Bacteria and the viruses that infect them—known as phages—coexist in soils, oceans and living organisms. Scientists have long thought this coexistence reflects a microscopic version of an evolutionary arms race: As parasites and hosts continually adapt to one another, both evolve ways to survive. But a joint study by researchers at the University of Maryland and the University of Exeter shows different pathways to coexistence.
The researchers found that when bacteria move together to find nutrients, their collective motion helps them stay ahead of phages. Yet collective bacterial migration has an unexpected vulnerability: it enables phages to infect a fraction of cells near the migrating front edge, while the remainder remain out of reach. The result is continual co-migration of parasites and hosts, even without coevolutionary adaptations. The findings were published in the journal Science Advances on October 21, 2026.
"This migration-mediated mechanism shows how viruses and their microbial hosts can coexist in the absence of arms races," said study co-lead Joshua Weitz , a professor of biology at UMD with a joint appointment at the University of Maryland Institute for Health Computing . "The coexistence relies on the collective migration of the bacteria that continually brings phages nearly to the leading edge of the front—so that neither bacteria nor phage can overtake the other. In a sense, migration and infection generate a détente between parasite and its host."
A key factor in this phenomenon is chemotaxis, or bacteria moving in response to changing chemical levels in the environment. As bacteria deplete a nutrient resource, the resulting chemical change compels them to move forward to find more nourishment—creating a "chemotactic wave."
For this study, the scientists used agar plates more than 50 times larger in area than conventional culture dishes as the study landscape, introduced bacteria and their phages and used time-lapse imaging to watch the bacteria migrate.
"The study began with a question: What if a small number of phages were added in front of a wave of migrating bacteria?" said Weitz, who also holds the Clark Leadership Chair in Data Analytics. "Would the phages take over? Could the bacteria outrun the phages? We expected to figure out the conditions when one of those alternatives would happen—end of story."
To the scientists' surprise, the phages formed a path down the middle of the bacterial colony.
"Our collaborators at the University of Exeter observed what appeared to be a lytic highway of infection that moved at the speed of collective migration but always remained just short of the bacteria's leading edge," Weitz explained. "The phages were killing enough bacteria to progress, but not enough to overtake the bacterial population."
The scientists also noted that the wave carried the phages much farther and faster than they would progress in isolation.
To investigate the mechanisms underlying this unexpected phenomenon, the researchers developed a mathematical model that precisely reproduced the dynamics observed in their experiments.
"Our model clearly showed that the balance between viral killing and bacteria chemotaxis produces a stable propagation of phage infections, transported by migrating bacteria for remarkable distances for different conditions and phage-bacteria pairs," noted the paper's co-lead author, UMD Biology Post-doctoral Associate Jacopo Marchi .
The model was crucial, Marchi said, for confirming that chemotaxis, which couldn't be changed experimentally, was needed for the movement-mediated system to function.
The study authors pointed out that the system is not inevitable—some computer-simulated adjustments to key parameters (for example, the amount of nutrients available to the bacteria) allowed the bacteria to escape the phages altogether or caused local extinction of the host population. Migration-mediated coexistence occurs in a variety of systems, including phages that infect clinically relevant bacterial pathogens like Escherichia coli and Pseudomonas aeruginosa.
"The use of a spatially explicit phage-bacteria infection model helped reveal the drivers of this robust balance between infection and chemotactic escape," Weitz said.
Understanding this mechanism is a step toward treating stubborn infections.
"In a case where a bacterial pathogen colonizes a surface, say a prosthetic or wound, it is essential to understand microbial movement," Weitz said. "In this case, migration mediates both the potential of and limits to phage-based elimination of bacterial pathogens and suggests new directions for exploring the dynamics of coexistence beyond the world of shaken flasks."
