Physical Characterization and Cellular Toxicity Studies of Commercial NiO Nanoparticles

Nickel oxide (NiO) nanoparticles from several manufacturers with different reported sizes and surface coatings were characterized prior to assessing their cellular toxicity. The physical characterization of these particles revealed that sizes often varied from those reported by the supplier, and that particles were heavily agglomerated when dispersed in water, resulting in a smaller surface area and larger hydrodynamic diameter upon dispersion. Cytotoxicity testing of these materials showed differences between samples; however, correlation of these differences with the physical properties of the materials was not conclusive. Generally, particles with higher surface area and smaller hydrodynamic diameter were more cytotoxic. While all samples produced an increase in reactive oxygen species (ROS), there was no correlation between the magnitude of the increase in ROS and the difference in cytotoxicity between different materials.

Interlaboratory Comparison on the Quantification of Total and Accessible Amine Groups on Silica Nanoparticles with qNMR and Optical Assays

Risk assessment of nanomaterials requires not only standardized toxicity studies but also validated methods for nanomaterial surface characterization with known uncertainties. In this context, a first bilateral interlaboratory comparison on surface group quantification of nanomaterials is presented that assesses different reporter-free and labeling methods for the quantification of the total and accessible number of amine functionalities on commercially available silica nanoparticles that are widely used in the life sciences. The overall goal of this comparison is the identification of optimum methods as well as achievable measurement uncertainties and the comparability of the results across laboratories. We also examined the robustness and ease of implementation of the applied analytical methods and discussed method-inherent limitations. In summary, this comparison presents a first step toward the eventually required standardization of methods for surface group quantification.

Characterization of Commercial Metal Oxide Nanomaterials: Crystalline Phase, Particle Size and Specific Surface Area

Physical chemical characterization of nanomaterials is critical to assessing quality control during production, evaluating the impact of material properties on human health and the environment, and developing regulatory frameworks for their use. We have investigated a set of 29 nanomaterials from four metal oxide families (aluminum, copper, titanium and zinc) with a focus on the measurands that are important for the basic characterization of dry nanomaterials and the determination of the dose metrics for nanotoxicology. These include crystalline phase and crystallite size, measured by powder X-ray diffraction, particle shape and size distributions from transmission electron microscopy, and specific surface area, measured by gas adsorption. The results are compared to the nominal data provided by the manufacturer, where available. While the crystalline phase data are generally reliable, data on minor components that may impact toxicity is often lacking. The crystal and particle size data highlight the issues in obtaining size measurements of materials with broad size distributions and significant levels of aggregation, and indicate that reliance on nominal values provided by the manufacturer is frequently inadequate for toxicological studies aimed at identifying differences between nanoforms. The data will be used for the development of models and strategies for grouping and read-across to support regulatory human health and environmental assessments of metal oxide nanomaterials.

Acellular oxidative potential assay for screening of amorphous silica nanoparticles

Silica nanoparticles (SiNPs) are used in a wide range of consumer products, engineering and medical applications, with likelihood of human exposure and potential health concerns. It is essential to generate toxicity information on SiNP forms and associated physicochemical determinants to conduct risk assessment on these new materials. To address this knowledge gap, we screened a panel of custom synthesized, well-characterized amorphous SiNPs pristine and surface-modified (-C3-COOH, -C11-COOH, -NH₂, -PEG) of 5 different sizes: (15, 30, 50, 75, 100 nm) for their oxidative potential using an acellular assay. The assay is based on oxidation of dithiothreitol (DTT) by reactive oxygen species and can serve as a surrogate test for oxidative stress. These materials were characterized for size distribution, aggregation, crystallinity, surface area, surface modification, surface charge and metal content. Tests for association between oxidative potential of SiNPs and their physicochemical properties were carried out using analysis of variance and correlation analyses. These test results suggest that the size of amorphous SiNPs influenced their oxidative potential irrespective of the surface modification, with 15 nm exhibiting relatively higher oxidative potential compared to the other sizes. Furthermore, SiNP surface area, surface modification and agglomeration in solution also appeared to affect oxidative potential of these SiNPs. These findings indicate that physicochemical properties are critical in influencing the oxidative behaviour of amorphous SiNPs, with potential to trigger cellular oxidative stress and thus toxicity, when exposed. This information advances our understanding of potential toxicities of these amorphous SiNPs and supports risk assessment efforts and the design of safer forms of silica nanomaterials.

