A low dose of cell-wall targeting antibiotic can promote aggregation of <i>Escherichia coli</i> bacteria

Exposure to antibiotic at low doses has been shown to stimulate biofilm formation. To study how low-dose antibiotic can promote aggregation and biofilm formation, we developed an experimental assay where E. coli bacteria form aggregates in a shaken suspension upon exposure to a low dose of cell-wall targeting antibiotic. We quantified the dynamical transition from a planktonic culture to a suspension of aggregates using a combination of microscopic, spectroscopic and microbiological methods. Our results support a picture in which cluster formation happens primarily due to the release of DNA by antibiotic-induced cell lysis, with a secondary effect due to the presence of pili . This aggregation process happens even in a strongly shaken suspension, and we speculate that such aggregates might form precursors to biofilm formation.