variant. In vitro experimental models confirm that some antibiotic resistant variants are selected only at certain selective concentrations of antibiotics. The correspondence between selective compartments and selectable variants could offer a way of describing more accurately the antibiotic selective landscapes and for taking measures to prevent the development of a major threat to the future of modern medicine.

Strategies to minimize the development of antibiotic resistance

The choice of appropriate antimicrobial agents should take into consideration not only the interests of the individual patient, but also the ecological impact of different drugs and their delivery schedules. Selection of antibiotic-resistant organisms is a key aspect to remember. Bacterial populations harboring determinants of antibiotic resistance will be selected for by a range of antibiotic concentrations which are able to suppress or slow the growth of susceptible populations. These concentrations (selective concentrations) will be achieved within the human body in a series of compartments (selective compartments), where the potential selective power will be roughly proportional to the time of exposure of the bacteria to the drug (selective period). The duration of the expected exposure of bacterial populations to these concentrations of the drugs and the number of challenges they experience are probably the most important factors in predicting the potential selective activity of an antibiotic regimen. Such a risk analysis procedure may be used to propose guidelines for minimizing the development of antibiotic resistance.

The antibiotic selective process: concentration-specific amplification of low-level resistant populations

The biochemistry and genetics of antibiotic resistance are far better known than the equally important events underlying the selection of resistant populations. The hidden selection of low-level resistant variants may be a key process in the emergence of high-level antibiotic resistance. Different low-level resistant bacterial subpopulations may be specifically selected by different low antibiotic concentrations. The space in the environment (human body) where a given selective concentration exists represents the selective compartment. For pharmacokinetic reasons, low antibiotic concentrations occur in a larger selective compartment and persist longer than high antibiotic concentrations. The specific selection of low-level variants by low concentrations of antibiotic can be reproduced in experimental in vitro models using mixtures of susceptible and low-level resistant populations. We demonstrated this in Escherichia coli strains harbouring TEM-1, TEM-12 and TEM-10 beta-lactamases challenged by cefotaxime, and also Streptococcus pneumoniae strains with various levels of penicillin resistance challenged by amoxicillin or cefotaxime. In both cases, four hours of antibiotic challenge produced selective peaks of low-level resistant variant populations at low-level antibiotic concentrations. We conclude that variants with small decreases in antibiotic susceptibility may be fully selectable under in vivo circumstances; on the other hand, low-level antibiotic concentrations may have a considerable selective effect on the emergence of antibiotic resistance.

Characterization of a nosocomial outbreak involving an epidemic plasmid encoding for TEM-27 in Salmonella enterica subspecies enterica serotype Othmarschen

A ceftazidime-resistant Salmonella enterica subspecies enterica serotype Othmarschen strain, harboring the plasmid-mediated new extended-spectrum beta-lactamase TEM-27, was involved in a nosocomial outbreak (8 patients) at the Pediatric Cardiology Department of the Ramón y Cajal Hospital in Madrid. Genomic DNA polymorphism analysis, using an arbitrarily primed polymerase chain reaction, demonstrated the clonal nature of all Salmonella isolates. The plasmid encoding TEM-27 (pJMM1) was characterized by EcoRI, SacI, and BglI restriction. An identical restriction pattern was found in plasmid from all S. enterica strains. Possible in vivo intergeneric plasmid spread was suggested by the identification of an identical plasmid in Escherichia coli RYC5H and Enterobacter cloacae RYC39737 strains isolated during the outbreak. Results indicate that this outbreak involved dissemination of a single ceftazidime-resistant Salmonella strain and the spread of a single TEM-27-encoding plasmid among different Enterobacteriaceae.