A Multi-Method Approach for Quantification of Surface Coatings on Commercial Zinc Oxide Nanomaterials

Surface functionalization is a key factor for determining the performance of nanomaterials in a range of applications and their fate when released to the environment. Nevertheless, it is still relatively rare that surface groups or coatings are quantified using methods that have been carefully optimized and validated with a multi-method approach. We have quantified the surface groups on a set of commercial ZnO nanoparticles modified with three different reagents ((3-aminopropyl)-triethoxysilane, caprylsilane and stearic acid). This study used thermogravimetric analysis (TGA) with Fourier transform infrared spectroscopy (FT-IR) of evolved gases and quantitative solution 1H nuclear magnetic resonance (NMR) for quantification purposes with 13C-solid state NMR and X-ray photoelectron spectroscopy to confirm assignments. Unmodified materials from the same suppliers were examined to assess possible impurities and corrections. The results demonstrate that there are significant mass losses from the unmodified samples which are attributed to surface carbonates or residual materials from the synthetic procedure used. The surface modified materials show a characteristic loss of functional group between 300-600 °C as confirmed by analysis of FT-IR spectra and comparison to NMR data obtained after quantitative release/extraction of the functional group from the surface. The agreement between NMR and TGA estimates for surface loading is reasonably good for cases where the functional group accounts for a relatively large fraction of the sample mass (e.g., large groups or high loading). In other cases TGA does not have sufficient sensitivity for quantitative analysis, particularly when contaminants contribute to the TGA mass loss. X-ray photoelectron spectroscopy and solid state NMR for selected samples provide support for the assignment of both the functional groups and some impurities. The level of surface group loading varies significantly with supplier and even for different batches or sizes of nanoparticles from the same supplier. These results highlight the importance of developing reliable methods to detect and quantify surface functional groups and the importance of a multi-method approach.

Dye-doped silica nanoparticles: synthesis, surface chemistry and bioapplications

Background

Fluorescent silica nanoparticles have been extensively utilised in a broad range of biological applications and are facilitated by their predictable, well-understood, flexible chemistry and apparent biocompatibility. The ability to couple various siloxane precursors with fluorescent dyes and to be subsequently incorporated into silica nanoparticles has made it possible to engineer these fluorophores-doped nanomaterials to specific optical requirements in biological experimentation. Consequently, this class of nanomaterial has been used in applications across immunodiagnostics, drug delivery and human-trial bioimaging in cancer research.

Main body

This review summarises the state-of-the-art of the use of dye-doped silica nanoparticles in bioapplications and firstly accounts for the common nanoparticle synthesis methods, surface modification approaches and different bioconjugation strategies employed to generate biomolecule-coated nanoparticles. The use of dye-doped silica nanoparticles in immunoassays/biosensing, bioimaging and drug delivery is then provided and possible future directions in the field are highlighted. Other non-cancer-related applications involving silica nanoparticles are also briefly discussed. Importantly, the impact of how the protein corona has changed our understanding of NP interactions with biological systems is described, as well as demonstrations of its capacity to be favourably manipulated.

Conclusions

Dye-doped silica nanoparticles have found success in the immunodiagnostics domain and have also shown promise as bioimaging agents in human clinical trials. Their use in cancer delivery has been restricted to murine models, as has been the case for the vast majority of nanomaterials intended for cancer therapy. This is hampered by the need for more human-like disease models and the lack of standardisation towards assessing nanoparticle toxicity. However, developments in the manipulation of the protein corona have improved the understanding of fundamental bio–nano interactions, and will undoubtedly assist in the translation of silica nanoparticles for disease treatment to the clinic.

