A trail of slime may have helped scientists solve the century-old mystery of how some bacteria travel: by jet propulsion. According to a report in this week's issue of Current Biology, tiny pores in the outer membranes of two species of bacteria secrete a stream of slime that propels them forward. Microbiologists suspect that many other species may be jet-setters as well.
Many bacteria swim with miniature propellers called flagella. But others seem to glide, sticking to a surface and then mysteriously oozing across it. Because these bugs have no external organs to propel themselves, some biologists have suggested other mechanisms, such as rolling along like tank treads or scooting like inchworms using the trails of slime as some sort of lubricant.
The slow pace of most bacteria has made the mystery even harder to crack, says Egbert Hoiczyk of The Rockefeller University in New York City. Most experimental bacteria crawl just fractions of a millimeter per day. Moreover, Hoiczyk says, the usual preparation of microscope samples tends to alter a cell's shape and destroy some of its outer layer.
To speed things up, Hoiczyk and Wolfgang Baumeister of the Max Planck Institute for Biochemistry in Martinsreid, Germany, worked on two species of cyanobacteria that can move at a brisk clip of 30 centimeters a day. By flash-freezing the bacteria, Hoiczyk could see that fragile pores extend all the way through the cell wall. Other microscope images suggest that the pores act as nozzles through which the bacteria excrete streams of slime. In a new twist, one of the species that glides along in a corkscrew motion has pores equipped with nozzlelike structures covered with helical protein channels, like the grooves in a rifle barrel. The other species lacks these nozzles and moves ahead in a straight line.
"This is the first solid evidence that bacteria move by a kind of jet propulsion," says microbial geneticist Phil Youderian of the University of Idaho, Moscow. He suspects that other bugs may use the same trick. But other microbiologists who have studied gliding bacteria remain skeptical. Edward Leadbetter of the University of Connecticut, Storrs, calls the new study "provocative" but hardly conclusive. "They haven't established whether the excretion of slime is a cause or a result of the movement."