Biofilms – multicellular bacterial communities embedded in a protective extracellular polymeric substance – are the predominant form of microbial life on earth. Biofilms are a determinant of bacterial virulence in human infection. For example, biofilms that form on implanted medical devices are recalcitrant to treatment, including antibiotic regimens. Biofilms can contribute to systemic infection, including bacteremia and sepsis. Here we investigate the biomechanics of biofilms of Staphylococcus epidermidis and Staphylococcus aureus, two species that commonly contribute to hospital-acquired infections. We examine the microstructural factors that control the viscoelasticity of biofilms, in pursuit of the idea that biofilm mechanics are a determinant of bacterial persistence. We measure the rheological properties of biofilms in response to variables that might be useful in treatment, including antibiotic dosage, ionic strength, pH, and temperature. We use dynamic light scattering to characterize associations in the staphylococcal exopolysaccharide and microrheology to probe the rheology of the biofilm. We furthermore generate a fibrin model of an infected clot – such as might be formed during bloodstream infection – and examine the effect of bacteria on its microstructure and rheology.