Quantification of surface functional groups on silica nanoparticles: comparison of thermogravimetric analysis and quantitative NMR

Thermogravimetric analysis (TGA) coupled with evolved gas analysis-FT-IR has been examined as a potential method to study the functional group content for surface modified silica nanoparticles. A comparison with a quantitative solution NMR method based on analysis of groups released after dissolution of the silica matrix is used to provide benchmark data for comparison and to assess the utility and limitations of TGA. This study focused primarily on commercially available silicas and tested whether it was possible to use a correction based on bare silica to account for the significant mass loss that occurs due to condensation of surface hydroxyl groups and loss of matrix-entrapped components at temperatures above ∼200 °C. Although this approach has been used successfully in the literature for in-house prepared samples, it was problematic for commercial silicas prepared by the Stöber method. For these materials the agreement between estimates from qNMR and TGA mass loss was poor in many cases. However much better agreement was observed for samples for which the mass loss above 200 °C is relatively low, such as non-porous silica, or samples for which the mass fraction of functional group is large (e.g., high molecule weight groups or multilayers). FT-IR was useful in identifying the likely structure of the components lost from the surface at various temperatures and in some cases provided evidence of contaminants in the sample. Nevertheless, in other cases correlation of thermograms and FT-IR with NMR data was necessary, particularly for samples where multi-step modification of the silica surface results in incomplete functionalization that gives a mixture of products. Overall the results indicate that TGA provides reliable results for silicas of low porosity or those for which the functional group accounts for a significant fraction of the total sample mass. It is also suitable as a supplementary or screening technique to indicate the presence of coatings or covalent surface modification, prior to applying other techniques or for routine analyses where sensitivity is not critical.

Polycarboxylated Dextran as a Multivalent Linker: Synthesis and Target Recognition of the Antibody-Nanoparticle Bioconjugates in PBS and Serum

Nanoparticles (NPs) functionalized with antibodies on their surface are used in a wide range of research applications. However, the bioconjugation chemistry between the antibodies and the surface of nanoparticles can be very challenging, often accompanied by several undesired effects such as nanoparticle aggregation, antibody denaturation, or poor target recognition of the surface-bound antibodies. Here, we report on a synthesis of fluorescent silica nanoparticle-antibody (NP-Ab) conjugates, in which polycarboxylated dextran is used as the multivalent linker. First, we present a synthetic methodology to prepare polycarboxylated dextrans with molecular weights of 6, 40, and 70 kDa. Second, we used water-soluble, polycarboxylated dextrans as a multivalent spacers/linkers to immobilize antibodies onto fluorescent silica nanoparticles. The prepared NP-Ab conjugates were tested in a direct binding assay format in both phosphate-buffered saline buffer and whole serum to investigate the role of the spacer/linker in the capacity of the NP-Ab to specifically recognize their target in "clean" and also in complex media. We have compared the dextran conjugates with two standards: (a) NP-Ab with antibodies attached on the surface of nanoparticles through the classical physical adsorption method and (b) NP-Ab where an established poly(amidoamine) (PAMAM) dendrimer was used as the linker. Our results showed that the polycarboxylated 6 kDa dextran facilitates antibody immobilization efficiency of nearly 92%. This was directly translated into the improved molecular recognition of the NP-Ab, which was measured by a direct binding assay. The signal-to-noise ratio in buffered solution for the 6 kDa dextran NP-Ab conjugates was 81, nearly 3 times higher than that of PAMAM G4.5 conjugates and 9 times higher than the physically adsorbed NP-Ab sample. In whole serum, the effect of 6 kDa dextran was more hindered due to the formation of protein corona but the signal-to-noise ratio was at least double that of the physically adsorbed NP-Ab conjugates.

