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Screening Assays
Screening Assays

High throughput screening is the term given to large-scale assays, in which many experimental samples are subjected to simultaneous testing under a given condition or conditions. The samples themselves may take the form of biochemical agents such as enzymes, immobilised nucleic acids, or live cells.

Almost any conceivable condition can be tested – for example enzyme concentration, reaction temperature, or the presence of toxins or drugs in the samples. The development of automated lab robotics has driven the use of high-throughput screening, allowing large data sets to be acquired quickly and easily.

Principals

  • High-throughput screening normally requires samples to be prepared and arranged in a regular array, either in multiwall dishes, or in the case of live cells, growing in an orthogonally-arranged pattern.
  • Tens to thousands of samples can be screened during a single experiment, allowing the measurement of multiple experimental replicates, test conditions and differences between samples.
  • Automated robotics can reduce the need for manual manipulation of the samples; loading, sampling, measuring, and documenting the output data. This greatly improves accuracy and reproducibility of the screen, as well as the volume of samples that can be tested.
  • Data acquired from high-throughput screens can be processed and analysed immediately by integrated software, or stored for later analysis.

Theory

The development of powerful, affordable computing and robotics has driven the use of high-throughput screening as a viable tool for research labs around the world. Previously unattainable types and volumes of data can now be acquired, providing deep insight into complex biological processes.

Three common functions can be found in the robotics and software that perform screening experiments:

Colony-picking

It is sometimes necessary to pick, replicate, and re-array microbial colonies growing on solid or in liquid media. The ability to quickly and accurately manipulate arrays of yeast, bacteria and fungi allows high-density experimental comparison between any combination of mutant microorganisms.

Kinematic robotic pinning heads can rapidly pick and redistribute microbial colonies between multiple plates using sterile pins. An example of this is the RoToR screening robot which can be seen in action here.

The ability to easily and quickly prepare an array of samples greatly increases the efficiency and utility of high-throughput screening.

Plate Elevation

An example of a 96-format array of yeast cells growing on a rectangular agar plate. The grid-like arrangement of colonies allows robotic systems to select, pick, and replate onto new dishes and medium.

Plate Elevation

Robotic arm positions itself over a plate containing a collection of yeast colonies.

The arm descends, picks up colonies on attached sterile pins, raises again, and rapidly moves to a new target plate position.

The arm descends once more, the pins contact with the new plate, and colonies are ‘printed’ in the same orthogonal arrangement as the source plate.

Pinning

Liquid-handling

Biochemical reactions often require the dispensing of precise volumes of liquid reagents. Dedicated liquid-handling robots can rapidly pipette specific volumes into large-format multi-well plates.

The ability to add, mix, or decant liquid reagents to many wells simultaneously, allows high-throughput screening of drugs, toxins, chemicals, or bioactive compounds. It also allows cells suspended in liquid culture to be diluted, sampled, or otherwise manipulated, in a highly precise manner.

Robotic head collects fresh pipette tips and lowers tips into wells of a multiwall plate. Liquid is aspirated from source wells and robot moves to new target plate.

Robotic head lowers the pipette tips into the target plate wells and dispenses the desired volume.

Liquid Handling

Optical Measurement

Screens
Colometric assays Turbidity measurements Cell marker expression Colony growth & cell death

Many high-throughput screens are interpreted though optical measurements – colour changes in cells and liquid reactions, the turbidity of a liquid culture, or the growth and size of microbial colonies.

Very sensitive spectrophotometric devices are sometimes built-in to screening robotics, and automatic measurements made and recorded for analysis. Other assays may require whole-plate imaging using flatbed or other scanning systems.


An example of a high-throughput screen

A yeast researcher may be interested in performing a high-throughput, yeast 2-hybrid assay, investigating the interactions between their query protein, and a collection of 1536 other proteins. Positive interactions will be shown through the expression of a GFP reporter gene.

1536 yeast strains in liquid culture are selected from several plates using a robotic liquid-handling module. The robot spots a small volume of yeast onto solid agar then transfers the plate into an incubation module. When the colonies have grown sufficiently, the plate is transferred onto the RoToR screening robot, which pins the strains onto a library of 1536 query-protein colonies. Following mating, the robot re-pins the diploids onto selection media and the plate is once again incubated.

After a suitable incubation period the robot picks the 1536 test strains and re-plates into liquid media in several multiwell dishes. These dishes are fed into a colorimeter which detects and records differing levels of GFP emission between colonies. In parallel, the original plate is fed into a scanning unit which images the entire plate for GFP expression and colony size.

This data is collated and analysed to flag positive 2-hybrid protein interactions and measure any effect on colony growth.

Robotics and modularity

The drive to create fully-automated, integrated or modular robotics has produced a large range of instruments for high-throughput screening. In addition, the development of microarray technology has allowed researchers to combine visual screening data with expression information from thousands of genes.

Bolt-on modular robotic components expand the range of possible screening assays whilst maintaining flexibility for labs involved in this type of experimentation.

The ability to perform high-density, high-throughput screening experiments has yielded huge quantities of data, but has also given researchers deep insight into complicated information networks and biological systems.

High Throughput Screening: RoToR HDA

For more information regarding screening assays using the RoToR HDA high throughput screening robot click here...

Read more...
 
Synthetic Genetic Array (SGA)
Synthetic Genetic Array

Synthetic Genetic Array (SGA) analyses high-density genetic screens that identify synthetic interactions and genetic networks

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Yeast Two-Hybrid
Yeast Two-Hybrid

2-hybrid assays are powerful screening tools to identify protein-protein interactions

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Algae Screening
Algae Screening

Chlamydomonas reinhardtii has become a useful model organism to do high-throughput genetic screens in green algae

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Bacteria Screening
Bacteria Screening

eSGA analysis adapts the SGA principles to study genetic networks and synthetic interactions in bacteria

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