Veni, Vidi, Vici: A fitness trade-off in biofilms

A review of: Nadel CD and Bassler BL (2011) A fitness trade-off between local competition and dispersal in Vibrio cholerae biofilms. PNAS, 108 (34): 14181-14185.

Biofilms are critical for processes ranging from biogeochemical cycling, bacterial pathogenesis and industrial biofouling. Bacteria living in biofilms secrete extra-cellular polymeric substances (EPS) that form a matrix for bacteria to adhere to. Previous studies suggest that EPS secretion benefit the producer and neighbouring cells while enhancing multi-cellular development within the bacterial community, including cells which do not produce EPS. Consequently, non-EPS producing cells are able to invade, exploit and compromise the structural integrity of wild type biofilms because they do not pay the cost of EPS production.

Simulation models of biofilm growth indicate that bacteria that produce EPS can occupy locations with better access to nutrients compared to bacteria that do not produce EPS, suggesting that EPS secretion may be competitively advantageous in natural biofilms. This is contrary to the idea that such environments easily succumb to exploitation by non-EPS producing microbes.

To test the theoretical model, Vibrio cholerae was used for subsequent experiments, based on their ability to initiate biofilm growth upon surface adherence and their capacity to utilise quorum sensing to activate or suppress EPS production at low cell densities and high cell densities respectively. V. cholerae EPS⁺ and EPS^- mutants expressing different fluorescent proteins were used for microscopy analysis. Monoculture results reveal that EPS⁺ strains pay a substantial cost for diverting resources away from biomass production in favour of EPS synthesis when compared to EPS^- strains. In co-inoculated biofilm cultures, EPS⁺ strains proliferated whereas EPS^- strains decreased by more than 80%, indicating subjugation of EPS^- growth. The opposite result was observed in a mixed liquid environment where primary fitness is based on growth rate which is slower in EPS⁺ strains due to EPS production costs.

Local competition is not the only determining factor for long term evolutionary success as micro-organisms must disperse to new resource patches after old ones have been depleted. Whether EPS production affects dispersal ability was assessed using adjacent microfluidic chambers. Frequency of EPS⁺ and EPS^- strains in the biofilm, the liquid effluent and the adjacent chamber was measured at regular intervals. Results show the dominance of EPS⁺ strains in the biofilm while remaining a minority in the liquid, correlating to poor colonization of the adjacent resource patch.

These findings suggest a trade-off between competition and colonization governed by EPS production. Bacteria have evolved to find a balance between local competition and dispersal ability with long term competition dynamics depending on resource availability. Bacteria like V. cholerae have finely tuned regulatory mechanisms such as quorum sensing which modulates the timing and strength of EPS production relative to environmental conditions.