Bacterial community diversity of bulk water and corresponding biofilms of four intensive care units' (ICUs) drinking water systems were studied and compared using 16S rRNA gene amplicons and next generation sequencing. Proteobacteria, mainly Alphaproteobacteria and Betaproteobacteria were dominant in the bulk water and biofilms. Principal component analysis showed different bacterial communities characterizing each of the bulk water and the biofilms in three of the studied ICUs. Taxonomic classification and comparison of different genera between samples highlighted the dominance of Aquabacterium (80%) and Novosphingobium (72%) in bulk water while biofilms harbored different bacteria affiliated to Pelomonas (97%) and Caulobacter (96%), Porphyrobacter (78%) and Staphylococcus (74%). Staphylococcus aureus was the only possible pathogen found with low percentage (2.32%) in three of the ICUs' biofilm and only in one of the ICU's bulk water. This study sheds light on the prevalence of unculturable bacterial flora in the biofilm ignored by the microbiological standard methods. This study was performed on tap and bulk water from ICUs; however, it indicates the need for further studies to investigate the function and activity of the microbial diversity in order to assess the real risk presented by this water microflora on patients' health.