How can you see them?


Having collected the samples let the samples settle.

Using a pasteur pippette, suck up a few millilitres of liquid and a little bit of soil from your sample, near the soil/water interface. You want a little soil, as this is where we think they 'live', but too much soil will make viewing difficult.

Rest your pippete on the bench with the end of the pippete facing one pole of a bar magnet.

Wait for a few minutes to allow the bacteria to migrate towards the magnet. Bacteria typically travel between 0.1 and 1 millimetre per second.

diagram of hanging dropPippette the end drop of liquid (which should have attracted many of the critters) onto a cover slip- be mindfull of where the magnet is when you do this. Pippetting on the wrong side of the magnet may make some of the bacteria migrate away from the end drop. The drop should be small enough so that it will hang on the bottom of the cover slip and not touch the rubber or glass slide below.

Place a rubber seperator (an 'O' ring seal from a hardware store is ideal) on a microscope slide. Thicker seperators are preferred (more distance between the cover slip and microscope slide), and serve to keep the water off the microscope slide.

The idea is to focus on the edge of a hanging drop of liquid. Turn the cover slip upside down, in a fluid motion. Surface tension should stop the drop from falling off the underside of the cover-slip. If not, either your drop is too big or you need to practice inverting the slide. An arcing motion is best.

The rubber seperator is to keep the drop from touching the microscope slide. Place the cover slip on the rubber seperator such that the drop is hanging in the middle of the seperator- if the drop touches either the seperator or the microscope slide, start again.

Refer to the diagram to check your setup.

Place the setup on your microscope.

Professor Frankel looks for magnetic bugsMove the slide around - with the slide out of focus you should notice two large dark circles as you move the slide around. The outer circle is the rubber seperator, while the inner circle is due to the thin film at the air/water interface at the edge of the drop. Centre on one part of the inner circle. At first this may be larger than the field of view of the microscope. As you raise the platform (or lower the lenses) the cover-slip should come into focus, and the dark ring caused by the edge of the drop will become thinner (This is not the case for the seperator).

Bring the edge of the drop into sharp focus- when the dark outline of the water drop just disappears. Place the same pole of the magnet that you used to attract the bacteria to the end of the pippette near the edge that you are focused on (remember that microscopes give images up-side down and back to front).

After a minute you should see some of the bacteria swimming at the appropriate edge of the water drop with just 100 times magnification- try adjusting the focus so you can see them better. Check the other side of the drop - are there any bacteria on the other side? What does this say about them? Remember that the bacteria will respond to the direction of the magnetic field- you need the magnet in place to see them migrate to one edge. Try moving the magnet away and back again.

It is thought that the drop, with its high surface area to volume ratio, admits too much oxygen to support the bacteria longer than about an hour. The bacteria are trying to escape the drop of water, and are using the magnetic field as a guide to the plane in which they are " tasting " the toxicity. Even the centre of the drop is too oxygenated for the bacteria, and this is why they can be attracted to the edge, despite this area being the more toxic site.

Remember that in the drop of water, bacteria will survive only an hour or so. In the jar, correctly lidded and kept in a dark place, the bacteria will survive for months, perhaps years. Some samples will become more populated as they age, others will become less populated.