Low concentrations of antibiotics have numerous effects on bacteria. However, it is unknown whether ecological factors such as trophic interactions and spatial structuring influence the effects of low concentrations of antibiotics on multispecies microbial communities. Here, we address this question by investigating the effects of low antibiotic concentration on community composition and horizontal transfer of an antibiotic resistance plasmid in a 62-strain bacterial community in response to manipulation of the spatial environment and presence of predation. The strong effects of antibiotic treatment on community composition depend on the presence of predation and spatial structuring that have strong community effects on their own. Overall, we find plasmid transfer to diverse recipient taxa. Plasmid transfer is likely to occur to abundant strains, occurs to a higher number of strains in the presence of antibiotic, and also occurs to low-abundance strains in the presence of spatial structures. These results fill knowledge gaps concerning the effects of low antibiotic concentrations in complex ecological settings.

Community compositionRelative abundance of different taxa in 40 day serial transfer microcosm experimentCommunity_composition.txtRP4 recipientsRP4 plasmid recipient taxa at the end-point of 40 day serial transfer microcosm experiment (identified using epicPCR)RP4_recipients.txtRP4 recipients pooled replicatesRP4 recipient taxa based on pooled replicates from end-point of 40-day serial transfer microcosm experimentRP4_recipients_pooled_replicates.txt