Science - November 29, 2011

Small droplets, big potential

Wageningen researchers have invented a device for isolating bacteria in water droplets. They hope eventually to use it to research the evolution of bacteria.

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The device, which looks like a small black chest, makes the droplets by pumping oil and water through a tube in turns. This creates a chain of water and oil droplets each containing 100 nanolitres (which fits into one millilitre 10,000 times). Because the bacteria are so diluted, each water droplet contains no more than one bacterium. And because you do not change the order of the droplets, it is easy to track individual bacteria throughout the course of an experiment; That, in a nutshell, is the secret of the device.
The device proved its worth by determining the minimum dose needed for an antibiotic to kill all bacteria. To do this the researchers filled more than 1,300 droplets with fluorescent bacteria. The droplets were then dosed with decreasing doses of the antibiotic cefotaxime. For twelve hours the researchers monitored how many bacteria were still alive by moving the droplets in the tube past a light detector. 
'It is an advantage to be able to look at so many bacterial cultures in parallel', says Merijn Salverda, researcher at the Laboratory of Genetics. The large number of readings makes for faster and more precise results.

Isolating individual bacteria is advantageous in itself, too. 'Then you can look at physiological heterogeneity', says associate professor Arjan de Visser from the Laboratory of Genetics. 'That is  variety that is not genetic'. This comes about, for instance, through the way the DNA is wound up and the unequal distribution of proteins among daughter cells.
 In spite of the device's advantages, it has not yet been fully developed. 'We would very much like to maintain the bacteria growth continuously', says Salverda.  At present the bacteria still die out en masse when their nutrient supply is finished. This makes it impossible to monitor them over a long period.
In a few years' time there should be a new model with more capacity and fewer teething troubles. De Visser thinks that this will open all sorts of doors to evolutionary research: 'For example, we want to look at the evolutionary capacity of bacteria. How quickly can strains adapt?' It will also be possible to determine more quickly which combination of mutations is useful: the 'fitness landscape'. De Visser: 'Traditionally we do this kind of experiment in ten test tubes. Because of the great variety you only find one solution. This is more exploratory, and here you find them all.'  
The developments involved various European partners but mainly a group of French physicists. The results will be published in December in the scientific journal Lab on a Chip.
 

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