Alladi Ramakrishnan Hall

Cooperation, resistance, and spatial patterning of microbial communities

Anupama Sharma

University of Michigan, Ann Arbor, Michigan, USA

Antibiotic resistance occurs as a consequence of a combination of processes operating at different scales. The mechanisms behind antibiotic resistance at a single cell level are well understood, but there is still a great deal that is unknown about how cells in a microbial community collectively respond to antibiotic stress, particularly in case of spatial expansion. Spatial expansion is particularly interesting because it enhances the genetic drift at the front of an expanding microbial colony, which leads to loss of genetic diversity due to the segregation of cells into regions (sectors) of the same genetic lineage. Existing theory suggests that in the case of mixed colonies of drug-sensitive and drug resistance cells, this spatial organization prevents the exploitation of 'public good' (cooperative resistance) due to de-mixing of cells, thereby favoring only drug-resistant cells to survive under antibiotic stress.

In this talk, I will present results from the colony-growth experiments that I performed on the semi-solid agar plate using two fluorescently labeled strains of E. faecalis, an opportunistic bacterial pathogen. Contrary to the theoretical expectations, we observe that the emergent spatial organization of bacterial communities coupled with cooperative resistance can help the evolutionarily unfit drug-sensitive bacteria to co-exist with the evolutionarily beneficial drug-resistant bacteria and can sometimes even dominate the microbial community due to the random population selection. Our model confirms that frequency-dependent cooperation can help the non-resistant cells to exploit the public good, thereby reducing the propensity of resistance cells in the community.