The present investigation is centered on the study of the growth curves of E. coli and C. xerosis bacteria in the presence of nanosize particles of Zinc Oxide. Previous works1,2 demonstrated the sensitivity of the bacteria, when these were reproduced in mediums that contain nanoparticles of luminescent silicon and Cobalt Ferrite. Doped ZnO nanocrystals were synthesized by conventional precipitation in ethanol solutions as reported by Spanhel and An der son for bare ZnO, In our case, the syntheses were carried out under room-temperature conditions. The experimental results of E. coli bacteria in contact with a stable suspension of nanoparticles of Zinc Oxide, showed a growth curve without adaptation period, Moreover a short and slowly logarithmic stage have been observed, reaching the stationary stage after approximately four hours compared with one in absence of the nanoparticles (standard curve). During the observations, a change in the lifetime of the bacteria (metabolism) with particulate was noticed, as well as the beginning of the mortality stage. However different results were recorded for silicon and ferrite. For the case of the bacteria C. xerosis, the curve with particles is above its standard curve for all times with none of the oscillations which occurred in the nanometer silicon. For these bacteria the beginning of the mortality stage is observed when they have particles. For both bacteria with Zinc Oxide nanoparticles this occurs approximately after nine hours. © 2006 Materials Research Society.