Commonest organism giving rise to urinary tract infection -

• Most UTIs are caused by a single bacterial species. • At least 80% of the uncomplicated cystitis and pyelonephritis are due to E. coli, with most of pathogenic strains belonging to the O serogroups. • Other less common uropathogens include Klebsiella, Proteus, and Enterobacter spp. and enterococci. • In hospital acquired UTIs, a wider variety of causative organisms is found, including Pseudomonas and Staphylococcus spp. • UTIs caused by S. aureus often result from hematogenous dissemination. • Group B beta-hemolytic streptococci can cause UTIs in pregnant women. • In children, Klebsiella and Enterobacter spp. are common causes of UTI. • Anaerobic bacteria, lactobacilli, corynebacteria, streptococci (not including enterococci) and S. epidermidis are found in normal periurethral flora. They do not commonly cause UTIs in healthy individuals and are considered common urinary contaminants.