Scientists at the Naval Research Laboratory (NRL) and Purdue University have shed light on one of microbiology's most fascinating mysteries--why some bacteria are naturally magnetic.
Their description of how being magnetic "helps" the bacteria is reported in the August 2006 issue of the Biophysical Journal.
Magnetic bacteria are found in a variety of aquatic environments, such as ponds and lakes. The strain of bacterium the research team studied, Magnetospirillum magneticum, was originally found in a pond in Tokyo, Japan. Magnetic bacteria typically live far below the surface, where oxygen is scarce. (They do not grow well where oxygen is plentiful.) What makes them fascinating is that they naturally grow strings of microscopic magnetic particles called magnetosomes. When placed in a magnetic field, the bacteria align like tiny swimming compass needles, a phenomenon call magnetotaxis.
The research team is using genetic engineering to create a strain of the bacteria that become magnetic only when exposed to specific toxic chemicals, with the goal of using them as living chemical sensors. As a first step, they have created a strain that cannot make magnetosomes and therefore is not magnetic. Dr. Lloyd Whitman from NRL, who led the research team, explains that "during the course of our research, we realized that nobody had ever really demonstrated that being magnetic actually helps the bacteria." "Genetic modification allowed us to directly observe differences in behavior between magnetic and non-magnetic versions of the same bacterium," adds Professor Bruce Applegate. Professor Applegate directed the genetic engineering at Purdue, with the assistance of Professor Lazlo Csonka, Dr. Lynda Perry, and Ms. Kathleen O'Connor.
In the past, scientists had suspected that being magnetic helps a bacterium find the oxygen concentrations it prefers more quickly by swimming only up and down in the earth's magnetic field rather than randomly in all directions. An analogy would be a blind-folded mountain climber searching for a specific altitude. If she only climbs straight up or down the mountain, she should find it more quickly. "But by observing how the bacteria moved away from oxygen that we added to their environment," reports Dr. McRae Smith, a member of the team while a postdoctoral researcher at NRL, "we directly measured how much magnetotaxis helps." NRL researcher Dr. Paul Sheehan adds, "by mathematically modeling their motion, we determined that being magnetic actually makes the bacteria much more sensitive to oxygen when in a magnetic field, so that they swim away from oxygen at much lower concentrations." It is as if the climber gets tired and turns around sooner when heading up the mountain, keeping her from heading too far in the wrong direction. And the stronger the magnetic field, the bigger the effect. The scientists do not yet know how the magnetic field has this affect on the bacteria, and are currently conducting additional experiments to help answer that question.
What was particularly interesting to the scientists was that the affect of being magnetic was too small for them to measure in the earth's natural, but weak, magnetic field. "Therefore," concludes Dr. Whitman, "the advantage to these bacteria in nature must be very small." "But over millions of years, this very subtle advantage has somehow produced bacterial magnetism."