Evaluation of filterability and taxonomic identification of ultramicrobacteria from an urban drinking-water source

Currently, filter sterilisation is recognized as a standard method to remove bacteria from fluid solutions. However, ultramicrobacteria that can pass through 0.22-μm membrane filters pose a great challenge to this technique. In this study, we identified that ultramicrobacteria enriched from an urban drinking-water source are members of the Bacteroidetes, Proteobacteria, Spirochaetae, Firmicutes, and Actinobacteria. Ultramicrobacteria displayed superior filterability during the entire bacterial growth phase, especially at the exponential phase. Maximal passage percentages were comparable at different cell densities, and were saturated in advance at high cell density. Filter retention for bacteria is independent of ambient temperature of filtering. However, cultivation temperature could affect the growth of some specific ultramicrobacteria and lead to variability in the passage percentage. Filter materials and pore size greatly affected the passage percentage of ultramicrobacteria, which ranked as polycarbonate >polyvinylidene fluoride (PVDF) ≈ polyethersulfone (PES) > mixed cellulose esters (MCE) for 0.45/0.22-μm filters. The majority of Hylemonella and SAR324 ultramicrobacteria could pass through 0.10-μm PVDF/PES filters but could not pass through 0.10-μm polycarbonate/MCE filters. Our results demonstrated that the intrinsic filterability of ultramicrobacteria, membrane characteristics and filtration operational conditions could challenge the validity of the 0.22/0.10-μm sterilizing grade filters in providing bio-safety barriers.