**12. Transmission of VFs**

The genes encoding specific UPEC VFs can be exclusively chromosomal (e.g., *pap* and *hly*); exclusively or principally plasmid-associated (e.g., *iss* and *traT*); or can occur in either location (e.g., the aerobactin operon and afa/drab). Plasmid-borne VFs have an obvious vehicle for horizontal transmission among *E. coli* lineages. Such VFs tend to be distributed more broadly but more sporadically within the species than are chromosomal VFs [129]. Studies have demonstrated that certain VFs commonly occur together, in a way that suggests co-selection or direct genetic linkage [130, 131]. Direct genetic linkage of VFs has been shown on PAIs and plasmids [132–134].
