Deadly bacteria attack not only us, but each other with remarkable precision.

Vibrio cholerae, the scourge of nations lacking clean water. Pseudomonas aeruginosa, the microbe that plagues people with cystic fibrosis. Acinetobacter species, opportunistic organisms that can infect vulnerable people. Escherichia coli, a culprit in food-borne illnesses.

When these bacteria invade their human hosts, they can cause misery and death. But these pathogens also do battle with each other—if provoked. New research sheds light on the tiny war machines that bacteria wield in surprisingly precise and selective counterattacks against their bacterial foes. Real-time fluorescent microscopy catches what HMS scientists call “bacterial tit-for-tat.”

John Mekalanos, HMS Adele Lehman Professor of Microbiology and Molecular Genetics and head of the Department of Microbiology and Immunobiology, last year noted “T6SS dueling” among bacteria of the same species. The name describes the interactions between two sister cells’ type 6 secretion systems. These dynamic nanomachines can deliver a toxic protein by piercing the threatening cell. A previous postdoctoral fellow in the Mekalanos lab, Joseph Mougous (now at the University of Washington, Seattle) discovered that immunity proteins likely protect these sister cells from such attacks. But the fact that the sisters fight back was appreciated only when Marek Basler, a research fellow in Microbiology and Immunobiology, and Mekalanos watched these pitched battles under the microscope.

View source publication