Biofilms facilitate cheating and social exploitation of β-lactam resistance in Escherichia coli

Gram-negative bacteria such as Escherichia coli commonly resist beta-lactam antibiotics using plasmid-encoded beta-lactamase enzymes. Bacterial strains that express beta-lactamases have been found to detoxify liquid cultures and thus to protect genetically susceptible strains, constituting a clear laboratory example of social protection. These results are not necessarily general; on solid media, for instance, the rapid bactericidal action of beta-lactams largely prevents social protection. Here, we tested the hypothesis that the greater tolerance of biofilm bacteria for beta-lactams would facilitate social interactions. We used a recently isolated E. coli strain, capable of strong biofilm formation, to compare how cooperation and exploitation in colony biofilms and broth culture drives the dynamics of a non-conjugative plasmid encoding a clinically important beta-lactamase. Susceptible cells in biofilms were tolerant of ampicillin-high doses and several days of exposure were required to kill them. In support of our hypothesis, we found robust social protection of susceptible E. coli in biofilms, despite fine-scale physical separation of resistant and susceptible cells and lower rates of production of extracellular beta-lactamase. In contrast, social interactions in broth were restricted to a relatively narrow range of ampicillin doses. Our results show that beta-lactam selection pressure on Gram-negative biofilms leads to cooperative resistance characterized by a low equilibrium frequency of resistance plasmids, sufficient to protect all cells.