Bacterial cells were resuspended to yield working concentrations of 1.67 × 107CFUmL−1 in PDB dilutions (1:50) containing 5%, 10%, 20%, 30% or 40%v/v bacterial cell-free culture filtrates. These solutions were used to assay trap formation. Four control plates were included in each test run. Two control plates were PDB dilutions (PDB 1:50) containing 5% NB (v/v) without or with bacterial cells (final concentration, 1.67 × 107CFUmL−1). And the other two were PDB dilutions (PDB 1:50) containing 40% NB (v/v) without or with bacterial cells (final concentration, 1.67 × 107CFUmL−1). The experiments were prepared in triplicate and repeated three times.