Dynamic fluid layer around immotile yeast colonies mediates the spread of bacteria

The survival of microorganisms crucially depends on the nature of their interactions with other cohabiting microorganisms. Often these interactions are mediated via chemical signals, and the role of physical factors is overlooked. In this study, we probe into the spreading characteristics of Pseudomonas aeruginosa, a flagellated bacteria in moisture-limited conditions and in the presence of immotile yeast colonies of Cryptococcus neoformans-a duo commonly known to cohabitate in nature. We find that bacteria spread faster in the presence of yeast, caused by the enhanced motility of bacterial cells in the vicinity of the yeast microcolonies. This enhancement is not a result of chemical signaling but due to the formation of a fluid layer around the yeast cells, which locally allows the flagellated bacteria to swim. Proliferation of yeast changes this fluid landscape around it and leads to the dynamic spread of bacteria. Factors such as the growth rate ratios and the seeding composition of the microorganisms determine the optimal spread of the bacteria. Physical factors such as these may have a crucial role in the dynamics of polymicrobial communities in various ecological environments.