A thin line between plankton and biofilm

Planktonic bacterial cells are by definition not aggregated. However, our previous work, where we have demonstrated the invisible mechanical connections between bacterial cells in dilute planktonic suspensions, challenged this assumption. Here we provide an experimental evidence using autocorrelation analysis of micrographs that in planktonic suspensions of B. subtilis a size continuum of aggregated structures is formed. In the microbial aggregates viable cells were embedded in the nucleic acid network. The eDNA was released during regular cell lysis events. To determine the size distribution of planktonic bacterial aggregates a pair-wise spatial correlations of bacterial cells in microscopic images were calculated. The monotonously decreasing shape of the autocorrelation function indicated a continuous distribution of bacterial aggregate sizes from monomer to multimers. Soft bacterial aggregates in dilute suspensions provide a missing link in a continuum of organic matter in aqueous environments and can significantly improve our understanding how non-attached biofilms form during planktonic growth.