The CDC Biofilm Reactor® is widely used due to it’s repeatability, reusability, and reliability in growing a well-adhered biofilm. Many different organisms have been successfully grown using this reactor, and it is commonly used to test disinfectants, treatments, antimicrobials, among other things. Due to the well-adhered, tough-to-kill nature of the type of biofilm grown using the the CDC Biofilm Reactor®, it has become the gold standard for testing biofilm-killing products.
The characteristics found in the biofilm grown in this reactor can be attributed to, among other things, the relatively high surface shear stress found at the coupon surface. This high shear stress is created by the PTFE stir baffle rotating at 125 RPM (the suggested rate according to ASTM E2562) past the coupon surface. High shear can cause a biofilm to grow tighter and thinner, since less-well-adhered cells tend to get pulled off of the biofilm. In contrast, a reactor like the Drip Flow Biofilm Reactor® grows a much fluffier, less robust biofilm, partially due to the lack of surface shear stress created by the slow-flowing fluid.
To study the surface shear stress experienced by the coupon surface, and thus the biofilm, calculations can be done using some assumptions. By assuming that the system consists of two frictionless cylinders, and the fluid consists entirely of water at 20° Celsius, an estimate of the Reynold’s number (which shows that the system is in the turbulent regime) can be found and used to then find friction factors and shear stress on the coupon surface. This calculation, while in the right ballpark and usable for most applications, is not a perfect representation of the system in question.
To properly and accurately assess the entire system, including the complex geometry of coupon holder rods, glass vessels, and the stir bar and stir baffle, a computational method must be used. In the paper “Characterizing the shearing stresses within the CDC biofilm reactor using computational fluid dynamics” by a team at the Montana State University Center for Biofilm Engineering (see citation below), researchers used Computer-Aided Design (CAD) models of the CDC Biofilm Reactor® provided by BioSurface Technologies Corporation to run advanced computational models of fluid dynamics found within the reactor. The result was a more detailed view of the shear stress found on the coupon and biofilm surfaces than has ever been found before.
The authors (Johnson, E., Petersen, T., & Goeres, D. M.) found an average shear stress of 0.365 +/- 0.074 pascals (Pa) across all twenty-four (24) coupons. While significantly higher than what has been calculated using the simplified assumptions outlined above, the computer models are likely to be much more accurate. Additionally, this study found higher spikes or pulses of shear stress across the surfaces of two to three pascals (2-3 Pa), a result of the stir baffle passing by twice per full rotation.
This study was the first of it’s kind to use the actual CAD models for complex computational analysis of fluid shear stress inside the CDC Biofilm Reactor®. To read more details found in the study, and view incredible computer models created from the analysis, take a look at the link provided below.
Read the full publication:
Johnson, E., Petersen, T., & Goeres, D. M. (2021). Characterizing the shearing stresses within the CDC biofilm reactor using computational fluid dynamics. Microorganisms, 9(8), 1709. https://doi.org/10.3390/microorganisms9081709.