Quantification of amine functional groups on silica nanoparticles: a multi-method approach

Surface chemistry is an important factor for quality control during production of nanomaterials and for controlling their behavior in applications and when released into the environment. Here we report a comparison of four methods for quantifying amine functional groups on silica nanoparticles (NPs). Two colorimetric assays are examined, ninhydrin and 4-nitrobenzaldehyde, which are convenient for routine analysis and report on reagent accessible amines. Results from the study of a range of commercial NPs with different sizes and surface loadings show that the assays account for 50-100% of the total amine content, as determined by dissolution of NPs under basic conditions and quantification by solution-state ¹H NMR. To validate the surface quantification by the colorimetric assays, the NPs are modified with a trifluoromethylated benzaldehyde probe to enhance sensitivity for quantitative ¹⁹F solid state NMR and X-ray photoelectron spectroscopy (XPS). Good agreement between the assays and the determination from solid-state NMR is reinforced by elemental ratios from XPS, which indicate that in most cases the difference between total and accessible amine content reflects amines that are outside the depth probed by XPS. Overall the combined results serve to validate the relatively simple colorimetric assays and indicate that the reactions are efficient at quantifying surface amines, by contrast to some other covalent modifications that have been employed for functional group quantification.

‘Overloading’ fluorescent silica nanoparticles with dyes to improve biosensor performance

Using dye-doped silica nanoparticles (DSNP) as reporter probes, we describe a simple method of enhancing fluorescent signal and the extension of the detectable target concentration range in a proof-of-concept ‘dissolution immunoassay’.

Stabilizing silica nanoparticles in hydrogels: impact on storage and polydispersity

For successful nanomedicine, it is important that the unique, size-dependent physico-chemical properties of the nanomaterial remain predictably constant during both the storage and the manipulation of the material.

Salicylanilide carbamates: antitubercular agents active against multidrug-resistant Mycobacterium tuberculosis strains

A series of 27 salicylanilide-based carbamates was prepared as a part of our ongoing search for new antituberculosis drugs. These compounds exhibited very good in vitro activity against Mycobacterium tuberculosis, Mycobacterium kansasii and Mycobacterium avium and, in particular, against five multidrug-resistant strains, with MIC values between 0.5-2 micromol/L. Moreover, they displayed moderate toxicity against intestinal cells with the selectivity index being up to 96. Furthermore, acid stability and a half-life of 43h at pH 7.4 were shown. Thus, these novel salicylanilide derivatives are drug candidates which should be seriously consider for further screening.

New antituberculotics originated from salicylanilides with promising in vitro activity against atypical mycobacterial strains

A new series of 30 N-protected amino acid esters were prepared as a part of ongoing search for new anti-tuberculosis active salicylanilides. The esters possess high in vitro activity against Mycobacterium tuberculosis, Mycobacterium avium, and two strains of Mycobacterium kansasii, where one is an isolate from the patient, with MIC in the range 1-32 micromol/L for all tested strains. The prepared esters can be considered as prodrugs with better bio-availability and as more efficient transport forms through the mycobacterial cell membranes due to the higher lipophilicity. The experimental and calculated lipophilicity, stability, antituberculotic activity, cytotoxicity as well as the quantitative structure-activity relationships (QSARs) explored by the Intelligent Problem Solver (IPS) in Trajan Neural Network Simulator 6.0 are presented.

A2A adenosine-receptor-mediated facilitation of noradrenaline release in rat tail artery involves protein kinase C activation and betagamma subunits formed after alpha2-adrenoceptor activation

