The graphenic surface was modified by oxygen plasma treatment at room temperature varying the parameters (power: 2-60 W, oxygen pressure: 0.14-0.5 mbar, time: 2 s-20 min). The modification was confined to the surface without changing the bulk. The modified surfaces were characterized in terms of carbon structure (mu Raman spectroscopy), surface morphology (scanning electron microscopy), wettability (water contact angle), surface oxygen concentration (X-ray photoelectron spectroscopy), electronic properties (work function) and bacteria adhesion (fluorescence microscopy). Generation of surface oxygen groups leads to substantial increase in work function (from 4.4 eV to 6.0 eV) and decrease in water contact angle (from 93.8 degrees to 7.0 degrees). It was also found that the introduction of surface oxygen groups promotes the Staphylococcus aureus adhesion (increase in the area occupied by bacteria from 3.2% to 9.2%). The discovered strong relationship between S. aureus adhesion, surface oxygen groups and electrodonor properties of the graphenic surfaces allows to propose the work function as a suitable parameter for the evaluation of graphenic surfaces against bacteria colonization.