The adhesion of dissimilatory metal reducing bacteria (DMRB) to iron (oxy)(hydr)oxides may play an important role in their respiration on ferric iron-containing minerals, but few quantitative surface cell density measurements have been made thus far. We used confocal microscopy to examine the adhesion of a common DMRB species, Shewanella oneidensis MR-1, onto iron (oxy)(hydr)oxide particulate-coated glass slides across a broad range of bulk (i.e., solution phase) cell densities from 10(5) cells/mL to 2 x 10(9) cells/mL. At bulk cell densities less than 1 x 10(7) cells/mL, cells adhered to the slide surface formed an evenly distributed, homogeneous monolayer, while at the bulk cell densities higher than 2 x 10(8) cells/mL the adhered cells formed distinct microcolonies. As a result of this complex adhesion behavior, simple Langmuir or Freundlich adsorption isotherms do not capture the relationship between the surface cell density and the bulk cell density over the entire range of bulk cell densities. Thus a new, two-step isotherm was developed that incorporated both isolated attached cells at low cell densities as well as microcolonies at higher cell densities.