This work aimed to investigate the molecular mechanisms involved in the interaction of alpha2-adrenoceptors and adenosine A2A-receptor-mediated facilitation of noradrenaline release in rat tail artery, namely the type of G-protein involved in this effect and the step or steps where the signalling cascades triggered by alpha2-adrenoceptors and A2A-receptors interact. The selective adenosine A2A-receptor agonist 2-p-(2-carboxy ethyl) phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680; 100 nM) enhanced tritium overflow evoked by trains of 100 pulses at 5 Hz. This effect was abolished by the selective adenosine A2A-receptor antagonist 5-amino-7-(2-phenyl ethyl)-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-triazolo [1,5-c]pyrimidine (SCH 58261; 20 nM) and by yohimbine (1 microM). CGS 21680-mediated effects were also abolished by drugs that disrupted G(i/o)-protein coupling with receptors, PTX (2 microg/ml) or NEM (40 microM), by the anti-G(salpha) peptide (2 microg/ml) anti-G(betagamma) peptide (10 microg/ml) indicating coupling of A2A-receptors to G(salpha) and suggesting a crucial role for G(betagamma) subunits in the A(2A)-receptor-mediated enhancement of tritium overflow. Furthermore, phorbol 12-myristate 13-acetate (PMA; 1 microM) or forskolin (1 microM), direct activators of protein kinase C and of adenylyl cyclase, respectively, also enhanced tritium overflow. In addition, PMA-mediated effects were not observed in the presence of either yohimbine or PTX. Results indicate that facilitatory adenosine A2A-receptors couple to G(salpha) subunits which is essential, but not sufficient, for the release facilitation to occur, requiring the involvement of G(i/o)-protein coupling (it disappears after disruption of G(i/o)-protein coupling, PTX or NEM) and/or G(betagamma) subunits (anti-G(betagamma)). We propose a mechanism for the interaction in study suggesting group 2 AC isoforms as a plausible candidate for the interaction site, as these isoforms can integrate inputs from G(salpha) subunits (released after adenosine A2A-receptor activation; prime-activation), G(betagamma) subunits (released after activation of G(i/o)-protein coupled receptors) which can directly synergistically stimulate the prime-activated AC or indirectly via G(betagamma) activation of the PLC-PKC pathway.

Unusual chromatographic behavior of oligonucleotide sequence isomers on two different anion exchange HPLC columns

The retention behavior of the unmodified phosphodiester oligonucleotide sequence isomers was investigated on two different anion exchange columns: Biospher GMB 1000Q (based on DEAE-modified glycidyl methacrylate) and PolyWAX LP (based on silica with a crosslinked coating of linear polyethyleneimine). There was a notable difference in retention of oligonucleotides of the same composition but differing in the position of a single base. The most pronounced difference was observed between the oligonucleotides with the variable base in the end and in the center of the sequence. The use of either acetonitrile or 2-propanol as a mobile phase organic modifier did not markedly affect the retention time patterns. Prediction of the retention times of oligonucleotides must take into account the base position as well as identity. This is the first report of such a "same composition different sequence" effect, described for the short peptides, for synthetic oligonucleotides.

Metabolism of 2-chlorobenzoic acid in Pseudomonas stutzeri

3-Chloropyrocatechol is formed as a result of oxidation of 2-chlorobenzoate by Pseudomonas stutzeri. 2-Chloro-cis,cis-muconic acid is the product of oxidation of 3-chloropyrocatechol. A catabolic pathway for the degradation of 2-chlorobenzoate by a newly isolated strain of P. stutzeri is proposed.

Degradation of 2-chlorobenzoic and 2,5-dichlorobenzoic acids in soil columns by Pseudomonas stutzeri

The heterocontinuous flow cultivation technique was used for the study of 2-chlorobenzoic and 2,5-dichlorobenzoic acid degradation in soil columns inoculated with Pseudomonas stutzeri. 2-Chlorobenzoic and 2,5-dichlorobenzoic acids disappeared from the soil columns within 8 and 12 d, respectively. The presence of the haloaromatics increased the survival of strain KS25 in soil. Viable cell numbers in the soil columns flushed with 2-chlorobenzoic and 2,5-dichlorobenzoic acids were 1.3 and 2 times higher, respectively, than those without the chlorobenzoic acids after 30 d of incubation.

