Published: November 15th, 2012
We demonstrate a method to collect magnetotactic bacteria (MTB) that can be applied to natural waters. MTB can be isolated and enriched from sediment samples using a relatively simple setup that takes advantage of the bacteria's natural magnetism. Isolated MTB can then be examined in detail using both light and electron microscopy.
Magnetotactic bacteria (MTB) are aquatic microorganisms that were first notably described in 19751 from sediment samples collected in salt marshes of Massachusetts (USA). Since then MTB have been discovered in stratified water- and sediment-columns from all over the world2. One feature common to all MTB is that they contain magnetosomes, which are intracellular, membrane-bound magnetic nanocrystals of magnetite (Fe3O4) and/or greigite (Fe3S4) or both3, 4. In the Northern hemisphere, MTB are typically attracted to the south end of a bar magnet, while in the Southern hemisphere they are usually attracted to the north end of a magnet3,5. This property can be exploited when trying to isolate MTB from environmental samples.
One of the most common ways to enrich MTB is to use a clear plastic container to collect sediment and water from a natural source, such as a freshwater pond. In the Northern hemisphere, the south end of a bar magnet is placed against the outside of the container just above the sediment at the sediment-water interface. After some time, the bacteria can be removed from the inside of the container near the magnet with a pipette and then enriched further by using a capillary racetrack6 and a magnet. Once enriched, the bacteria can be placed on a microscope slide using a hanging drop method and observed in a light microscope or deposited onto a copper grid and observed using transmission electron microscopy (TEM).
Using this method, isolated MTB may be studied microscopically to determine characteristics such as swimming behavior, type and number of flagella, cell morphology of the cells, shape of the magnetic crystals, number of magnetosomes, number of magnetosome chains in each cell, composition of the nanomineral crystals, and presence of intracellular vacuoles.
1. MTB Collection
A magnet is an effective tool that can be used to isolate magnetotactic bacteria (MTB) contained in environmental samples (Figure 1A). A capillary racetrack (Figure 1B) uses the magnetic properties of MTB to attract them through a cotton plug where they can be separated from non-magnetotactic microorganisms also contained within the environmental sample.
Magnetotactic bacteria are not necessarily found in every aquatic environment8 but when they do occur, they can be found on the order of 100 - 1,000 cells per milliliter2. In order to observe the MTB using optical microscopy, you will need approximately 50 bacteria/ml in your sample8. If there are no or few MTB in your sample, then you will either need to select a new environmental site to collect your samples or you will need to try one or more of the techniques discussed in the next sec.......
This work was supported by grants from the U.S. National Science Foundation (EAR-0920299 and EAR-0745808); U.S. National Science Foundation East Asian and Pacific Summer Institutes; the Geological Society of America Research Grant Program and the Alumni Grants for Graduate Research and Scholarship from The Ohio State University. We would like to thank the editor and two anonymous reviewers for their insightful comments.....
|Glass Pasteur pipets
|Any plastic or glass container that can hold at least 0.5 L and can be sealed
|Microscope with 60X dry lens
|A 60X dry lens is not absolutely necessary, but this gives a high NA without using oil
|0.22 mm filter
|1 ml syringe
|Formvar/Carbon 200 mesh, copper grids
|Ted Pella Inc.
|Ted Pella Inc.
|Tecnai Spirit TEM
|Tecnai F20 S/TEM
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