Bioavailability Research Area

Rationale for the biofilm mode of growth in oligotrophic environments is the accessibility of adsorbed organic matter. There has been a great deal of conjecture about the availability of sorbed organics to biofilm bacteria, but little data exists. We are addressing the issue of bioavailability in three contexts: (1) direct utilization of natural organic matter, predominantly as humic substances, by biofilms, (2) the availability of hydrophobic organic compounds to biofilms when these compounds are adsorbed to natural organic matter, and (3) the availability of imbedded antimicrobial agents to deter long-term biofilm growth. In all of these cases, conventional wisdom states that the availability of sorbed compounds is predominantly governed by the equilibrium of the sorbed compound in the aqueous phase. This possibility and alternative rate limiting steps in the biofilm/compound interactions¹ are being pursued.

A critical component in understanding bioavailability issues is the physiological changes that take place when a cell makes the transition from being in suspension to colonizing a surface. It is also necessary to adequately model biofilm colonization on surfaces with and without adsorbed compounds so that the influence of sorbed materials can be determined.

The issue of bioavailability has profound implications for diverse interest areas including global carbon cycling, bioremediation of hydrocarbon contaminated soils and aquifers, and the growth of biofilms in drinking water and ultrapure water distribution systems.

Goal

The goal of this research area is to determine how the interaction of surfaces modified with compounds and bacterial physiology promotes bioiflm formation. Specific objectives include:

Determining the ability and importance of adsorbed organic and inorganic molecules for the support of biofilm growth;

 

Identifying physiological changes that distinguish bioflm cells from planktonic cells;

 

Developing mathematically sound methods to predict the manner in which initial attachment on surfaces with and without sorbed materials influences the distribution of cells within a biofilm.

Highlights

Biofilms are capable of using humic substances extracted from soils as a sole carbon and energy source. Mixed natural populations of organisms could use 78% of added humics in 2 hrs, while scraped and suspended biofilms could utilize approximtely 20% in two days. Total numbers of biofilm cells grown on humics were nearly the same as the those produced on an equal amount of amino acids or carbohydrates. The humic substances were visibly sorbed to the biofilm matrix.

When bacteria are transferred from a chemostat to a fresh batch culture, growth continues with an average doubling time nearly equal to that in the chemostat (1000% increase in twelve hours). In contrast, cells attached to a surface and fed with the same medium as in the batch culture had only a 20% population increase in the same time period. Other research has shown that many new proteins are expressed during the first few hours after a cell from a chemostat attaches to a surface. It is probable that these bacteria are changing phenotype from a suspended to biofilm form.

FIGURE 1. This figure illustrates the concept of desorption-rate limited bioavailability. Experiment #1 (Biotic-no soil, green line) was performed in a batch culture degrading phenanthrene, where each point indicates the concentration of phenanthrene remaining in solution. The slope of the line (approx. 4.9E-4 1/hr) indicates the first order biotransformation rate coefficient of phenanthrene where desorption-limited bioavailability is not an issue. Experiment #2 (abiotic-with soil, blue line) was performed in a column packed with phenanthrene contaminated soil that was flushed continuously with microbiological media, where each point represents the concentration of phenanthrene remaining on the soil. Thus, the slope of the line (approx. 2.4E-3 1/hr) indicates the first order desorption rate coefficient of phenanthrene in the soil system. Experiment #3 (biotic-with soil, red line) was performed in a batch slurry containing phenanthrene contaminated soil, where each point represents the concentration of phenanthrene remaining in the slurry. Thus, the slope of the line (approx. 4.4E-3 1/hr) indicates the first order biotransformation rate coefficient of phenanthrene under desorption-limited bioavailability conditions. The hypothesis that desorption controls bioavailability is confirmed by noting the similarity in the desorption rate coefficient (Experiment #2) and the soil slurry biotransformation rate coefficient (Experiment #3), and noting that they are both much less than the aqueous phase biotransformation rate coefficient (Experiment #1).  Data provided by Ryan Jordan, MSU-CBE

Return to top