The existing antibiotics are incapable of killing resistant pathogens including E. coli. Extended-spectrum beta-lactamases (ESBL) genes are found as one of the main causes of the resistance as it degrades the β- lactam ring of the antibiotics. This study aims at the green synthesis of silver nanocolloids from endophytic fungi Lasiodiplodia theobromae (LtNc's), isolated from traditional medicinal plant Cinnamomum zeylanicum and biophysically characterized. Minimum susceptibility concentration, growth rate, Minimum Bactericidal Concentration and Biofilm inhibition assays were performed to evaluate the efficacy of LtNc's against biofilm forming urinary tract infection (UTI) causing clinical strains of E. coli. The LtNc's was found to be efficient antimicrobial agent which successfully controlled the growth and biofilm formation in E. coli. Inhibition of growth, biofilm formation, downregulation of CTX-M-15 and Amp C genes were observed when treated with LtNc's in Urinary Tract infection (UTI) causing pathogens. [ABSTRACT FROM AUTHOR]