Internalisation of PTECs by E. coli isolates. 16 isolates of E. coli from the urine of patients with clinical UTI (A and C) and 15 isolated from blood cultures when the source was the urinary tract (B and D) were assessed to determine whether they demonstrated C3-dependent internalisation. (A) and (B) shown the number of bacteria internalised by PTECs in the presences of 5% NHS or HIS (mean of 4 separate wells per isolate). C3-dependent internalisation was arbitrarily defined as a 5-fold increase in the number of bacteria internalised in the presence of NHS compared with HIS. 7 urine isolates (43.75%) (C) and 3 (20%) blood isolates (D) demonstrated C3-dependent internalisation. The results were reproducible in two independent experiments.