In the study, we explored the antimicrobial activities of single-walled carbon nanotubes (SWNTs) and multiwalled carbon nanotubes (MWNTs) with different Surface groups to bacterial pathogens, including Gram-negative vs Gram-positive species and rod-shaped vs round-shaped species. We report here for the first time that SWNTs' antimicrobial activity is buffer and concentration dependent, and the charge effect of functional groups on the surface of carbon nanotubes (CNTs) is not a critical factor. SWNTs with surface groups of -OH and -COOH exhibited extremely strong antimicrobial activity to both Gram-positive and Gram-negative bacterial cells in DI water and 0.9% NaCl solution regardless of cell shape, but they did not exhibit antimicrobial activity in PBS buffer and brain heart infusion broth. The antimicrobial activities of these two SWNTs increased with their concentration and treatment time. In DI water or 0.9% NaCl solution, SWNTs-OH and SWNTs-COOH started to show their antimicrobial activity at similar to 50 mu g/mL; when their concentration increased to 200-250 mu g/mL, they could inactivate 10(7) cfu/mL Salmonella cells in 15 min. The similar to 7 log reduction in viable cell count achieved by this CNTs-based method exceeded those of many reported antimicrobial methods. SWNTs-NH2 only exhibited antimicrobial activity at higher concentrations. MWNTs with Surface groups of -OH, -COOH, and -NH2 did not show any significant antimicrobial activity to all tested bacterial cells in any of the tested buffers at concentrations up to 500-875 mu g/mL Formation of cell-CNTs aggregates were studied using fluorescence and electron scanning microscopes. The possible mechanism of SWNTs' antimicrobial activities was also discussed.