To quantify metal adsorption onto bacterial surfaces, recent studies have applied surface complexation theory to model the specific chemical and electrostatic interactions occurring at the solution-cell wall interface. However, to date, the effect of ionic strength on these interactions has not been investigated. In this study, we perform acid-base titrations of suspensions containing Bacillus subtilis or Bacillus licheniformis in 0.01 or 0.1 M NaNO3, and we evaluate the constant capacitance and basic Stern double-layer models for their ability to describe ionic-strength-dependent behavior, The constant capacitance model provides the best description of the experimental data. The constant capacitance model parameters vary between independently grown bacterial cultures, possibly due to cell wall variation arising from genetic exchange during reproduction. We perform metal-B. subtilis and metal-B. licheniformis adsorption experiments using Cd, Pb, and Cu, and we solve for stability constants describing metal adsorption onto distinct functional groups on the bacterial cell walls. We find that these stability constants vary substantially but systematically between the two bacterial species at the two different ionic strengths.