Open Access

Melanie Broszat, Elisabeth Grohmann

• Horizontal gene transfer (HGT) is an important means to obtain and maintain plasticity of microbial genomes. Basically, bacteria apply three different modes to horizontally exchange genetic material: (1) conjugative transfer mediated by mobile genetic elements (MGE), (2) DNA uptake via transformation, and (3) transduction. The three modes rely on different prerequisites of the participating cells:Horizontal gene transfer (HGT) is an important means to obtain and maintain plasticity of microbial genomes. Basically, bacteria apply three different modes to horizontally exchange genetic material: (1) conjugative transfer mediated by mobile genetic elements (MGE), (2) DNA uptake via transformation, and (3) transduction. The three modes rely on different prerequisites of the participating cells: conjugative transfer depends on close cell to cell contact between a donor and a recipient cell and is mediated through multi-protein complexes, denominated type IV secretion systems (T4SS), and DNA transformation does not rely on cell–cell contact but is the uptake of free DNA from the environment by a competent bacterial cell. In some bacteria it is also mediated by a T4SS. The third mechanism depends on the presence of a bacteriophage, which can transfer genomic DNA from one host cell to another. Experimental evidence exists that all three modes occur in planktonic cultures and recent data have also been provided for the occurrence of all three ways in biofilms. Regulation of these HGT events and their consequences for the acting microbes and the biofilms they live in are discussed in this chapter. Additionally, we focus on modern techniques to visualize and to quantify HGT in planktonic and biofilm modes.…