THE FATE AND ECOLOGICAL RELEVANCE OF EXTRACELLULAR DNA IN SOIL

Chemical characteristics of organic molecules underlie the soil organic matter cycle. Recent studies report that under varying soil environmental conditions, the stable and labile carbon pool could respectively be less or more recalcitrant to degradation. These theories perfectly explain the fate of extracellular DNA (eDNA) in soil, as reported in several research and review papers published in the last years. The relevance of these studies results are clear when we consider the plethora of ecological functions in which eDNA is involved. This fraction can be actively extruded by living cells or discharged during cellular lysis and may exert a key role in the stability and variability of the soil bacterial genome. In fact, c eDNA the extracellular fraction of the mobile genetic information present in the soil environment, defined as mobiloma, available for bacterial competent cells capable to adopt transformation. This gene transfer strategy could permit to bacterial competent cell to acquire genes to increase its survival capacity, such as those encoding resistance to toxic molecules and/or or production of defensive molecules. Moreover, bacterial eDNA acquisition could also represent a mechanism to repair damaged genomic DNA fragments, contributing to genome stability. The adhesive properties of the DNA molecule confer to eDNA at high concentrations the capacity to inhibit or kill pathogenic bacteria by cation limitation induction, and to facilitate the formation of biofilm and extracellular traps (ETs), that may protect microorganisms inhabiting biofilm and plant roots against pathogens and allelopathic substances. Moreover, eDNA at sub-inhibitory concentration could create a cation limited environment that cause an inducible antibiotic resistance in biofilm inhabiting bacteria. Finally the cation limitation induction may also represent an eDNA self-defence from DNases degradation due to Mg2+ limitation, cation required as co-factor by DNases. The ecological relevance of ETs, actively extruded by root border cells when they are dispersed in the rhizosphere, consist in empower plants to extend an endogenous pathogen defensive system outside the organism. One of the most promising areas for future development is the manipulation of the rhizosphere to produce sustainable and efficient agriculture production systems. Using Omics approaches, to define the distinctive features of eDNA systems and structures, will facilitate progress in rhizo-enforcement and biocontrol studies. Finally the relevance of eDNA from soil bacteria could also be extended to human health applications. In fact, one of the most modern and sufficiently perspective trends in immunotherapy, particularly against cancer, is to culture selected soil inhabiting bacteria to obtain an eDNA rich in cytosine-guanine (CG) dinucleotides with respect to mammalian cells, that confer to these molecules a high immuno-stimulatory activity. An example is the so called CpG island rich eDNA (C and G are connected by a phosphodiester bond), obtained from soil Bacillus subtilis culture medium filtrate (CMF). Moreover the knowledge of the complex molecular mechanism that affect DNA release, the biofilm development and the induction of antibiotic resistance in biofilm inhabiting bacteria, may be useful for creating strategies to control biofilm formation and eradicate persistent infections.