A test plate shows the killing effect of some strains of Vibrio bacteria on another strain. The nutrient gel in this dish was evenly covered with a yellowish “target” strain. Then small spots of other bacterial strains were placed at random on top. “Halos” indicate where added strains killed the target bacteria. The size of a halo is related to the number of cells that died. The arrow indicates a spot of added bacteria that lacks a halo; the strain in this spot did not kill the target strain. (Photo courtesy of Hans Wildschutte)

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A test plate shows the killing effect of some strains of Vibrio bacteria on another strain. The nutrient gel in this dish was evenly covered with a yellowish "target" strain. Then small spots of other bacterial strains were placed at random on top. "Halos" indicate where added strains killed the target bacteria. The size of a halo is related to the number of cells that died. The arrow indicates a spot of added bacteria that lacks a halo; the strain in this spot did not kill the target strain. (Photo courtesy of Hans Wildschutte)