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RESEARCH PROFILES
Biofilms are all around us. For example, the plaque on your teeth is a biofilm. Rocks at the bottom of rivers and streams often are coated with slippery biofilms. Biofilms also can spoil food, clog drains, and cause many human infections, such as those of the urinary tract. Some of these microbial communities also are resistant to antibiotics and biocides. What are these strange substances? They are bacteria or other microorganisms that stick together and secrete an extracellular matrix, most commonly composed of polysaccharides. These microbial communities often are found on solid surfaces that are immersed in liquid. Although biofilms occur almost everywhere in nature, they are still shrouded in mystery. Recently, a controversy has erupted over whether biofilms are controlled by unique regulatory proteins or simply composed of normal cells at various growth stages. To address this issue, Volker Brözel and Sébastien Vilain at South Dakota State University used statistical methods to study the Bacillus cereus proteome. In this issue of JPR (pp 1924–1930), they report that the B. cereus biofilm proteome is different from the proteomes of other B. cereus populations. In addition, they found that free-floating, or planktonic, bacteria that live near a biofilm are different from planktonic cells that have not been exposed to a biofilm. To encourage biofilm formation, the researchers added B. cereus to an Erlenmeyer flask that contained a liquid medium and glass wool. A small percentage of the population formed a biofilm on the glass wool, whereas most of the cells remained planktonic; these free-floating cells were called planktonic with glass wool (PGW). This culturing method simulates real-world conditions better than other current methods, which involve washing a sterile culture medium over a surface that has a biofilm growing on it, says Brözel. With the glass-wool system, planktonic
cells are constantly present and can stick to or detach from the biofilm. As a first step, Brözel and Vilain compared the growth rates of the PGW and biofilm cells with those of planktonic cells grown in a separate culture flask without glass wool. The biofilm cells grew slowly, but both populations of planktonic cells grew heartily and with the same kinetics. Therefore, the VOLKER BRÖZEL AND SÉBASTIEN VILAIN
A unique biofilm proteome
Unique population. A B. cereus biofilm (green and red) on a piece of glass wool, cultured in batch. Cells that appear red are surrounded by an extracellular matrix that contains DNA and proteins.
presence of the biofilm and the glass wool did not affect PGW cell division or survival. But how are the biofilm and planktonic cells different on the molecular level? To answer this question, the researchers ran whole-cell extracts of biofilm and PGW cultures on 2DE gels. In addition, they analyzed extracts of exponential-, transient-, and stationary-phase planktonic cells that had been grown in a separate flask without glass wool. The researchers observed 823 spots on the gels, but no single spot was unique to the biofilm population. However, this population did exhibit altered levels of some proteins, says Brözel. Typically, the next step would be to identify each protein that was differentially expressed among the bacterial populations. “Identifying the proteins is very powerful, but then the focus
1824 Journal of Proteome Research • Vol. 5, No. 8, 2006
is on those specific proteins. You are studying a subset of the data,” explains Brözel. He and Vilain applied statistical methods to their data instead. With this approach, each of the 823 spots was given “an equal voice”, he says. Using principal components analysis (PCA), the researchers compared the protein expression of the five types of cells. The biofilm proteome was distinct from all of the other proteomes, including in silico mixtures of planktonic-cell proteomes at different stages. According to Brözel, these results indicate that the biofilm is not merely a group of planktonic cells of varying ages or stages that happen to stick together and to the glass wool. Finally, Brözel and Vilain wondered whether the PGW population was different from the planktonic cultures grown without glass wool and from the biofilm. They performed PCA with PGW, biofilm, and planktonic proteomes as well as with in silico mixtures of these proteomes. After reviewing the data, the researchers realized that the PGW proteome was distinct from the other proteomes; therefore, they renamed the PGW population “biofilm and surface-exposed planktonic”. Brözel explains, “Somehow, the biofilm or the surface influences the way that swimming cells behave.” Brözel and Vilain currently are following up on several other interesting findings from this project. For example, they have identified a few of the differentially regulated proteins, and they are investigating the composition of the B. cereus extracellular matrix. So far, DNA and three proteins have been found in the matrix. The researchers also plan to study the PGW population. “We think that if we can understand better how the biofilm influences its surroundings and how those surrounding cells behave, [then we may have] a better shot at understanding why cells detach from the biofilm,” says Brözel. “Studying that [aspect] in more detail may open a new window to finding mechanisms of controlling biofilms.” —Katie Cottingham
© 2006 American Chemical Society
