Genetic Exchange in Biofilms

Horizontal gene transfer is one of the principal mechanisms driving the evolution of microorganisms. One can induce from this “archeological evidence” that genetic exchange between bacteria occurs fairly frequently. This chapter describes the examples of genetic exchange in biofilms as they have been observed in the laboratory. Transformation in biofilms has, to date, only been demonstrated in two main in vitro model systems. Quorum sensing in gram-positive bacteria has been found to regulate a number of physiologic activities in different species, including competence development in Bacillus subtilis, Streptococcus mutans, Streptococcus gordonii, and Streptococcus pneumoniae. Optimal transformation frequencies were observed when fresh medium was fed to the biofilms, and the amount of surrounding planktonic cells was limited. The high rates of transformation observed with both S. mutans and Acinetobacter biofilms suggest that competence may be the normal state for surface-adherent populations. In addition to increasing the probability of donor-recipient contact it has been suggested that genes activated by contact with surfaces may be involved in the process of conjugation. To visualize actual mating events in biofilms, samples were examined by scanning electron microscopy. Furthermore, the fact that bacteria that are capable of forming biofilms are generally adept at genetic exchange should give us the impetus to study these processes in detail. A means of controlling genetic exchange in biofilms may help us not only to limit the transfer of undesirable characteristics such as antibiotic resistance in medical infections, but also to enhance the desirable processes in other applications.