Degradation of 2-chlorobenzoic and 2,5-dichlorobenzoic acids in pure culture by Pseudomonas stutzeri

A strain of Pseudomonas stutzeri KS25 utilizing 2-chlorobenzoic and 2,5-dichlorobenzoic acids as the sole carbon and energy source was isolated from polychlorophenol-contaminated soil and sewage, using the method of enrichment cultures. This strain was also able to grow on 2-fluoro-, 2-iodo-, 2-bromo- and 2,5-dihydroxybenzoate, but did not utilize 3-, 4-chloro-, 2,4- and 2,6-dichlorobenzoates as the sole carbon and energy source, however, it cometabolized 3-chloro-, 2,4- and 2,6-dichlorobenzoates, but not 4-chlorobenzoate. The yield of released chlorine during utilization of 2-chloro- and 2,5-dichlorobenzoates amounted to 100% of the theoretical. The concentration of 2-chloro- and 2,5-dichlorobenzoates, not substantially inhibiting the isolated microorganism, was within the range 0.25-0.5 and 2.5-3.0 g/L, respectively.

Effect of glucose and ribose on microbial degradation of the herbicide bromoxynil continuously added to soil

Bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) was continuously added to chernozem (Haplic typic) soil inoculated with a suspension of Pseudomonas putida capable of cometabolic decomposition of the compound in a hetero-continuous-flow cultivation setup. In the steady state, when glucose or ribose were simultaneously added, 90 and 47% of the added herbicide was degraded per day, respectively. If the saccharides were absent, only 10-27% of the herbicide was decomposed. Addition and removal of glucose feeding resulted in an increase and decrease, respectively, of the degradation intensity, irrespective of the amount of the bacterial decomposers present. Two degradation products, 3,5-dibromo-4-hydroxy-benzamide and 3,5-dibromo-4-hydroxybenzoic acid, were formed during cultivation. The total amount of bromine-containing compounds was reduced only in the presence of glucose.

Degradation of the herbicide bromoxynil in Pseudomonas putida

Biological conversion of the herbicide bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) was studied in a batch culture of Pseudomonas putida by using HPLC. The process had a cometabolic character and proceeded only in the presence of another, simultaneously metabolizable, carbon and energy source. The intensity of degradation correlated with the growth rate, the degradation stopping when the cosubstrate becomes exhausted or the pH value of the medium falls below 6.5. In a medium with glucose, no lag phase longer than one day was observed concerning growth, sugar and herbicide consumption and formation of metabolic herbicide derivatives (3,5-dibromo-4-hydroxybenzamide and 3,5-dibromo-4-hydroxybenzoic acid). In a medium with ribose, the initial lag of the above processes took 2 d. No formation of other degradation products was detected. Growth inhibition was proportional to the concentration of bromoxynil.

Degradation of 3-chlorobenzoate in soil by pseudomonads carrying biodegradative plasmids

Degradation of continuously added 3-chlorobenzoate (3-CB) was studied in samples of chernozem soil. Soil columns were inoculated with Pseudomonas putida growing on 3-CB and carrying the biodegradation plasmid and with Pseudomonas aeruginosa incapable of growth on 3-CB and carrying the inserted biodegradation plasmid pBS 2 determining ortho-cleavage of the aromatic ring. While the 3-CB degradation was observed in both inoculated variants, the native microflora of the soil under study was incapable to degrade 3-CB. Among pseudomonads isolated from inoculated soil at different stages of cultivation and growth on 3-CB, some had the taxonomic features of P. putida as well as those differing in 1-5 characteristics. The study of the activities of the enzymes cleaving the aromatic ring revealed the presence of pyrocatechol 1,2-dioxygenase in the isolated strains only, as estimated by means of benzoate and 3-CB as substrates.

Mineralization of 2,4-dichlorophenoxyacetic acid in soil simultaneously enriched with saccharides

