List of experiments

Magnetospirillum: A Microbial Magnet Environmental Microbiology Experiments for middle and high school Christine L. Case Skyline College

Objective

Define the following terms: chemotaxis, geotaxis, phototaxis, and magnetotaxis.

Background

Almost all organisms use taxis, movement toward or away from a particular stimulus. Such stimuli include chemicals (chemotaxis), light (phototaxis), gravity (geotaxis), and the geomagnetic poles (magnetotaxis). Responses to environmental cues is critical for survival. If the signal is positive, called an attractant, the organism will move toward the stimulus. If the signal is negative, called a repellent, the organism will move away from the stimulus. The response exhibited by plant shoots as they grow toward light is called positive phototaxis (toward light). This response allows the shoots to capture sunlight. When snails are held in a jar, they will climb to the top of the jar. The stimulus is gravity and the response is negative geotaxis (away from gravity). This response allows snails to avoid drowning or ground-dwelling predators. In 1975, Richard P. Blakemore reported that sulfide-rich mud contained highly motile bacteria that "swam in the same geographic direction even when the microscope was turned about, moved to another location, or covered with a pasteboard box. To my astonishment, when a magnet was brought near the microscope, the hundreds of swimming cells instantly turned and rushed away from the end of the magnet! They were always attracted by the end that also attracted the north-seeking end of a compass needle and they were repelled by its opposite end. Their swimming speed was fast, and the entire population swerved in unison as the magnet was moved about nearby." Magnetotactic bacteria synthesize magnetite-iron oxide (Fe3O4)-and store it in inclusions called magnetosomes. The magnetosomes act like magnets and cause the bacterial cells to align on the Earth's geomagnetic field. These bacteria are found in sediments throughout the world. When Magnetospirillum magnetotacticum are separated from an attachment site, they swim along what appear to be geomagnetic lines. They move downward, toward either the north or south pole, until reaching a suitable attachment site. TDK Corporation is developing culture methods to obtain large quantities of magnetite from bacteria. They hope to use the magnetic particles in the production of magnetic tapes for recording sound and data.

Materials

Collect some mud in a wide-mouth jar and fill it to the top with water from the same source. Cover loosely and leave undisturbed for several days (low light is best-not in direct sunlight). You might collect the mud in several different places-a pond where cattails grow is very good. This enrichment will invariably improve as a source of magnetic bacteria over the course of several weeks to months if kept in dim light. Some jars have abundant magnetic bacteria after years on a lab shelf.

Procedure

Use a dropper to transfer a small amount of mud from the sediment/H2O interface to a slide (Figure 1a). A depression slide works best. Focus on the edge of the drop of water. Bring a bar magnet near the drop as shown in Figure 3: the end of the magnet which attracts the north-seeking end of a compass needle will also attract the magnetic bacteria. In a minute, you should see an accumulation of bacteria. These will reverse their direction when you turn the magnet around. Put a magnet near the jar and see if you get an accumulation (a small white speck) after stirring up the water in the area of the magnet (Figure 1b). Wait 5-10 minutes. Using a dropper, transfer a sample of this pellet to a slide for microscopic examination.

Figure 1. Collect cells with a bar magnet.
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Question

Propose a mechanism for birds, bees, and dolphins to migrate at night when the stars are obscured by clouds.

Source

R. P. Blakemore, "Magnetotactic bacteria." Annual Review of Microbiology 36:217-238, 1982. R. P. Blakemore, Pers. comm.