Increased similarity of aquatic bacterial communities of different origin after antibiotic disturbance.

Stochastic or deterministic processes control the bacterial community assembly in waters and their understanding is a fundamental question to correctly manage aquatic environments exposed to the release of antibiotics from anthropogenic sources. It has been suggested that microdiversity (i.e. the rare biosphere) convers freshwater communities with stability, meaning that previously rare taxa bloom when the community is disturbed. Since there might be a seed bank of similar, but not abundant, bacterial taxa in different waters, we tested whether a disturbance by an antibiotic cocktail would increase similarity in bacterial communities from different freshwater systems (a wastewater effluent and two lakes). In a continuous culture set-up in chemostats, we show that disturbance with antibiotics causes communities from different environments to become more similar. Once the antibiotic pressure is released the communities tend to become more dissimilar again. This shows that there is a similar shift in community composition even in waters from very different origins when they are disturbed by antibiotics, even at low concentrations. Antibiotics impact the bacterial communities at the cell and the community level, independently by the original degree of anthropogenic stress they are adapted to, altering the original phenotypes, genotypes, and the relations between bacteria.