UC San Diego

The gram-positive rod-shaped bacterial species Bacillus subtilis exists in two morphological states: the vegetative cell and the spore. Which state B. subtilis exists is dictated by its environment. While this bacterial species is well studied, most research focuses on only one of its two morphological states at a time. Furthermore, bacteria live in diverse microbial environments that place different species in close proximity to each other. Yet, B. subtilis interactions with other microbial species is poorly understood. Using low magnification time-lapse imaging, we first characterized microbial interactions with respect to the spore entity of B. subtilis. We find that B. subtilis wild-type spores do not outgrow in the presence of P. aeruginosa wild-type cells. Next, we used high magnification time-lapse phase microscopy; we find that B. subtilis spores germinate but do not outgrow on P. aeruginosa acellular conditioned media pad. Then we investigated interspecies cellular interactions between B. subtilis wild-type vegetative cells and P. aeruginosa wild-type cells at the single cell level; we find that B. subtilis cells die in the presence of P. aeruginosa cells. This is the first study in microbial interactions to identify a molecular mechanism using P. aeruginosa acellular conditioned media to study the response of B. subtilis spores. This study demonstrates that the response of B. subtilis spores to P. aeruginosa is conserved in both the P. aeruginosa cellular condition and acellular conditioned media. We also demonstrate that B. subtilis cells die in the presence of P. aeruginosa cells; however, time did not permit for us to explore if this mechanism is dependent on the physical presence of P. aeruginosa cells or is also induced by P. aeruginosa acellular conditioned media. Mapping interactions of microbial species and deciphering the molecular mechanisms of these interactions adds depth to our understanding of microbial ecology.