Natural transformation in Streptococcus gordonii biofilm

Abstract

Natural transformation is one of the three mechanisms of horizontal gene transfer by which bacteria are able to take up free DNA from the surrounding environment. In natural ecosystems, bacteria preferentially attach to surfaces, forming matrix-enclosed communities known as biofilm. Biofilm cells differ phenotypically and physiologically from their free-floating counterparts. Objective: To estimate transformation frequency in Streptococcus gordonii biofilm and compare that to transformation in a planktonic type of growth. Methods: S. gordonii challis (ATCC 35105) was grown as biofilm in appropriate liquid media. The plasmid vector pVA838 and chromosomal DNA of S. gordonii were used as donor DNA in the transformation assay. Strain-specific synthetic Competence Stimulating Peptide (CSP) was used to induce competence in S. gordonii. The biofilm was grown in polystyrene microtiter wells. Cells in the biofilm were harvested to assess the transformation frequency. Results: The transformation frequency in biofilm bacteria was found to be approximately 6-fold higher than those of planktonic counterparts. Conclusion: This result may either indicate a potential stress-related role of CSP or the fact that CSP participates in the synthesis of bacteriocin-like protein. The finding of enhanced cells in the biofilm in the presence of CSP suggests the involvement of CSP-QS system in the process of biofilm formation. Probably the most confusing finding of this present study was the dramatic increase of the OD value in the liquid culture of S. gordonii after addition of CSP in the early growth phase. This study demonstrated that S. gordonii cells were able to acquire foreign plasmid DNA much more efficiently than their planktonic counterparts in vitro. This finding suggests that biofilm growth mode may provide optimal condition for genetic transformation.