Immigration and emigration of Burkholderia cepacia and Pseudomonas aeruginosa between and within mixed biofilm communities A.G. Al-Bakri, P. Gilbert and D.G. Allison School of Pharmacy and Pharmaceutical Sciences, University of Manchester, Manchester, England, UK 2003/0569: received 27 June 2003, revised and accepted 23 September 2003 ABSTRACT A.G. AL-BAKRI, P. GILBERT AND D.G. ALLISON. 2004. Aims: To investigate the dynamics of binary culture biofilm formation through use of both the Sorbarod model of biofilm growth and the constant depth film fermenter (CDFF). Methods and Results: Pseudo steady-state biofilm cultures of laboratory and clinical strains of Pseudomonas aeruginosa, selected on the basis of their ability to produce a Burkholderia cepacia growth-inhibitory substance, were established on Sorbarod filters and challenged with corresponding planktonic grown cultures of B. cepacia. Reverse challenges were also conducted. Both B. cepacia and P. aeruginosa were able to form steady-state monoculture biofilms after 48 h growth. When steady-state biofilms of B. cepacia NTCT 10661 were challenged with planktonically grown P. aeruginosa PAO1 known to produce a B. cepacia growth-inhibitory substance, the immigrant population was rapidly and almost completely bound to the biofilm, displacing B. cepacia. By contrast, established biofilms of P. aeruginosa PAO1 resisted immigration of B. cepacia 10661. Similar experiments conducted with a nongrowth inhibitory substance producing clinical pairing of P. aeruginosa 313113 and B. cepacia 313113 led to the formation of stable, mixed biofilm populations in both instances. Moreover, co-inoculation with these clinical isolates resulted in a stable, mixed steady-state biofilm. Similar observations were made for biofilms generated in CDFFs. In such instances following pan-swapping between two monoculture CDFFs, B. cepacia 313113 was able to integrate into an established P. aeruginosa 313113 biofilm to form a stable binary biofilm. Conclusions: Establishment of a mixed species community follows a specific sequence of inoculation that may either be due to some degree of match between co-colonizers or that P. aeruginosa predisposes uncolonized sections of the surface to permit B. cepacia colonization. Significance and Impact of the Study: Colonization of a surface with one bacterial species confers colonization resistance towards other species. Disinfection of a surface might well increase the probability of pathogen harbourage. Keywords: biofilms, Burkholderia cepacia, constant depth film fermenter, Pseudomonas aeruginosa, Sorbarod. INTRODUCTION In the majority of natural situations, monospecies biofilms are comparatively rare, primarily colonizing medical pros- theses and in-dwelling devices. By comparison, in most natural and industrial environments biofilms are complex communities consisting of more than one microbial species (Stoodley et al. 2002). Moreover, mixed species biofilms are often thicker and more stable than monospecies biofilms, a factor that may contribute towards their profound resistance to antimicrobial agents (Allison et al. 2000). Whilst there have been many studies made which investigate the Correspondence to: D.G. Allison, School of Pharmacy and Pharmaceutical sciences, University of Manchester, Oxford Road, Manchester M13 9PL, England, UK (e-mail: david.allison@man.ac.uk). ª 2004 The Society for Applied Microbiology Journal of Applied Microbiology 2004, 96, 455–463 doi:10.1111/j.1365-2672.2004.02201.x