Highly efficient genetic transformation of Bacillus subtilis attached to sand grains

Genetic transformation at the solid/liquid interface was studied using Bacillus subtilis 1G20 (trpC2) with a flow-through system of columns filled with chemically pure sea sand. Studies were done at 23 degrees C. In one type of experiment, competent cultures were incubated with sand-adsorbed DNA, and in another, competent cultures were exposed to sand and then incubated with dissolved DNA for transformation. Of the applied cells, around 10% were retained in columns filled with DNA-loaded sand and around 1% in columns with pure sand. Reversible attachment of some of the cells to surfaces of sand grains could be demonstrated. The overall transformation frequencies obtained were 25- to 50-fold higher than in a standard liquid culture procedure. In this standard procedure, transformation was sensitive to DNAase I concentrations above 50 ng ml-1, whereas in sand columns it was resistant to DNAase I concentrations up to 1 microgram ml-1. Quantification of transformants eluting from columns indicated that sand-attached cells detach at some point after DNA binding or uptake.