New extended-spectrum TEM-type beta-lactamase from Salmonella enterica subsp. enterica isolated in a nosocomial outbreak

A new extended-spectrum beta-lactamase was detected in a lactose-positive Salmonella enterica subsp. enterica strain that caused a nosocomial outbreak involving eight patients in a pediatric cardiology unit. This strain showed high levels of resistance to ceftazidime and aztreonam and relatively low levels of resistance to cefotaxime and ceftriaxone. Resistance was associated with a conjugative plasmid of 59 kb, which encoded a new beta-lactamase with an isoelectric point of 5.9 that strongly hydrolyzed ceftazidime and to a much lesser extent hydrolyzed cefotaxime. The enzyme activity was inhibited by clavulanate. The corresponding bla gene was cloned and sequenced. The deduced amino acid sequence showed three significant amino acid replacements with respect to the TEM-1 sequence: Arg-164-->His, Glu-240-->Lys, and Thr-265-->Met. This combination is unique among extended-spectrum beta-lactamases and served to characterize the new enzyme, TEM-27.

Single amino acid replacements at positions altered in naturally occurring extended-spectrum TEM beta-lactamases

By directed mutagenesis, we constructed a set of seven TEM-1 derivatives containing single replacements in each one of the amino acids substituted in naturally occurring extended-spectrum TEM beta-lactamases. The exact contribution of each mutation to the resistance phenotype was determined. In addition, mutant enzyme production and stabilities were studied. Five of seven mutations determined to some extent variations in cephalosporin and/or monobactam activity. Dramatic changes in the hydrolysis of ceftazidime and aztreonam occurred when a serine was at position 164. Changes at positions 104, 238, and 240 showed more leaky variation in activity towards cephalosporins and aztreonam. Replacements at positions 237 and 265 caused no variation in susceptibility to cephalosporins. Interestingly, the change from Gln to Lys at position 39 found in TEM-2, classically considered a neutral change, slightly but consistently increased the MIC of ceftazidime and aztreonam. The in vitro construction of mutations appearing in naturally occurring TEM-beta-lactamases, studied in the same genetic context, may help to understand the evolution of extended-spectrum beta-lactamases.

In vitro selective antibiotic concentrations of beta-lactams for penicillin-resistant Streptococcus pneumoniae populations

Therapeutic regimens containing beta-lactam antibiotics are selecting penicillin-resistant Streptococcus pneumoniae populations all over the world. The selective pressure after 4 h of exposure to different concentrations of amoxicillin, cefixime, cefuroxime, and cefotaxime for low-level or high-level penicillin-resistant S. pneumoniae was evaluated in an in vitro model with mixed populations with penicillin susceptibilities of 0.015, 0.5, 1, and 2 micrograms/ml. The antibiotic concentration selecting for low-level resistance strongly reduced the susceptible population. Increasing antibiotic concentrations tended to decrease the total proportion of penicillin-resistant bacteria because of reduced numbers of the low-level-resistant population. The antibiotic concentration selecting for high-level resistance produced fewer resistant populations, but most of the organisms selected represented high-level resistance. In general, amoxicillin was a good selector for the low-level-resistant population and a poor selector for high-level resistance; cefuroxime and cefotaxime were poor selectors for low-level resistance and better selectors than amoxicillin for high-level penicillin resistance. Cefixime was the best selector of low-level penicillin resistance. When only resistant populations were mixed, the strains with high-level resistance were selected even at low antibiotic concentrations. Determination of the effects of selective antibiotic concentrations on mixed cultures of bacteria expressing different antibiotic resistance levels may help researchers to understand the ecology and epidemiology of penicillin-resistant S. pneumoniae populations.

In vitro activity of cefpirome compared with other third generation cephalosporins against nosocomial isolates in Argentina

The in vitro activity of cefpirome was evaluated against strains that showed conflicting results for third generation cephalosporins. Against isolates with derepressed inducible chromosomal cephalosporinase (n = 40) cefpirome was the sole cephalosporin with an MIC90 in the susceptible range; Klebsiella spp. with plasmid-mediated beta-lactamases (broad spectrum SHV-2 or SHV-2 type) (n = 40) remained most susceptible to ceftizoxime and cefpirome; against aminoglycoside-resistant Pseudomonas aeruginosa (n = 50), cefpirome was as active as ceftazidime and cefoperazone; against oxacillin-susceptible and oxacillin-resistant Staphylococcus spp., (n = 40), cefpirome was more active than other third generation cephalosporins but killing was inadequate against both oxacillin-resistant staphylococci and enterococci.