Abstract
We revisited the mathematical model of the chemostat and examined consequences of considerably decreasing the concentration of limiting nutrient in the inflow for the growth of both the planktonic and biofilm cells in the chemostat tank (fermenter). The model predicts a substantially lower steady-state biomass of planktonic cells in response to decreasing inflowing nutrient concentration. Contrarily, the steady-state concentration of nutrient inside the fermenter is expected to remain the same, as long as the inflowing concentration does not fall below its value. This allows the biofilm cells to grow at a rate regulated only by the exchange rate of the medium (dilution rate). We maintained a strain of Enterococcus faecalis in a chemostat of our own design with limiting nutrient in the inflow set near saturation constant at three dilution rates (0.09, 0.28, and 0.81 h-1). The highest dilution rate was near the critical rate calculated by the model. The one-day total biofilm buildup was 21× larger and its estimated growth rate 2.4× higher at highest dilution rate than at the lowest one. This increased biofilm formation with increased dilution rates is in agreement with previously published data on pure and mixed continuous flow cultures.
Highlights
A large amount of both experimental data and mathematical description of biofilm buildup has accumulated in the past (Rittmann and McCarthy, 1980; Evans et al, 1994; Beyenal and Lewandowski, 2002; Vinogradov et al, 2004; Wessel et al, 2013; Fernandez et al, 2017), yet little attention has been paid to the general validity of the chemostat theory in the context of biofilm formation
We chose a strain of E. faecalis (ATCC 29212) for estimating initial rates of exponential growth at serial dilutions of its common culture medium, Trypticase Soy Broth (TSB)
Even though the original function had been designed for a single limiting substrate, it allowed estimating the concentration of sterile medium just sufficient to support population of bacteria growing at a given rate (Figure 4)
Summary
A large amount of both experimental data and mathematical description of biofilm buildup has accumulated in the past (Rittmann and McCarthy, 1980; Evans et al, 1994; Beyenal and Lewandowski, 2002; Vinogradov et al, 2004; Wessel et al, 2013; Fernandez et al, 2017), yet little attention has been paid to the general validity of the chemostat theory in the context of biofilm formation. Even though the chemostat model itself does not include a biofilm component, it calculates a “common currency” for both plankton and biofilm, i.e., concentration of substrate limiting the growth of the strain in question. As soon as a cell attaches to a submerged surface (underlying substratum material or previously attached cells), the current concentration of substrate ceases to be Dilution Rate and Nutrient Availability for Biofilms the limiting factor for its survival which creates a competitive advantage for attached cells over the cells still in suspension. Adhesion is a key trait that biofilm cells need to succeed in competition. It is accomplished using attachment factors and extracellular polymers. The production of extracellular matrix varies among strains and may result in eliminating the less producing cells from the systems by sloughing (Schluter et al, 2015)
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