Clinical significance of seeding dispersal in biofilms: A response

Abstract

We welcome the dialogue concerning the potential clinical significance of seeding dispersal (Purevdorj-Gage et al., 2005) in the life cycle of mucoid Pseudomonas aeruginosa biofilms. We had based our hypothesis that the seeding dispersal phenomenon may be more relevant for non-mucoid, environmental strains on (1) the propensity of diseases associated with reduced mucociliary clearance in the lung, such as cystic fibrosis (CF) or chronic obstructive pulmonary disease (COPD), to select for mucoid P. aeruginosa phenotypes, and (2) that conversion to mucoidy is usually associated with a concomitant down-regulation of flagella production and loss of swimming motility (see Garrett et al., 1999). However, as Kirov et al. discuss above, there may be greater diversity in mucoid CF isolates than generally acknowledged, and the perceived dichotomy between mucoidy and swimming phenotypes should be a topic for debate. More recently it has been shown that expression of flagellum genes in response to oxygen limitation precedes loss of mucoidy and is reversible during this state of transition (Wyckoff et al., 2002). Further, in an ongoing screen of mucoid CF isolates it was found that 6 out of 20 were motile (D.J. Wozniak, personal communication). However, it was suspected that growth medium may also play a role in the outcome of the motility assay and exact proportions may vary depending on culture conditions. The growth-condition-dependent transient switching between mucoid and swimming phenotypes is problematic when relating a particular biofilm behaviour with phenotype and stresses the importance of attempting to characterize the phenotypic state at various time points during biofilm development. Broadly though, the finding of Dr Wozniak is in agreement with that of Kirov et al. We agree that to fully assess the role of seeding dispersal (and other yet unidentified behavioural developmental phenotypes) in the context of lung infections, biofilm studies should include greater diversity in strains, growth conditions and be conducted over longer time scales. The interesting observation that seeding motility occurred in a mucoid CF isolate clearly demonstrates that the phenomenon may have clinical relevance.