Figure 1. Emergence of an azoreductase by which, in addition to dicarboxybenzene (DCAB), a more complex azo compound (carboxy-Orange II) also is reduced. This type of enzyme first emerged in a continuously growing bacterial culture after 318 days. (H.G. Kulla in: L. Leisinger et al., Xenobiotics, p. 387–399, Academic Press, 1981, modified.)

Figure 2. Transformation, an important mechanism for horizontal gene transfer. (a) Uptake of DNA fragments (red) by bacteria in soil. A few living organisms between dead and lysed cells (watercolor and gouache, Anne Kemmling, Goettingen, Germany). (b) Transformation experiment with a mutant of an Acinetobacter species that can no longer grow on p-hydroxybenzoate. The mutant lacks the gene for p-hydroxybenzoate hydroxylase. The lacking or defective gene is replaced by transformation with wild-type DNA. For this purpose, wild-type DNA is spotted on agar at points indicated by red arrows. The only carbon and energy source present in this agar is p-hydroxybenzoate. Mutant cells are now streaked out from the upper part of the Petri dish through the wild-type DNA spots downwards. Growth is only possible after contact of the cells with wild-type DNA (Beate Averhoff, Frankfurt/Main, Germany; original experiment: Nicolas Ornston, Beate Averhoff). (c) Model of the apparatus for DNA uptake by transformation in Gram-negative bacteria. The pilus consisting of helically arranged proteins is depicted in green. Double-stranded DNA (ds-DNA) is threaded into the transformation apparatus and pulled into the cell with concomitant hydrolysis of one strand. Single-stranded DNA then arrives in the cell. The whole process is driven by ATP hydrolysis. The space between the cell wall (a major part of it is the outer membrane) and the cytoplasmic membrane is called periplasm. (Model: Beate Averhoff, Frankfurt/Main, Germany.)

Figure 3. DNA transfer in E. coli by conjugation. (a) Electron micrograph, the F ⁺ cell carries fimbriae, and it produces the pilus by which contact is made with the F ^-cell (according to Charles C. Binton Jr. cited in: R.Y. Stanier, J.L. Ingraham, M.L. Wheelis, P.R. Painter. The Microbial World, Prentice-Hall, NY, USA, 1986). (b) Schematic presentation of a), showing the chromosome of both F cells and the F plasmid in the F ⁺cell (diagram: Anne Kemmling, Goettingen, Germany). (c) One strand of the F plasmid migrates via a plasma bridge to the F ^– cell. The single-stranded parts of the plasmid in both cells are immediately replenished to double strands. (Diagram: Anne Kemmling, Goettingen, Germany.)