Modeling of mixing in 96-well microplates observed with fluorescence indicators

Mixing in 96-well microplates was studied using soluble pH indicators and a fluorescence pH sensor. Small amounts of alkali were added with the aid of a multichannel pipet, a piston pump, and a piezoelectric actuator. Mixing patterns were observed visually using a video camera. Addition of drops each of about 1 nL with the piezoelectric actuator resulted in umbrella and double-disklike shapes. Convective mixing was mainly observed in the upper part of the well, whereas the lower part was only mixed quickly when using the multichannel pipet and the piston pump with an addition volume of 5 microL or larger. Estimated mixing times were between a few seconds and several minutes. Mixing by liquid dispensing was much more effective than by shaking. A mixing model consisting of 21 elements could describe mixing dynamics observed by the dissolved fluorescence dye and by the optical immobilized pH sensor. This model can be applied for designing pH control in microplates or for design of kinetic experiments with liquid addition.     

Turbo-mixing in microplates

How to effectively mix small volumes of liquids within microplate wells is a still underestimated and often neglected challenge. The method the authors introduce here relies on violent turbulent motion within a liquid caused by spotting an organic solvent drop onto its surface. The amount needed, less than 1 to 3 microL, is generally small enough not to alter bioactive molecules. Moreover, a solvent may be selected for its compatibility with assay components. The method was tested with layers of aqueous liquids that differ in pH and concentration of a pH-dependent dye, allowing mixing to be monitored optically. Rapid mixing was caused by spotting drops of alcohols, acetone, acetonitrile, and aqueous solutions of these, as long as the difference of surface tension between the drop and the uppermost layer of the bulk liquid surpassed 30 dynes/cm. Along with this difference, position and velocity of spotting, as well as viscosity and geometry of the bulk liquid volume, may influence the turbulence evoked. No significant difference was found for the activity of aspartate aminotransferase, alanine aminotransferase, and alkaline phosphatase when measured after mixing by shaking and after mixing by spotting 1 microL of methanol onto assays within 96-well microplates.     

A quantitative method for determining lectin activity on microplates

A modification of a known quantitative solidphase microtiter plate lectin assay elaborated by Hatakeyama and coauthors developed with biotinylated lectins and ExtrAvidin--alkaline phosphatase conjugate was proposed. The modification was performed in order to use a broad spectra of lectin ligands, including glycopeptides, peptides, etc. The optimal concentrations of the reagents and time of the reagents incubation were selected. Being known from the literature Kdiss for Concanavalin A and monosaccharides: methyl-Man, Man, Glc and Gal were determined in a model experiments. The results were in agreement with the range of monosaccharides specificity for the Concanavalin A. The modification described is relatively simple and sensitive.     

Approximate solution for capillary exchange models with and without axial mixing

Explicit expressions are obtained which give quite accurate values for the effluent blood concentration for the model with axial mixing in tissue for the cases in which the initial capillary concentration is either zero or one. These simple expressions are useful since the formal solutions of Gosselin and Gosselin (1987,Bull. math. Biol 49, 329–349) are cumbersome to use. Reasonably accurate expressions are also provided for the model without axial mixing since the formal solution for the case in which the initial capillary concentration is zero (Gosselin and Gosselin, 1987) and for the case in which the initial concentration equals that of the tissue, the solution for which is provided here, require numerical integration.     

Silver ion microplates for immunoassays.

Microplate wells can be coated with silver ions using glutaraldehyde as a spacer molecule and thiourea as a complexing ligand. Microwells containing surface silver ions are shown to immobilize biotin-labeled horseradish peroxidase (HRP) in active form, while showing very little affinity for the unlabeled enzyme. These plates can also immobilize biotin-labeled antibodies that exhibit bioactivity after immobilization. Silver ions are needed for the complexation of the biotinylated enzyme or antibody because microwells modified to contain surface amine or thiourea molecules do not immobilize appreciable amounts of the labeled proteins. A maximum surface coverage for biotin-labeled HRP of 40 ng/cm2 and an immobilization binding constant of Km = 8 x 10(9)/M are determined from serial dilutions in a microplate. Detection of as little as 6.7 fmol HRP is achieved using antibodies immobilized on the silver ion-modified microplates. Active antibody surface densities were estimated to be between 130 and 260 nm2/antibody molecule. Background binding of HRP to the modified silver ion microplates was very low, allowing for reasonably accurate detection between 10(-14) and 10(-11) mol HRP.     

