SPLIT STOPPER [+]Enlarge Credit: J. Am. Chem. Soc.

Bacterial cell division is a surprisingly complex and mysterious process. If scientists could uncover the mechanisms that split cells, they could develop new antibiotics against virulent pathogens by targeting those processes. Now, researchers report a small molecule that inhibits bacterial cell division through a novel mechanism (J. Am. Chem. Soc. 2013, DOI: 10.1021/ja404640f). The compound may help researchers piece together steps in the division process and serve as a first step toward a new class of antibiotics, the researchers say.

When a bacterium divides in two, it enlists a cast of more than a dozen proteins to help. The proteins assemble at the dividing line, arriving either in an early phase or a late one. And basically, that’s where biologists’ understanding stops, says Douglas B. Weibel of the University of Wisconsin, Madison. “Nobody knows what happens between the accumulation of proteins at the division site and the division.”

To help solve the mystery, Weibel and colleagues searched for small molecules that could gum up the works of these division proteins. Using a high-throughput screening process, they found divin, a weak inhibitor of an enzyme called MipZ that coordinates where the cell splits in two. The researchers tested divin’s effect on cell division by treating Caulobacter crescentus bacteria with the molecule. They saw something they’d never seen before: The cell starts to divide, but the two daughter cells never separate. “It ends up looking like a dumbbell,” Weibel says.

But divin doesn’t stop cell division by inhibiting MipZ, the researchers found. When they treated Escherichia coli, which lack MipZ, with divin, the team saw the same dumbbell shapes. Based on those results, they concluded that divin also targets other critical division proteins.

BACTERIAL DUMBBELLS [+]Enlarge Credit: J. Am. Chem. Soc.

So far, all other molecules found to arrest bacterial cell division work directly or indirectly through a protein called FtsZ. It is the first of the proteins to arrive at the site of cell division, and it helps recruit other necessary proteins. One theory is that it generates a force that pulls apart the dividing cell. Yet when the researchers mixed divin with FtsZ, they saw no change in its enzymatic activity.

Because divin doesn’t inhibit FtsZ but does halt division, the researchers think FtsZ activity alone isn’t enough to split cells. Instead, Weibel says, divin’s activities support an alternate theory that the cell-splitting force is generated as enzymes build and restructure the cell wall between the two daughter cells.

More importantly, the lack of an effect on FtsZ showed that divin uses “definitely a new mechanism of inhibiting bacteria,” Weibel says. In preliminary experiments, his team has found that several late-phase division proteins don’t arrive at their proper spots in cells treated with divin. That narrows the search for divin’s target, but Weibel says they are still looking.