In this paper, surface modification of various materials exposed to a nonthermal atm. plasma is investigated. The used source is the plasma needle: a radio-frequency-driven nonthermal atm. microplasma. A no. of substrates (Perspex and polystyrene) were treated with the plasma needle. The modification of materials was subsequently identified as hydrophilization of the surface and was exptl. validated by water-contact-angle measurements. Furthermore, the effect of this modification on the growth of two bacterial species, which are the Escherichia coli and Streptococcus mutatis, is studied. The bacteria were cultured on treated and nontreated polystyrene 96-well plates; the growth of E. coli on the treated substrates was enhanced, while for S. mutans, it was reduced. An explanation of these effects is provided.