A collection of 234 Pseudomonas aeruginosa strains isolated from hospitalized patients (n=20), inlet effluent of a local sewage treatment plant (STP) (n=136) and from six sampling sites of a river (n=78) downstream of the STP, were tested for their clonality using RAPD-PCR, the presence of seven virulence genes (VGs), their resistance against 12 antibiotics and their adhesion to three human cell lines. A high diversity was found among the isolates irrespective of their sources with only six common (C) types containing strains from clinical and environmental sources. Environmental strains belonging to these C-types showed a higher level of adhesion to A-498 cells than clinical strains (17±13 versus 13±11; p<0.001), whereas clinical strains adhered significantly greater to Calu-3 and Caco-2 cells than environmental strains (p<0.001 for both). The VGs and antibiotic resistance profiles of the strains were similar, however the prevalence of STP strains carrying both exoS and exoU was significantly (p<0.0368) higher than clinical strains. Whilst all strains were resistant to ticarcillin and ticarcillin/clavulanic acid resistance against aztreonam (98%), gentamicin (39%), amikacin (16%) and ceftazidime (8%) were higher among river than STP and clinical strains. In contrast resistance against pipercillin was higher amongst clinical strains (57%) than STP (27%) and river (16%) strains. These results suggest that environmental strains of P. aeruginosa carrying similar virulence properties including adhesion to various human cell lines which makes them capable of causing infection in humans under predisposing conditions.