Abstract
By transferring ecologically important traits between species, plasmids drive genomic divergence and evolutionary innovation in their bacterial hosts. Bacterial communities are often diverse and contain multiple coexisting plasmids, but the dynamics of plasmids in multi-species communities are poorly understood. Here, we show, using experimental multi-species communities containing two plasmids, that bacterial diversity limits the horizontal transmission of plasmids due to the ‘dilution effect’; this is an epidemiological phenomenon whereby living alongside less proficient host species reduces the expected infection risk for a focal host species. In addition, plasmid horizontal transmission was also affected by plasmid diversity, such that the rate of plasmid conjugation was reduced from co-infected host cells carrying both plasmids. In diverse microbial communities, plasmid spread may be limited by the dilution effect and plasmid–plasmid interactions, reducing the rate of horizontal transmission.
Highlights
Mobile genetic elements are an important source of potentially beneficial accessory traits for host bacteria, equipping these bacterial cells with new ready-to-use functions and thereby allowing them to expand their ecological niche [1,2,3]
In communities where plasmids rely on horizontal transmission for their maintenance [8, 11], plasmid dynamics could be affected by the diversity of the community, especially if the different host species differ in their proficiency and transmission rates
Studies focused on parasite transmission in host communities have shown that the transmission of multi-h ost parasites can be limited by species richness, which is termed the ‘dilution effect’ [12, 13]: a focal host species has a reduced risk of parasite infection when in a diverse community than would be expected from its intraspecific transmission rate, if transmission from other species in the community is less efficient [14]
Summary
Mobile genetic elements are an important source of potentially beneficial accessory traits for host bacteria, equipping these bacterial cells with new ready-to-use functions and thereby allowing them to expand their ecological niche [1,2,3]. The long-term persistence of plasmids in bacterial communities will depend both on the proficiency of host species to stably maintain plasmids in their populations by vertical transmission [7], and the rate of horizontal transmission of plasmids within and between species by conjugation [8]. Previous studies have shown that plasmids are not maintained across different host species [9, 10], while plasmid transmission dynamics are affected by bacterial community structure [8]. In communities where plasmids rely on horizontal transmission for their maintenance [8, 11], plasmid dynamics could be affected by the diversity of the community, especially if the different host species differ in their proficiency and transmission rates. We hypothesize that the dilution effect may apply to plasmids in communities where hosts differ in their ability to maintain and transmit plasmids
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