Group-beneficial traits, frequency-dependent selection and genotypic diversity: an antibiotic resistance paradigm

The evolution of group-beneficial traits potentially allows the survival of 'cheaters' that would otherwise be unfit. Here we describe experimental work on group-beneficial traits and the consequences of frequency-dependent selection in the context of bacterial antibiotic resistance. We constructed a 'self-limited antibiotic resistant' (SLAR) strain of Escherichia coli in which a TEM-1 beta-lactamase was anchored to the inner membrane. In pairwise competition experiments between the SLAR strain and ampicillin-sensitive strains, only the SLAR strain survived in the presence of ampicillin. We also constructed a 'shared antibiotic resistant' (SAR) strain in which TEM-1 beta-lactamase protected both the SAR strain and nearby sensitive cells, thus acting as a model for a genetically defined group-beneficial trait. In pairwise competition experiments of the SAR strain against two different sensitive strains of E. coli, we found that the sensitive strains maintained themselves at frequencies of 5-12% in the presence of ampicillin. When the relative cost of the SAR strain was lowered, its equilibrial frequency rose. Sensitive strains also arose from pure cultures of the SAR strain. In these cases, too, the sensitive 'cheaters' were maintained in ampicillin at frequencies comparable to those observed in the previous competitions. These results suggest that traits which benefit other group members can permit survival of genotypes that otherwise would be eliminated by natural selection, and allow the maintenance of greater genetic variation upon which evolution can operate.