A rapid screening method for micro-organisms degrading polycyclic aromatic hydrocarbons in microplates

A method for enumerating micro-organisms degrading polycyclic aromatic hydrocarbons (PAHs) was developed. The micro-organisms present in water samples are incubated in 96-well microplates, in which the desired PAHs are available as sole carbon source in a liquid mineral-salts medium (MSM). Walls and bottoms of the wells in the microplates are covered with PAHs by dissolving them in a non-polar solvent and pipetting this solution into the wells. After solvent elimination under vacuum, the PAHs remain on the surface of the wells. The formation of coloured products during microbial degradation of PAHs causes colouring of the MSM, thus allowing evaluation of the cell titre by determining the most probable number. Usage of an electronic multichannel pipette makes the work faster and more effective. This allows the inoculation of several microplates pre-treated with different PAHs out of one serial dilution. On the one hand, this method is very effective in screening the usability spectrum of different PAHs microorganisms; on the other hand it allows the additional employment of other sources of hydrocarbons. Correspondence to: M. Stieber     

A continuous-flow capillary mixing method to monitor reactions on the microsecond time scale.

A continuous-flow capillary mixing apparatus, based on the original design of Regenfuss et al. (Regenfuss, P., R. M. Clegg, M. J. Fulwyler, F. J. Barrantes, and T. M. Jovin. 1985. Rev. Sci. Instrum. 56:283-290), has been developed with significant advances in mixer design, detection method and data analysis. To overcome the problems associated with the free-flowing jet used for observation in the original design (instability, optical artifacts due to scattering, poor definition of the geometry), the solution emerging from the capillary is injected directly into a flow-cell joined to the tip of the outer capillary via a ground-glass joint. The reaction kinetics are followed by measuring fluorescence versus distance downstream from the mixer, using an Hg(Xe) arc lamp for excitation and a digital camera with a UV-sensitized CCD detector for detection. Test reactions involving fluorescent dyes indicate that mixing is completed within 15 micros of its initiation and that the dead time of the measurement is 45 +/- 5 micros, which represents a >30-fold improvement in time resolution over conventional stopped-flow instruments. The high sensitivity and linearity of the CCD camera have been instrumental in obtaining artifact-free kinetic data over the time window from approximately 45 micros to a few milliseconds with signal-to-noise levels comparable to those of conventional methods. The scope of the method is discussed and illustrated with an example of a protein folding reaction.     

UV measurements in microplates suitable for high-throughput protein determination

An UV spectrophotometric method for protein determination using microplates is described. Using the SPECTRAmax PLUS reader, the UVStar 96- and 384-well microplates and a 96 or 384 parallel channel liquid handling technique, large-scale determinations can be performed with intraassay precision better than 3% CV (coefficient of variation) in the range from 1 to 8000 microg of protein/ml, measuring at 205, 215, and 280 nm and using different volume-dependent light-path lengths. Since the absorbance coefficient at 205 nm is found to be 30 ml/(mgxcm) for eight different proteins with a CV of 5.6% only with the Path Check option of the reader, protein concentration can be determined without any individual calibration. Samples in the volume range of 60-250 microl can be analyzed without time-consuming and expensive treatment and without sample loss. Using a special 96 or 384 parallel dialyzing device, low molecular weight substances which interfere with the analysis by their UV absorbance, such as buffers and detergents, can effectively be removed. Application examples for serum protein separation are also shown in the presence of the strongly UV absorbing detergent Triton X-100.     

