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Containers made with PET (Polyethylene-terephthalate) are currently more and more employed in packing industry, particularly for the storage of mineral waters. The increasing utilization of such containers is due to the intrinsic properties of this polymer, which was shown particularly suitable for making bottles devoted to the storage of gassed drinks. The resistance of the PET to high pressure, hits by falls and top-to-down loads indeed makes PET bottles unbreakable; their gas-tightness warrants a good gas maintenance during the storage period; the high transparency of the PET allows a good vision of the contents; the light weight of the bottles and the low temperatures required for their production allow a remarkable saving of energy; lastly PET bottles can easily be recycled. Previous microbiological investigations carried out on several mineral waters bottled in glass bottles and non-PET plastic (i.e. PVC) bottles, had shown higher microbial counts in the water samples stored in plastic bottles. In the present work we have studied the growth rates of the bacterial flora in a sample of non gassed medio-mineral water stored in PET bottles, with respect to a control of the same kind of water, stored in glass bottles. Before using, both PET and glass bottles were washed with 5% Desogen, and sterilized by 100 vol. hydrogen peroxide. After the appropriate sterility checks, the bottles were filled directly from the spring with a non gassed medio-mineral water, and then subdivided into four groups, each consisting of the same number of bottles. A the time of bottling, a bacterial count on such water samples at 20 degrees C and 37 degrees C was performed, in order to establish the "zero" value. One of the two groups of PET bottles, and one of the two groups of glass bottles were stored in the darkness, while the other two groups were stored in the light. Afterwards, one bottle from each group was drawn once a week over one year, in order to measure the bacterial concentration in the water. 20 degrees C and 37 degrees C bacterial counts were done after plating in standard agar. The findings of our study show that the bacterial count in both PET- and glass-stored water increases first, but decreases afterwards, though in a non-uniform rate. Further, light exposure weakly don't affects significantly the bacterial growth, even though, in the average, the bacterial count is lower in the bottles exposed to the light.(ABSTRACT TRUNCATED AT 400 WORDS)