One ml aliquots of B. subtilis and P. fluorescens overnight cultures (OD A600=1.0) were mixed together in 10 ml co-aggregate buffer (20 mM Tris–HCl buffer (pH 7.8), 0.01 mM CaCl2, 0.01 mM MgCl2, 0.15 M NaCl, 0.02% NaN3) and vortexed for 10 s. The mixture was incubated in a rotary shaker for three min and left undisturbed for 24 h ( Grimaudo and Nesbitt, 1997).