pairs on swarm agar. Three methods were used to detect interactions between the two species. (A)
B. pseudomallei and B. thailandensis were spotted on different poles of a swarm agar plate and the growth observed daily over a 72 h incubation time course. An example of “Complete inhibition” is shown in which a clear zone was present around the Bp1710a strain. In contrast, no clear zone was observed around BpK96243 (“No inhibition”) or control strain B. thailandensis E264. (B) Inhibitory effect of B. pseudomallei cell-free supernatant on B. thailandensis E264 motility on swarm agar. A B. thailandensis colony was spotted onto the center of swarm agar and incubated at 37°C for 16 h to allow initial swarming to begin, after which three drops of B. pseudomallei cell-free supernatant were inoculated at 2, 6 and 10 o’clock followed by further incubation for 16 hours. Swarming of B. thailandensis E264 was blocked around three spots of supernatant from B. pseudomallei 1710a supernatant (arrows), and is representative of results using strains that inhibited B. thailandensis swarming. B. pseudomallei K96243 is representative of strains that did not inhibit B. thailandensis swarming. Controls: B. thailandensis E264 colony with no supernatant added (top left panel), and B. thailandensis E264 colony with B. thailandensis E264 supernatant added (bottom right panel). (C) The inhibitory effect of B. pseudomallei cell-free supernatant on B. thailandensis E264 colony swarming. One hundred microliters of cell-free supernatant from B. pseudomallei was deposited at the center of a swarm agar plate and left to dry in air for 15 min. Subsequently, B. thailandensis were spotted at the center of the plate. Growth of the swarm colony was observed after incubation at 37°C for 18 h.