Drug evaluation and genome analyses are two scientific fields that can benefit from performing studies at the single cell level. This is because biological effects observable at the single cell level may be lost when averaged over a collection of cells.
For example, a scientists may screen a drug against a population of bacteria, observe that only 10% of the bacteria died, and conclude that the drug is ineffective. However, screening against a single bacterium may tell the scientist that the drug is 99% effective against bacteria that lack a certain protein in the cell wall.
This would tell the scientist not only that the drug perhaps shows some kind of promise after all, but also provide clues as to the drug's mechanism of action (in this case, it has something to do with that protein in the cell wall). Such information would have been lost in the average behavior of all bacteria (in this case, most of which possess that critical protein in the cell wall).
A certain amount of technical knowledge and equipment is currently required to fabricate devices that enable scientists to screen individual cells, providing knowledge such as in the above hypothetical example. A device that requires minimal knowledge and equipment for fabrication would enable more scientists to carry out such experiments, especially those working in resource-limited settings.
Shih-hui Chao (Arizona State University) and coworkers have tackled this challenge. They utilized a cheaply fabricated patterned surface to seed approximately 3000 small individual droplets with bacterial cells in minutes.
Fabricating cell arrays.
The scientists call their device a microscale oil-covered cell array. They used ionized gas to turn a hydrophilic (water-attracting) glass microscope slide into a hydrophobic (oil-attracting) microscope slide.
They covered this hydrophobic microscope slide with an aluminum sheet patterned with an array of holes, each approximately 250 micrometers in diameter (a typical bacterium may be a few micrometers or less in diameter). Ionized gas was used to make the uncovered parts of the slide hydrophilic again.
This generated an array of hydrophilic spots on the microscope slide, each large enough to hold a cell, each surrounded by a hydrophobic boundary. Mineral oil and a cell suspension were mixed together on this patterned slide.
The cells are attracted to the hydrophilic regions of the slide, forming an array of cell droplets defined by the patterns on the slide. The mineral oil isolates the cell droplets, as well as both preventing water from the cell suspensions from evaporating (which would kill the cells) and contamination from the atmosphere.
The procedure requires only two minutes, about as fast as you're ever going to get (or need). The variation in droplet volume is estimated at approximately 23% from drop to drop, due to imperfections in the slide patterns.
Cell image quality.
It is obviously important that images of cells obtained from this technique are of high quality. This is potentially an issue, because mineral oil negatively impacts certain types of optical microscopy.
The scientists found that conventional microscopic imaging and fluorescence imaging were unaffected by the mineral oil. However, phase contrast imaging, a technique commonly utilized to highlight certain structural features in cells by displaying them as differences in reflected light intensity, was negatively impacted by the mineral oil.
Controlling the number of cells per spot.
The purpose of the scientists' work was to develop a cheap and easy method to image individual cells. They therefore wanted to determine the conditions necessary to place approximately 1 cell in each spot.
This depends simply on the concentration of cells one adds to the array. They found that a cell concentration of 210,000 cells per milliliter gives the highest proportion (32%) of spots possessing one cell, with on average a little over two cells per spot (close to single-cell occupancy).
Implications.
Chao and coworkers have reported an easy, cheap method of generating arrays of individual cells, desirable for many biological analyses. The needed equipment includes an ionized gas generator, glass microscope slides, mineral oil, and a patterned metal screen, all of which are available commercially at relatively low cost.
Although the goal of this research was to lower the cost of single cell analyses, their design is compatible with fast, automated imaging equipment (which is not cheap). This design should be very useful for all scientists, not just those on a limited budget.
for more information:
Lin, L.-I., Chao, S.-h., & Meldrum, D. R. (2009). Practical, Microfabrication-Free Device for Single-Cell Isolation PLoS ONE, 4 (8) DOI: 10.1371/journal.pone.0006710