Spatio-Temporal Dynamics of Phase Variants in Expanding Gonococcal Populations

Abstract

Various bacterial species carry mutation hotspots on their genomes that act at high frequencies to reversibly switch off the generation of surface molecules. This process of phase variation is believed to be a major adaptive strategy to escape the immune system. However, most new clones are likely to get lost within a population due to the stochastic fluctuations in the early fixation process.Here, we directly visualised the spatio-temporal dynamics of a new clone and its progeny within a growing microcolony of the human pathogen Neisseria gonorrhoeae. At a high rate N. gonorrhoeae switches off the production of a major virulence factor, the type IV pilus. Imitating this, we designed DNA to replace an essential pilus gene with the gene for an egfp reporter and plated it on a surface. Stochastically, individual P+ cells within an expanding colony growing on this surface integrated this DNA into their genome, lost their pili (P-) and became fluorescent.Hardly any clones arising more than six bacterial diameters behind the expanding front fixed. Closer to the front, we found the probability of fixation of P- clones to be strongly increased compared to the probability for neutral P+ clones to reach the front. P- bacteria were expelled from the P+ microcolony. Due to high surface tension of P+ colonies, P- bacteria accumulated at the front of expanding colonies. This could be confirmed by characterising spontaneous mutations from the P+ to the P- state in range expansion assays.We conclude that the reduced physical interaction between P- and P+ cells, leads to an increased probability of fixation for P- gonococci within a microcolony. It is tempting to speculate that gonococci can shield their major antigen, the type IV pilus, by surrounding themselves with P- cells.