title: In siturheology ofStaphylococcus epidermidisbacterial biofilms.
abstract: We developed a method to growStaphylococcus epidermidisbacterial biofilms and characterize their rheological propertiesin situin a continuously fed bioreactor incorporated into a parallel plate rheometer. The temperature and shear rates of growth modeled bloodstream conditions, a common site ofS. epidermidisinfection. We measured the linear elastic (G') and viscous moduli (G″) of the material using small-amplitude oscillatory rheology and the yield stress using non-linear creep rheology. We found that the elastic and viscous moduli of theS. epidermidisbiofilm were 11 ± 3 Pa and 1.9 ± 0.5 Pa at a frequency of 1 Hz (6.283 rad per s) and that the yield stress was approximately 20 Pa. We modeled the linear creep response of the biofilm using a Jeffreys model and found thatS. epidermidishas a characteristic relaxation time of approximately 750 seconds and a linear creep viscosity of 3000 Pa s. The effects on the linear viscoelastic moduli of environmental stressors, such as NaCl concentration and extremes of temperature, were also studied. We found a non-monotonic relationship between moduli and NaCl concentrations, with the stiffest material properties found at human physiological concentrations (135 mM). Temperature dependent rheology showed hysteresis in the moduli when heated and cooled between 5 °C and 60 °C. Through these experiments, we demonstrated that biofilms are rheologically complex materials that can be characterized by a combination of low modulus (~10 Pa), long relaxation time (~103seconds), and a finite yield stress (20 Pa). This suggests that biofilms should be viewed as soft viscoelastic solids whose properties are determined in part by local environmental conditions. Thein situgrowth method introduced here can be adapted to a wide range of biofilm systems and applied over a broad spectrum of rheological and environmental conditions because the technique minimizes the risk of irreversible, non-linear deformation of the microbial specimen before analysis.
authors.affiliation: Department of Chemical Engineering, University of Michigan, Ann Arbor, MI, 48109, USA. ; Tel: +1 734 764 3119.
authors.forename: Leonid
authors.initials: L
authors.lastname: Pavlovsky
authors.affiliation: Department of Emergency Medicine, University of Michigan, Ann Arbor, MI, 48109, USA. Tel: +1 734 647 7564 ; Biointerfaces Institute, University of Michigan, Ann Arbor, MI, 48109, USA ; Michigan Center for Integrative Research in Critical Care, University of Michigan, Ann Arbor, MI, 48109, USA.
authors.forename: John G
authors.initials: JG
authors.lastname: Younger
authors.affiliation: Department of Chemical Engineering, University of Michigan, Ann Arbor, MI, 48109, USA. ; Tel: +1 734 764 3119 ; Biointerfaces Institute, University of Michigan, Ann Arbor, MI, 48109, USA.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Solomon