Induced Natural Transformation of Acinetobacter calcoaceticus in Soil Microcosms

Factors affecting natural transformation of Acinetobacter calcoaceticus BD413 with homologous chromosomal DNA in a silt loam soil microcosm were investigated. Inducible transformation of declining populations of noncompetent A. calcoaceticus cells was detectable for up to 6 days when a simple carbon source, salts, and freshly added DNA were used. In two different experimental setups, the residence time in soil of induced cells could be increased to either 11 or 24 h before DNA addition without reduced transformation frequency; 200-to 1,000-fold fewer transformants were observed following the addition of water. These observations suggest that A. calcoaceticus remains transformable for several hours after its activation by nutrients in soil. Increasing the levels of phosphate salts significantly enhanced the numbers of transformants without increasing the recipient counts correspondingly. Variable levels of ammonium or divalent cations (Mg(sup2+) and Ca(sup2+)) did not have a similar major influence. Soil moisture content significantly affected the transformation frequency of A. calcoaceticus cells, with a general tendency of higher frequencies in drier soil. A minimal frequency was observed at around 35% soil moisture. The data indicate that A. calcoaceticus cells in soil which cannot be detectably transformed are easily induced by nutrients to undergo natural transformation with chromosomal DNA. Access to nutrients seems to be critical for the development and maintenance of competence in soil, which is also affected by abiotic factors like moisture level and phosphate salt concentration.