Fig. 3.

Schematic representation of the two proposed evolutionary processes (based on the results presented in Figs 1 and 2, and Table 1) leading to the dissemination of trimethoprim-resistance determinants. Left panel: upon the introduction of trimethoprim, mobilization events involving pre-existing resistant chromosomal folA genes can be favourably selected. Right panel: following the introduction of trimethoprim, mobilization events involving folA genes with novel mutations that confer resistance to this chemotherapeutic agent may be selected for and disseminated among closely related bacteria.