Detoxication of 2,4-dichlorophenoxyacetic acid (2,4-D) in samples of chernozem soil was determined by a biological test and the time course of production of 14CO2, a product of microbial degradation of 2-14C-2-4-D, was measured during 38-d incubation at 28 degrees C in the dark. Enrichment of the soil with glucose (1000 ppm), two exocellular bacterial glucan and glucomannan polysaccharides (750 ppm), or a mixture of glucose with (NH4)2SO4 (C : N = 5 : 1) brought about acceleration of both detoxication and mineralization of 2,4-D (50 ppm) added simultaneously with the saccharides. Mineralization of the saccharides always preceded the degradation of the herbicide. The lag phase of 2,4-D mineralization did not exceed 3 d. In samples with saccharides the doubling time of the mineralization activity in the exponential phase of the process was substantially shortened and the mineralization of 2,4-D was accelerated even when the soil was inoculated with a suspension of soil in which microbial 2,4-D decomposers had accumulated. The extent of mineralization was not affected by the presence of saccharides (about 1/3 of the introduced radioactive carbon was transformed into 14CO2). All saccharides had a similar effect which reflected an increase in the overall bacterial count and in the relative abundance of bacterial 2,4-D decomposers . The role of other mechanisms such as co-metabolism in the stimulation of the degradation process is discussed.

Mineralization of 2,4-dichlorophenoxyacetic acid in soil previously enriched with organic substrates

Samples of chernozem soil were enriched with vanillic acid, protocatechuic acid glucose, a mixture of glucose and (NH4)2SO4 (C : N = 5 : 1), ethanol and 2,4-dichlorophenoxyacetic acid (2,4-D). After a 6-d (with 2,4-D 35-d) incubation during which primary oxidation of the introduced substrates occurred, the soil was supplied with a solution of 2-14C-2,4-D (50 ppm; 6.7 kBq) and production of 14CO2 (product of microbial degradation of 2,4-D) was measured. Previously enriched samples exhibited a higher degradation rate; both the lag phase and doubling time of mineralization activity in the exponential phase of the process were markedly higher. This reflected an overall proliferation of bacteria and the increased relative proportion of bacterial strains capable of mineralizing 2,4-D in enriched samples. The stimulation of 2,4-D degradation may involve specific adaptation and selection mechanisms (as in the case with samples previously enriched with 2,4-D or its structural analogues--aromatic monomers, ethanol) as well as non-specific mechanisms. The extent of mineralization of 2,4-D was not affected by soil pretreatment, about 1/3 of introduced radioactive carbon being invariably transformed to 14CO2.

Effect of glucose on the amount of bacteria mineralizing 2,4-dichlorophenoxyacetic acid in soil

Plate numbers of bacteria and relative incidence of strains capable of mineralization of 2,4-dichlorophenoxyacetic acid (2,4-D) in chernozem samples incubated for 14d with the herbicide (50 ppm) in the presence or absence of glucose (1000 ppm) were compared. Whereas the total number of bacteria increased 1.2-fold in the variant with 2,4-D and 2.4-fold in the variant with glucose and the herbicide, the number of 2,4-D-mineralizing bacteria increased 12.1-fold and 34.2-fold, respectively. In a collection of 96 isolates of soil bacteria substantially more strains capable of degradation of 2,4-D in the presence of glucose were detected as compared with the variant without it, indicating that processes of cometabolic type are involved during the degradation of this herbicide in the soil.

Microbial transformations of inorganic sulphur compounds in soil under conditions of heterocontinuous cultivation

Development and activity of the association of the sulphur cycle bacteria, represented by Thiobacillus thioparus and Desulfovibrio sp., were followed in chernozem soil continuously supplemented with sodium thiosulphate. The technique of heterocontinuous cultivation made it possible (i) to determine changes in the individual components of microflora involved in successive metabolic steps, their time and space sequence, (ii) to follow changes in the transformations of substrate and formation of metabolic products, and (iii) to reach a steady state in the system. A possible use of this approach for the evaluation of the effect of ecological factors, for modelling microbiological processes of the sulphur cycle, for the investigation of trophic relationships among microorganisms in natural and artificial association and for the evaluation of the geochemical activity of sulphur bacteria is discussed.

