Typical Causative Organisms and Antibiotic ResistanceUrinary tract infections are primarily caused by gram-negative bacteria, but gram-positive pathogens may also be involved. More than 95% of uncomplicated UTIs are monobacterial.The most common pathogen for uncomplicated UTIs is E. coli(75%–95%), followed by Klebsiella pneumoniae, Staphylococcus saprophyticus, Enterococcus faecalis, group B streptococci,and Proteus mirabilis (Sobel 2014). Distribution of uropathogensmay differ by type of infection or patient population(Table 1-2). E. coli can cause both uncomplicated and complicated UTIs. P. mirabilis, Pseudomonas aeruginosa, and Enterococcus spp. predominantly cause complicated infections and are more commonly isolated in hospitals and long-term care facilities.