A comparison of silver ion to streptavidin coated microplates

Direct comparisons are made between covalently linked streptavidin and silver ion coated microplates. Both coatings can immobilize biotinylated molecules. Silver ion coated microplate wells can immobilize 1.8 times higher amounts of biotin labeled horseradish peroxidase. The quantitation range and capacity for the capture of horseradish peroxidase using biotin labeled horseradish peroxidase are also greater for silver ion coated microplates. Approximately twice as many anti-horseradish peroxidase antibodies can be immobilized per well using silver ion coated microplates. Higher capacities are presumed to be due to the smaller footprint of silver ions as compared to streptavidin. A direct comparison between the two coatings for a beta-galactosidase ELISA showed that while the silver ion coated microplates gave higher readings, the streptavidin coated microplates exhibited smaller well-to-well variation. However, higher well to well variation for the silver microplates is attributed to the high density of anti-beta-galactosidase antibodies on the microplates and the weak binding of clone GAL-13 to beta-galactosidase, rather than the silver coating. These studies suggest silver ion coated microplates are a desirable alternative to streptavidin plates for quantitative immunoassays.     

Myocardial capillarity and maximal capillary diffusion capacity in exercise-trained dogs

Our purpose was to determine whether changes in myocardial capillarity underlie the exercise training-induced increases in coronary transport capacity previously observed in dogs (J. Appl. Physiol. 58: 468-476, 1985). The approach was to measure capillary diffusion capacity (PS) in working hearts and then measure capillary numerical density (CD), capillary surface area density (CSA), and capillary volume density (CV) in specimens from perfused-fixed hearts. Eight dogs (20-30 kg) were exercise trained (ET) for 12-18 wk and compared with a group of seven control dogs. PS for 51Cr-labeled ethylenediaminetetraacetic acid was determined during maximal adenosine coronary vasodilation with perfusion pressures equal to 100 mmHg in both groups. The trained dogs' maximal PS averaged 58 +/- 10 ml.min-1.100 g-1, which was significantly greater than the control value (31 +/- 6). Maximal PS was linearly related to CV (r = 0.61) and CSA (r = 0.78) in the ET group. However, there was no difference between control and trained average left ventricular CD, CSA, CV, or intercapillary distance. The data indicate that although coronary blood flow capacity and capillary transport capacity may be improved in exercise-trained dog hearts, these changes are not the result of an increase in myocardial capillarity. Rather, the increased maximal PS appears to be due to changes in the determinants of capillary blood flow and/or the relationship between capillary area available for exchange and capillary perfusion.     

Standardized measurements and differential spectroscopy in microplates

Microplates (MPs) are excellent devices for the parallel processing of multiple samples for the spectroscopic analysis of chromophores and turbidity, for luminometric measurements, for cell culture applications, or simply for sample storage, library organization, and other high-throughput (HTP) processes. Disadvantages include an ill-defined pathlength, meniscus formation, evaporation, and cross-contamination. Here, we have developed a novel MP and lid system which can serve to minimize these drawbacks. Cup-like lids are inserted into MP wells. Thereby, liquid is pushed aside. The flat bottoms of the cup-like lids guarantee a planar interface and a defined pathlength. In addition, the devised MP system allows for differential spectroscopic analysis of multiple samples comparable to measurements in tandem cuvettes. This was shown by the investigation of the binding of reduced nicotinamide adenine dinucleotide to dihydrolipoamide dehydrogenase. The MP lid system described offers a low-cost solution for standardized spectrophotometric quantitations in any solvent compatible with the MP/lid material. In addition to the system's suitability for routine MP application, it should be advantageous as a simple and noninvasive method, i.e., no labeling and immobilization of analytes is required for detection of the interaction of molecules, for various HTP applications and drug screening purposes.     

Pulmonary diffusing capacity and capillary blood volume in aging dogs.

Single-breath carbon monoxide diffusing capacity (DLco), pulmonary capillary blood volume (Vc), and membrane diffusing capacity (Dm) were measured in 24 beagle dogs aged 289-3,882 days. DLco and Vc were a function of age and alveolar volume (Va). Vc decreased with age resulting in changes in DLco. Changes in Vc may have been due to pulmonary morphological changes or to an exaggerated decrease in pulmonary blood flow in old dogs in response to 20-30 cmH-2O transpulmonary pressure. There was no age-related change in Dm.     

