Bacterial chromosome size is more strongly associated with plasmid size than the number of plasmids per cell across different species of Enterobacterales

Plasmids are mobile genetic elements and are responsible for the horizontal spread of genes within and between species. Plasmid genomes are known to be vary in size from less than a thousand base-pairs to several mega-bases. What determines the size of a plasmid remains unclear, as the fitness cost of a plasmid does not appear to be strictly related to its size. The size of the bacterial host chromosome may be a key factor, as the greater capacity to replicate the host chromosome could lead to greater plasmid sizes or diversities. Using a collection of over 7000 host chromosomes and 14 000 plasmids from 23 species of Enterobacterales, average host chromosome size was found to correlate with average plasmid size. This was primarily driven by changes at a deep taxonomic scale and the pattern is not found between neighbouring species. The average number of different plasmids carried was also found to correlate to host chromosome size, but no more strongly than expected from the shape of phylogeny alone. Mean plasmid size was not related to the mean number of plasmids carried. Overall this highlights how different species of bacteria have contrasting relationships with plasmids, and how plasmids may have divergent roles in the ecology and evolution of different species. This finding is discussed in relation to the traits carried by plasmids and the relationship with other mobile genetic elements.