Acceleration by montmorillonite of nitrification in soil

A soil naturally containing montmorillonite (M) was amended with 10% M and sequentially perfused with glyeme, with fresh glyeme being added every 16--17d after nitrification of the previously added glycine-nitrogen had reached a plateau. In some systems, the old perfusates were replaced each time with a fresh glycine solution; in others, the initial perfusate was not replaced but only adjusted each time to the original 200 ml volume and a comparable glycine concentration (140 micrograms NH2-N/ml). The incorporation of M enhanced the rates of heterotrophic degradation of glycine and subsequent autotrophic nitrification, but these stimulatory effects decreased with each successive perfusion. The reasons for these decreases are not known, but they did not appear to be related to inorganic nutrition, as perfusion with a mixed cation solution after five perfusion cycles did not significantly enhance nitrification in either the check or M-amended soils during three subsequent perfusions with glycine. The enhancement of nitrification by M appeared to be a result, in part, of the greater buffering capacity of the M-amended soil, as indicated by lesser reductions in the pH of perfusates from the M-amended soil, by titration curves of the soils, and by the greater and longer stimulation of nitrification in the check soil amended with 1% CaCO3, which had a greater buffering capacity than did M. The stimulation by CaCO3 may also have been partially the result of providing CO2 for the autotrophic nitrifyers. Significant concentrations of nitrite accumulated only in perfusates from soil amended with CaCO3. Air-drying and remoistening the soils enhanced nitrification of subsequently added glycine, especially in the check soil. The importance of pH-mediation, of the production of inhibitors, and/or of feed-back inhibition was indicated by the lower rate and extent of nitrification in systems wherein the perfusates were not replaced between successive additions of glycine. Although the results of these studies confirmed previous observations that M enhances the rate of nitrification in soil, the mechanisms responsible for this stimulation are still not known.

The priming effect of glucose in soil sterilized by gamma-radiation and reinoculated with Cellulomonas sp

Mineralization of native organic matter and U-14C-glucose was followed by measuring the formation of CO2 and its radioactivity in chernozem soil presterilized by gamma-radiation and inoculated with a washed suspension of Cellulomonas sp. cells. The introduced bacteria mineralized the soil organic component to a higher extent in variants enriched with glucose. This so-called priming effect of glucose was observed also in the presence of chloramphenicol, inhibiting the growth of the bacteria. The increased mineralization of the native soil organic fraction was also detected in samples that were not enriched with glucose when the bacterial suspension was first disintegrated ultrasonically and the material then used for the inoculation. Possible participation of phenomena of the type of cometabolism and activation of cell membrane transport mechanisms on the occurrence of the priming effect of glucose in the soil is discussed.

Microbial activity in soil enriched with preparations of AS-lignin and lignofulvonic acid

In chernozem soil, enriched with preparations of AS-lignin or lignofulvonic acid, an increased production of carbon dioxide was observed during a 4-week incubation, and, as compared with the non-amended control, an increased number of bacteria but not of actinomycetes was detected. Increased numbers of fungi were detected only in the variant with AS-lignin at the end of the incubation. The relative incidence of bacteria utilizing vanillin, syringic acid or protocatechuic acid as the only carbon sources increased in the enriched medium. Oxidation of vanillic acid, syringic acid and to a lesser extent of coumarin increased in suspensions of soils incubated with AS-lignin or fulvic acid. The results obtained indicate that bacteria are involved in the mineralization of the added substrates and confirm the relationship between metabolism of these compounds and simple aromatic derivatives.

Biological decomposition of fulvic acid preparations

Decomposition of preparations of various fractions of fulvic acids in pure cultures of bacteria and in the soil was investigated. In the soils enriched with fulvic acids the amount of bacteria increased, oxygen consumption and formation of carbon dioxide followed a typical sigmoid curve. The above measurements indicated that mineralization occurred after a very short or negligible lag phase. During the decomposition of fulvic acids the ability of microorganisms to oxidize aromatic compounds, e.g. vanillic and p-hydroxybenzoic acid, increased. The presence of aromatic structures in the used preparations of fulvic acids was demonstrated on the basis of their IR spectra and according to the results of chromatographic analyses of their hydrolysates. The results indicated a relationship between metabolism of fulvic acids and aromatic compounds. In samples of the soil preincubated with glucose with fulvic acids decomposed more rapidly than in untreated samples.