A fitness trade off between local competition and dispersal in V. cholerae biofilms

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

Bacteria lead highly interactive lives within densely populated and often heterogeneous communities, termed biofilms, that typically grow on solid-liquid or liquid-air interfaces, and are critical for processes ranging from biogeochemical cycling and bacterial pathogenesis to industrial biofouling. The secretion of extracellular polymeric substances (EPS) is a cooperative trait that benefits all neighboring bacteria, even those that do not contribute to production of the polymer matrix. Because these nonproducing strains do not pay the cost of EPS production, they invade wild-type biofilms on liquid surfaces and compromise their structural integrity. However, bacteria that do produce EPS can occupy spatial locations with superior access to nutrients, relative to non-producing cells, indicating that EPS secretion may be a competitively advantageous phenotype in natural biofilms.

Vibrio cholera was used as the model organism in this experiment. Biofilm growth of V. cholera is initiated after adhering to surfaces and shedding its flagella, using quorum sensing to activate EPS production at low cell density and to repress its production at high cell density. flaA and hapR genes which encode the flagellin core protein and the quorum sensor master regulator respectively were deleted. The resulting ΔflaAΔhapR double mutant constitutively produces EPS and was named EPS+ in this experiment. An isogenic strain lacking vpsL, a gene required for EPS biosynthesis, was also engineered. This ΔflaAΔhapRΔvpsL triple mutant never produces EPS and is known as EPS-.

A number of experiments were carried out using both strains and the results indicate that EPS production is disadvantageous in mixed liquid environments, where the primary fitness currency is simply growth. In Biofilm environments however, a strain that produces EPS readily outcompetes an isogenic strain that does not. The authors conclude that EPS-producing cells and their offspring gain competitive advantage when growing on solid surfaces, due to their capacity to build structures that adhere to the substratum and resist shear stress, and they inherently avoid exploitation by non-producers.

Local competition is not the only factor contributing to long-term evolutionary dynamics: organisms must disperse to new resource patches as old ones are depleted or destroyed, and so the authors considered and conducted a further experiment to test whether constitutive EPS production could affect dispersal ability. Competition between EPS-producing and nonproducing cells fundamentally depend on how often resource patches are created and destroyed. Cells that produce EPS dominate local competition but are rarely dislodged for the structures they generate within biofilms. If resource patches are long-lived and new patches are rarely generated, EPS production will be favored. When resource patches are short-lived and new patches are generated often, however, cells that do not produce EPS will be favored. The study establishes that a fundamental trade-off between local competition and dispersal exists amoung bacteria, and may be governed by a single phenotype.

I though this paper was interesting although there is quite a bit of tricky genetic info to get your head around. The study also carried out quite a few experiments but the paper is presented in a logical order (apart from the method and material bit at the end) and the reasons for experiments stated clearly so is therefore easy to follow.