The waning black crescent is all that remains from an Escherichia coli sample. "If it could scream, it would," says University of Iowa microbiologist John Kirby, who led a recent study on bacteria behavior. The E. coli has fallen victim to Myxococcus xanthus, a type of bacteria that forms unique rippling waves as it feasts on other bacteria. During an attack, M. xanthus secretes enzymes to break down E. coli, and then each bacterium moves back and forth like a vacuum cleaner to suck up its food. Kirby coined the term "predataxis" to refer to behavior -- in this case, the rippling -- that is altered by the presence of prey. He thinks M. xanthus could provide a nontoxic way to get rid of unwanted microorganisms: "I can imagine that even antibiotic-resistant bacteria might be dealt with by a natural predator."