Photosynthetic capacity at the surface microlayer during the mixing period

The pbytoplankton biomass and primary production of the seasurface microlayer (upper 3 mm) and the bulk water (1, 10, 20,30 and 40 m) were investigated in Saronicos Gulf, Aegean Sea,during the period of November-December 1987. The experimentswere performed by using newly developed sampling and incubationin situ techniques regarding the surface microlayer. In thewell mixed water column, maximum assimilation ratios rangingfrom 2.63 to 10.97 mg C mg Chla^–1 h^–1 were recordedat 1 m depth and these values were reduced to {small tilde}45%at the surface microlayer regardless of the uniform distributionof chlorophyll a in these layers. The 1% light level variedbetween 20 and 40 m depth and the assimilation ratio at thislevel averaged 12% in relation to the maximum value at 1 m depth.The problems associated with the static bottle incubations andthe type of glass of the bottles used are discussed with respectto the photosynthetic measurements at the surface microlayer.     

Oxidative capacity and capillary density of diaphragm motor units

Motor units in the cat diaphragm (DIA) were isolated in situ by microdissection and stimulation of C5 ventral root filaments. Motor units were classified based on their isometric contractile force responses and fatigue indexes (FI). The muscle fibers belonging to individual units (i.e., the muscle unit) were identified using the glycogen-depletion method. Fibers were classified as type I or II based on histochemical staining for myofibrillar adenosine triphosphatase (ATPase) after alkaline preincubation. The rate of succinate dehydrogenase (SDH) activity of each fiber was determined using a microphotometric procedure. The location of capillaries was determined from muscle cross sections stained for ATPase after acid (pH = 4.2) preincubation. The capillarity of muscle unit fibers was determined by counting the number of capillaries surrounding fibers and by calculating the number of capillaries per fiber area. A significant correlation was found between the fatigue resistance of DIA units and the mean SDH activity of muscle unit fibers. A significant correlation was also observed between DIA unit fatigue resistance and both indexes of muscle unit fiber capillarity. The mean SDH activity and mean capillary density of muscle unit fibers were also correlated. We conclude that DIA motor unit fatigue resistance depends, at least in part, on the oxidative capacity and capillary density of muscle unit fibers.     

MTT方法评价微生物细胞活性的探讨

对MTT比色法用于评价微生物细胞活性进行了探讨。本文以大肠杆菌为模式菌株,研究了不同浓度MTT、不同用量、在不同时间对试验结果OD570值的影响,结果表明细菌数在4.9×107－4.9×108个/mL范围内测出的OD570值与细菌浓度呈良好的正相关,0.5 mg/mL MTT用量20 L,反应时间20 min时效果最佳,其相关回归方程为y = 0.1769x + 0.03,R2 = 0.9983。     

A biological system for studies on mixing in bioreactors

A method for studying bioreactor inhomogeneity is suggested. Oxygen depletion in an E. coli cultivation leads to mixed acid fermentation with hydrogen gas production. Using a palladium metal-oxide semiconductor (Pd-MOS) hydrogen sensor on line in the effluent gas, it is shown that in a batch culture of 1 m³ hydrogen gas was evolved even before the dissolved oxygen tension (DOT) as measured by the probe reached zero. This suggests an appreciable degree of inhomogeneity with respect to DOT. Using measurements performed off line, it was observed that the lag time for hydrogen to appear became greater if the cells had been subjected to oxygen depletion. Lack of oxygen also resulted in a lowered hydrogen evolution capacity of the cells.     

A modified capillary assay for chemotaxis

Summary A modification is described of the capillary assay for chemotaxis. It employs a 96-well dilution plate and its cover. Capillary tubes are inserted through the cover and are supported by small rubber collars. The method is faster and less tedious and gives more precise results than earlier methods.