Plasmids survive despite their cost and male-specific phages due to heterogeneity of bacterial populations

Problem: Conjugative plasmids manipulate their bacterial hosts to express sex-pili – long, filamentous organelles whose genes are carried by the plasmids themselves. Sex-pili locate and attach to another bacterial cell for plasmid transfer. Bacteria differ in their ability to express sex-pili and hence to donate plasmids. Does this diversity in sex-pilus expression contribute to the survival of conjugative plasmids? Method: Simulations of a chemostat containing bacterial cells, some harbouring a conjugative plasmid. Certain viruses, called male-specific phages (MSPs), enter the chemostat. These viruses infect only donor cells. I monitor whether plasmids continue to survive in each simulation. Key assumption: A bacterium’s ability to donate is proportional to its susceptibility to MSPs. Conclusions: Bacterial diversity in sex-pilus expression strongly increases the size of the parameter space for plasmid survival. A host with low expression of sex-pili – and hence a low plasmid transfer rate – represents a sink habitat for the conjugative plasmid. However, hosts with high expression of sex-pili also constitute sink habitats – although they have a high plasmid transfer rate, they are the most susceptible to MSPs. A system consisting of both types of bacteria (two sink habitats) constitutes a source habitat for the conjugative plasmid. At the same time, this diversity helps to maintain the MSP itself because extinction of the conjugative plasmids would also doom the MSP.