Background: Conjugation plays a major role in the transmission of plasmids encoding antibiotic resistance genes in both clinical and general settings. The conjugation efficiency is influenced by many biotic and abiotic factors, one of which is the taxonomic relatedness between donor and recipient bacteria. A comprehensive overview of the influence of donor-recipient relatedness on conjugation is, however, still lacking, but such an overview is important to quantitatively assess the risk of plasmid transfer and the effect of interventions which limit the spread of antibiotic resistance, and to obtain parameter values for conjugation in mathematical models. Therefore, we performed a meta-analysis on reported conjugation frequencies from Escherichia coli donors. Results: 32 studies reporting 313 conjugation frequencies for liquid broth matings and 270 conjugation frequencies for filter matings were included in our meta-analysis. The reported conjugation frequencies varied over 11 orders of magnitude. Decreasing taxonomic relatedness between donor and recipient bacteria, when adjusted for confounding factors, was associated with a lower conjugation frequency in liquid matings. The conjugation frequency for bacteria of the same order, the same class, and other classes was 10, 20, and 789 times lower than the conjugation frequency within the same species, respectively. This association between relatedness and conjugation frequency was not found for filter matings. The conjugation frequency was furthermore found to be influenced by temperature in both mating types, and in addition by plasmid incompatibility group in liquid matings, and by recipient origin and mating time in filter matings. Conclusions: In our meta-analysis, taxonomic relatedness is limiting conjugation in liquid matings, but not in filter matings, suggesting that in general taxonomic relatedness is not a limiting factor for conjugation in environments where bacteria are fixed in space. One such environment is the intestinal mucus, which is considered an important hotspot for the transmission of resistance plasmids with consequences for public and veterinary health. Conjugation of plasmids carrying extended-spectrum beta-lactamase genes is more efficient in the intestine than in liquid mating. Together this suggests that distantly related bacteria which live together in the gut mucus could exchange resistance plasmids through conjugation over large taxonomic distances.
