The dilution effect limits plasmid horizontal transmission in multispecies bacterial communities

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.

Interspecific competition can drive the loss of conjugative plasmids from a focal species in a microbial community

“…In turn, plasmid maintenance is decreased in communities with low conjugation frequencies, where interspecific competition can increase the cost of plasmid carriage. That said, recent work indicates that community context can also limit conjugation to focal species due to the dilution effect [23].…”

Ecological dynamics of plasmid transfer and persistence in microbial communities

“…Diverse multispecies communities will contain a diversity of possible hosts, each of which will vary in their ability to harbour and spread conjugative plasmids [43] ; less-permissive hosts with low conjugation rates will introduce barriers to the infectious spread of conjugative plasmids within the community and may ultimately destabilise the maintenance of purely infectious plasmids. An elegant study by Kottara et al [44] demonstrated this effect within communities, establishing that the dilution effect — a hypothesis in disease ecology [45] whereby disease risk is reduced as a result of less efficient disease vectors diluting the impact of highly competent vectors — can equally apply to the infectious spread of plasmids. Through tracking the spread of a highly conjugative plasmid, pQBR57, within multispecies communities, it was shown that the presence of a less permissive host with significantly lower conjugation rates reduced the overall infectivity of the plasmid within the focal species [44] .…”

Estimating the transfer rates of bacterial plasmids with an adapted Luria–Delbrück fluctuation analysis

“…In this paper, we have highlighted the possibility that the rate of conjugation may change (substantially) with the identity of the plasmid-bearing cell [32][33][34]. For example, as a plasmid moves from the original donor strain to the recipient background (forming a transconjugant), the transfer rate can change (i.e., γ D 6 ¼ γ T ).…”