There are conflicting reports on the action of different extracts of stinging nettle (Urtica dioica) on pathogenic bacteria. In a 2014 study, Samoilova et al. (1) describe an experiment where Urtica dioica reduced biofilm formation by E. coli.
Selected references:
1. Samoilova Z, Muzyka N, Lepekhina E, Oktyabrsky O, Smirnova G. Medicinal plant extracts can variously modify biofilm formation in Escherichia coli. Antonie Van Leeuwenhoek. 2014;105(4):709–722. doi:10.1007/s10482-014-0126-3