Rhamnolipid Surfactant Production Affects Biofilm Architecture in Pseudomonas aeruginosa PAO1

Abstract

In response to certain environmental signals, bacteria will differentiate from an independent free-living mode of growth and take up an interdependent surface-attached existence. These surface-attached microbial communities are known as biofilms. In flowing systems where nutrients are available, biofilms can develop into elaborate three-dimensional structures. The development of biofilm architecture, particularly the spatial arrangement of colonies within the matrix and the open areas surrounding the colonies, is thought to be fundamental to the function of these complex communities. Here we report a new role for rhamnolipid surfactants produced by the opportunistic pathogen Pseudomonas aeruginosa in the maintenance of biofilm architecture. Biofilms produced by mutants deficient in rhamnolipid synthesis do not maintain the noncolonized channels surrounding macrocolonies. We provide evidence that surfactants may be able to maintain open channels by affecting cell-cell interactions and the attachment of bacterial cells to surfaces. The induced synthesis of rhamnolipids during the later stages of biofilm development (when cell density is high) implies an active mechanism whereby the bacteria exploit intercellular interaction and communication to actively maintain these channels. We propose that the maintenance of biofilm architecture represents a previously unrecognized step in the development of these microbial communities.Although bacteria are commonly viewed as solitary life forms, these organisms are more typically colonial creatures. In their natural settings, bacteria persist within microbial communities, where they exploit elaborate systems of intercellular interaction and communication to adjust to changing environmental parameters. Moreover, biofilm formation has also been linked to the emergence of a variety of opportunistic human pathogens (5). For example, organisms such as Staphylococcus epidermidis and Pseudomonas aeruginosa form biofilms on implants and dead or living tissue, thereby contributing to a variety of persistent infections.Thinking about bacterial populations as connected organisms capable of concerted multicellular activities has provided researchers with novel insights into microbial biology. The key to such multicellular behavior lies in the ability of each individual cell to sense and respond to information from nearby cells, and this behavior requires a certain population size or quorum of cells. The development of biofilms is a process that involves both a quorum of cells and multicellular behavior (6). Single-species biofilms are of particular interest due to their clinical importance and the monospecies biofilms formed by P. aeruginosa has become a prominent model for studying this aspect of microbial biology.The complex structure of microbial biofilms has only recently been determined. Detailed analysis by scanning confocal laser microscopy has shown that biofilms of P. aeruginosa formed on solid surfaces and exposed to a continuous flow of fresh nutrients are open, highly hydrate...

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