Biofilm Formation

When microorganisms reach a certain density, many begin to form aggregates and attach to surfaces. First microorganisms establish a biopolymeric matrix in which the cells become embedded. Biofilm formation leads to drastic physiological changes of the individual planktonic cells; for example, they lose their flagellum that is not needed within the biofilm. Often biofilms are formed as mixed communities of different

A/-Acylhomoserine lactone (AHL) y-Butyrolactone (A-factor) 1 2

OH O

O OH Virginia lactone 3

HO OH B

3-Hydroxypalmitic acid methyl ester

HO HO

H HN

O Autoinducer 2 (AI-2)

Diketopiperazine 6

O Autoinducer 2 (AI-2)

2-Acyl-3-hydroxy-4-quinolone (PQS) 7

2-Acyl-3-hydroxy-4-quinolone (PQS) 7

Figure 1 Microbial quorum-sensing molecules.

microorganisms, leading to synergetic advantages for nutrient uptake as well as for defense.

A biofilm offers multifaceted protection against attack: First, the structure of the biofilm provides good shelter against toxic compounds because biocides cannot penetrate efficiently into the inside of the biofilm. Thus many cells are not exposed in high enough concentration to toxins that kill them. In addition, some microbial cells deep inside the biofilm differentiate into metabolically dormant states. Due to their downregulated metabolism, toxins that usually act by blocking the cell's metabolism become inefficient. Because of their high stability against drastic conditions, such cells have been named persisters. After the destruction of metabolically active cells in the biofilm, life in the biofilm may be reestablished by the persister cells. Also the diversity of mixed microbial communities helps the individual microorganisms to survive attack because deleterious foreign compounds are more likely to be detoxified by the combined enzymatic arsenal of many microorganisms. Furthermore, the chances for exchange of antibiotic resistance mechanisms via plasmid transfer are favored in close vicinity compared to diluted planktonic cells.

Consequently, microbial biofilms are extremely durable and thus also cause severe economical and medicinal problems (e.g., biofilms on ship hulls, cystic fibrosis, or caries) because standard disinfection or antibiotic treatments often fail.

A promising way to fight biofilms is to prevent their establishment by confusing microbial quorum-sensing processes in the first place. For example, the alga Delisea pulchra produces brominated furanones that interfere with microbial quorum sensing and thus prevent their attachment to the alga (see Defense Strategies of Marine and Aquatic Organisms).

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