The Physics of Plasmid Transfer on Microbial Colonies

In addition to chromosomal DNA, many bacteria contain small pieces of DNA called plasmids. Plasmids often carry genes that provide resistance to antimicrobial drugs. Consequently, plasmid transfer (conjugation) between unrelated bacteria is an important factor contributing to the spread of antibiotic resistance. Using a computer model and simple calculations I have studied how the physics of inter-bacteria interactions affects the rate of plasmid transfer between adjacent cells in expanding bacterial colonies. In my talk, I will show that, although neutral (no fitness cost) plasmids may eventually spread to the whole colony, this process is hindered by spatial expansion and is thus very slow. In addition, mechanical forces acting between the cells may further decrease the rate of plasmid transfer to negligible levels.