Background and Objectives:
A range of on-site physical and chemical treatments for secondary disinfection have been proposed with the aim of controlling microbial water contamination, differing in effectiveness, application methods (in continuous and shock), costs and management. Each method can modify the composition of the bacterial community of water systems. Nowadays, Bioinformatics and Next Generation Sequencing (NGS) technologies offer high-throughput, rapid and accurate methods of determining all cultivable and non-cultivable bacteria within a water distribution system. Our study aims to characterise the microbial community _pre-post_ introduction of a continuous hydrogen peroxide disinfection system applied in the hot water network of an Italian hospital for _Legionella _control.

Methods:
_pre-post_ study is performed. The main building includes 10 blocks fed by the same water plant. Municipal cold water, cold and hot hospital water before treatment and after 1, 6 and 12 months are analysed for microbiome characterization by 16S amplicon sequencing. Here, we present the _pre_-treatment and _post_-one-month results.

Results:
A total of 32 samples was collected: 14 cold water and 18 hot water samples (9 before treatment and 9 after 1 month). Cold water bacterial profiles at genus level did not vary significantly between the two sampling sessions. On the contrary, the hot water profiles differed between _pre_-treatment and _post_-one-month treatment samples. Before treatment the two most abundant genera were _Sphingorhabdus_ and _Thermus_, while after one month were _Porphyrobacter_ and _Blastomonas_.

Conclusion:
Determining the microbial profiles of hospital water networks through 16S amplicon sequencing is a promising approach for Public Health. Important strength of our study is the _pre-post_ treatment approach to investigate the possible modifications of bacterial community due to hydrogen peroxide disinfection. Moreover, over time monitoring allows to evaluate microbiome stability in the treated water.