Induced Natural Transformation of Acinetobacter calcoaceticus in Soil Microcosms

Abstract

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; 200to 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 2؉ and Ca 2؉) 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.

Barriers to horizontal gene transfer by natural transformation in soil bacteria

“…And, indeed, development of competence in a soil residing bacterium @. calcoaceticus) was recently shown for the first time in soil (Nielsen et al, 1997~).…”

Exploring the immediate and long-term impact on bacterial communities in soil amended with animal and urban organic waste fertilizers using pyrosequencing and screening for horizontal transfer of antibiotic resistance

“…Transfer frequency of antibiotic resistance is increased in fertilizer-amended soil Sludge, MSW-compost and manure may be regarded 'hot spots' for HGT due to the high number of bacteria and high nutrient levels. A laboratory experiment has shown increased frequency of transfer of heavy metal resistance genes due to nutrient amendment (Top et al, 1990), and in another study, natural transformation of Acinetobacter calcoaceticus was induced by nutrients, carbon and salts (Nielsen et al, 1997). Finally, manure has been shown to promote the horizontal transfer of antibiotic resistance genes in soil due to high loads of broad-host-range plasmids (Smalla et al, 2000).…”

Cues and regulatory pathways involved in natural competence and transformation in pathogenic and environmental Gram-negative bacteria

“…Based on these in vitro studies described earlier, it is difficult to determine the environmental signals that could trigger competence in Acinetobacter. Nielsen et al (1997) performed soil microcosm studies ('in situ') and observed the induction of competence by nutrient upshifts, as well as a transformation-enhancing effect of phosphate. These authors concluded that 'poorly transformable A. calcoaceticus cells can be induced to undergo natural transformation with chromosomal DNA in soil' and that 'their level of competence is influenced by their metabolic state' (Nielsen et al, 1997).…”