Abstract
Conjugation is the primary mechanism of horizontal gene transfer that spreads
antibiotic resistance among bacteria. Although conjugation normally occurs in
surface-associated growth (e.g., biofilms), it has been traditionally studied
in well-mixed liquid cultures lacking spatial structure, which is known to
affect many evolutionary and ecological processes. Here we visualize spatial
patterns of gene transfer mediated by F plasmid conjugation in a colony of
Escherichia coli growing on solid agar, and we develop a quantitative
understanding by spatial extension of traditional mass-action models. We found
that spatial structure suppresses conjugation in surface-associated growth
because strong genetic drift leads to spatial isolation of donor and recipient
cells, restricting conjugation to rare boundaries between donor and recipient
strains. These results suggest that ecological strategies, such as enforcement
of spatial structure and enhancement of genetic drift, could complement
molecular strategies in slowing the spread of antibiotic resistance genes.
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