Speaker: Bruce E. Rittmann,
Director, Center for Environmental Biotechnology,
Biodesign Institute at Arizona State University

Title: Modeling Multi-Component Biofilms - Why Some Biofilms Are Strong,While Others are Weak

Abstract:More than 90% of bacteria live in biofilms, which are microbial aggregates attached to surfaces. Many biofilms benefit humans and the environment. "Good" biofilms mediate nutrient cycling in natural rivers,lakes, and wetlands; furthermore, environmental engineers exploit biofilms in treatment technologies for water, wastewater, and contaminated gases. On the other hand, other biofilms cause great harm in human society. For example, "bad" biofilms accelerate N loss from fertilizers applied in agriculture, deplete oxygen in streams by oxidizing reduced N and C, cause infections of artificial prostheses, and foul pipes and ship hulls. Whether we want to promote "good" biofilm or eliminate "bad" biofilm, its accumulation depends on the biofilm's mechanical strength, or its ability to resist being physically removed by forces that scour or abrade the biofilm off the surface.
The mechanical strength depends on the composition and density of the microbial biomass in the biofilm. The common wisdom is that EPS (extracellular polymeric substances) is a bacterially secreted "glue" that binds the active bacteria and inert (or dead) bacteria to each other and the surface. Despite the importance of EPS, no one up to now has included it in mathematical models of biofilm development. In the work I summarize here,I describe how we created and used a mathematical model for a multi-component biofilm containing active bacteria, inert biomass, and EPS. The development demanded that we accomplish these major breakthroughs in biofilm modeling: (1) quantitatively describe the formation and consumption of EPS by active bacteria; (2) create a two-dimensional, cellular-automaton model that tracks the growth and movement of the three biomass components, along with several soluble chemical species; (3) computes a composite density for the sum of all biomass components; (4) incorporate a consolidation phenomenon to represent the slow "densification" of biomass; and (5) solve the model to gain new insights and compare its outputs to experimental data, where they exist. This talk will highlight each of the steps and point out how the findings shed light on what controls the mechanical strength of a biofilm.