3-day-old biofilm of wild-type P. aeruginosa PAO1 (green) growing in the presence of the flagellate R. nasuta (red). Image courtesy of Staffan Kjelleberg, University of New South Wales.

The genus Pseudomonas is a physiologically diverse group of bacteria that is present in a wide range of environments, ranging from soil and sewage to plants and animals. A recent report from Environmental Microbiology sheds light on one factor underpinning this ubiquity — resistance to consumption by predatory protozoa.

The most notorious member of this genus, the opportunistic pathogen Pseudomonas aeruginosa, causes several nosocomial infections. In particular, P. aeruginosa, growing as a biofilm, is a major cause of death and morbidity in cystic fibrosis patients. In recent years, it has become well established that biofilm formation is an integral component of the bacterial life cycle, and is a key factor for survival in diverse environments. But do biofilms contribute to the survival of bacteria subject to intense predatory pressure? To investigate this question, Staffan Kjelleberg and colleagues analysed cellular characteristics that are important for P. aeruginosa biofilm development for their roles in resisting protozoan feeding.

Using wild-type and mutant strains of the microorganism with altered biofilm development, the authors examined biofilm formation in response to the presence of the common surface-feeding flagellate, Rhynchomonas nasuta. Their initial observations showed that, in the presence of the predator, bacterial microcolonies formed that were resistant to feeding by the flagellate. The authors also tested strains of P. aeruginosa that were deficient in quorum sensing, or that lacked type IV pili, and found that these mutant bacteria formed significantly fewer microcolonies compared with the wild-type microorganism. When flagella-deficient P. aeruginosa (ΔfliM) were exposed to the protozoan, no microcolony formation was observed, indicating a key role for flagella in the development of microcolonies.

Following initial colonization, microcolonies develop into mature biofilms in a differentiation process associated with the production of alginate. When Matz et al. assessed an alginate-overproducing strain of P. aeruginosa, the development of larger microcolonies in response to feeding by R. nasuta was noted, and the number of protozoa observed was considerably lower than that observed with wild-type bacteria. This finding clearly indicates that alginate production is beneficial for the microorganism as a method of inhibiting predator feeding. The authors were also able to show that mature biofilms formed by wild-type bacteria were acutely toxic to the predator. By contrast, protozoan growth on mature biofilms formed by rhlR/lasR mutant bacteria was very efficient. As the RhlR/LasR quorum-sensing systems are known to regulate the expression of several toxins, these findings indicate that quorum-sensing-mediated toxin production in mature biofilms is an effective mechanism that allows P. aeruginosa to resist protozoan feeding.

In conclusion, the authors have convincingly shown that the formation of microcolonies in the early stages of biofilm development, the production of toxins at later stages, and the regulation of these processes by cell-to-cell communication combine to promote resistance of P. aeruginosa to ingestion by protozoan predators. This protection mechanism might also partly explain the widespread presence of this microorganism, and its persistence in both natural and clinical environments.