For the first time, pristine ZnO nano-particles can be used as effective catalyst for water disinfection by killing and complete mineralization of two gram negative bacteria with direct solar light. Just like in earlier studies, pristine nano-size ZnO particles have shown anti-bacterial activity against two types of gram negative bacteria, E. coli and P. aeruginosa, where up to 20% of the former and 25% of the latter have been killed in the dark. Under direct solar radiation, the pristine ZnO particles readily catalyzed bacterial photo-degradation. While earlier studies were mostly limited to bacterial death and growth inhibition by pristine ZnO particles, the results describe for the first time how bacteria and their organic content can be completely photo-mineralized by direct solar radiations in 60 min. Only the bacterial cell wall fragments resisted the photo-degradation process. Under the reaction conditions, the degradation occurred by the UV tail of the direct solar light, where the ZnO nano-particles behaved as photo-catalysts. The results show the added value of using ZnO nano-particles as photo-catalysts in water disinfection strategies, leaving no resulting organic molecules in water.