Explore the vast range of titles available.

Abstract Natural transformation is the only process of gene exchange under the exclusive control of the recipient bacteria. It has often been considered as a source of novel genes, but quantitative assessments of this claim are lacking. To investigate the potential role of natural transformation in gene acquisition, we analyzed a large collection of genomes of Acinetobacter baumannii (Ab) and Legionella pneumophila (Lp) for which transformation rates were experimentally determined. Natural transformation rates are weakly correlated with genome size. But they are negatively associated with gene turnover in both species. This might result from a negative balance between the transformation's ability to cure the chromosome from mobile genetic elements (MGEs), resulting in gene loss, and its facilitation of gene acquisition. By comparing gene gains by transformation and MGEs, we found that transformation was associated with the acquisition of small sets of genes per event, which were also spread more evenly in the chromosome. We estimated the contribution of natural transformation to gene gains by comparing recombination-driven gene acquisition rates between transformable and non-transformable strains, finding that it facilitated the acquisition of ca. 6.4% (Ab) and 1.1% (Lp) of the novel genes. This moderate contribution of natural transformation to gene acquisition implies that most novel genes are acquired by other means. Yet, 15% of the recently acquired antibiotic resistance genes in A. baumannii may have been acquired by transformation. Hence, natural transformation may drive the acquisition of relatively few novel genes, but these may have a high fitness impact.

Abstract Bacterial lineages acquire novel traits at diverse rates in part because the genetic background impacts the successful acquisition of novel genes by horizontal transfer. Yet, how horizontal transfer affects the subsequent evolution of core genes remains poorly understood. Here, we studied the evolution of resistance to quinolones in Escherichia coli accounting for population structure. We found 60 groups of genes whose gain or loss induced an increase in the probability of subsequently becoming resistant to quinolones by point mutations in the gyrase and topoisomerase genes. These groups include functions known to be associated with direct mitigation of the effect of quinolones, with metal uptake, cell growth inhibition, biofilm formation, and sugar metabolism. Many of them are encoded in phages or plasmids. Although some of the chronologies may reflect epidemiological trends, many of these groups encoded functions providing latent phenotypes of antibiotic low-level resistance, tolerance, or persistence under quinolone treatment. The mutations providing resistance were frequent and accumulated very quickly. Their emergence was found to increase the rate of acquisition of other antibiotic resistances setting the path for multidrug resistance. Hence, our findings show that horizontal gene transfer shapes the subsequent emergence of adaptive mutations in core genes. In turn, these mutations further affect the subsequent evolution of resistance by horizontal gene transfer. Given the substantial gene flow within bacterial genomes, interactions between horizontal transfer and point mutations in core genes may be a key to the success of adaptation processes.

Natural transformation is the only mechanism of genetic exchange controlled by the recipient bacteria. We quantified its rates in 786 clinical strains of the human pathogens Legionella pneumophila (Lp) and 496 clinical and environmental strains of Acinetobacter baumannii (Ab). The analysis of transformation rates in the light of phylogeny revealed they evolve by a mixture of frequent small changes and a few large quick jumps across 6 orders of magnitude. In standard conditions close to half of the strains of Lp and a more than a third in Ab are below the detection limit and thus presumably non-transformable. Ab environmental strains tend to have higher transformation rates than the clinical ones. Transitions to non-transformability were frequent and usually recent, suggesting that they are deleterious and subsequently purged by natural selection. Accordingly, we find that transformation decreases genetic linkage in both species, which might accelerate adaptation. Intragenomic conflicts with chromosomal mobile genetic elements (MGEs) and plasmids could explain these transitions and a GWAS confirmed systematic negative associations between transformation and MGEs: plasmids and other conjugative elements in Lp, prophages in Ab, and transposable elements in both. In accordance with the hypothesis of modulation of transformation rates by genetic conflicts, transformable strains have fewer MGEs in both species and some MGEs inactivate genes implicated in the transformation with heterologous DNA (in Ab). Innate defense systems against MGEs are associated with lower transformation rates, especially restriction-modification systems. In contrast, CRISPR-Cas systems are associated with higher transformation rates suggesting that adaptive defense systems may facilitate cell protection from MGEs while preserving genetic exchanges by natural transformation. Ab and Lp have different lifestyles, gene repertoires, and population structure. Nevertheless, they exhibit similar trends in terms of variation of transformation rates and its determinants, suggesting that genetic conflicts could drive the evolution of natural transformation in many bacteria.

Natural transformation is the only process of gene exchange under the exclusive control of the recipient bacteria. It has often been considered as a source of novel genes but quantitative assessments of this claim are lacking. To investigate the potential role of natural transformation in gene acquisition, we analysed a large collection of genomes of Acinetobacter baumannii (Ab) and Legionella pneumophila (Lp) for which transformation rates were experimentally determined. Natural transformation rates are weakly correlated with genome size. But they are negatively associated with gene flow in both species. This might result from a negative balance between transformation's ability to cure the chromosome from mobile genetic elements (MGEs), resulting in gene loss, and its facilitation of gene acquisition. By focusing on the latter, we found that transformation was significantly associated with small gene acquisition events while MGEs-driven gene acquisition tend to be associated with larger ones. Events of gene gain by transformation were spread more evenly in the chromosome than MGEs encoding the ability to integrate autonomously. We estimated the contribution of natural transformation to gene gains by comparing recombination-driven gene acquisition rates between transformable and non-transformable strains. Natural transformation may have caused the acquisition of up to 6.4% (Ab) and 1.1% (Lp) of the novel genes. This low contribution of natural transformation to the acquisition of novel genes implies that most novel genes must have been acquired by other means. Interestingly, the ones potentially acquired by transformation include almost 15% of the recently acquired antibiotic resistance genes in A. baumannii. Hence, natural transformation may drive the acquisition of relatively few novel genes but these may have high fitness impact.Competing Interest StatementThe authors have declared no competing interest.

Natural transformation is a widespread molecular pathway of horizontal gene transfer involving the uptake and recombination of exogenous DNA. Exogenous DNA follows a pathway involving genes sequentially required for its capture, internalization, protection, and recombination with the chromosome. Most of these genes were identified through the isolation of transformation-defective mutants and/or based on their expression preceding natural transformation. Yet, genes required for key steps of the pathway remain elusive. We sought to identify any missing component by comparing Tn-seq data obtained in two distantly-related transformable diderm species, the human pathogen Legionella pneumophila and the cyanobacterium Synechococcus elongatus. We identified yraN, a widespread and highly conserved gene of unknown function required for natural transformation. We provide evidence that YraN is a nuclease associated with the ComM helicase, which cooperate to process the D-loop formed by the invasion of the transforming DNA in the chromosomal DNA strands. We propose a model in which cleavage of the displaced strand by YraN can promote the recombination of transforming DNA, leading to extended recombination events. The identification of this YraN/ComM nuclease/helicase system supports the hypothesis that bacteria possess a conserved pathway for the transport and recombination of exogenous DNA.Competing Interest StatementThe authors have declared no competing interest.Footnotes* Added missing label in Figure 1

Bacterial lineages vary in the frequency with which they acquire novel traits, like antibiotic resistance or virulence. While previous studies have highlighted the impact of the genetic background on the successful acquisition of novel traits through horizontal gene transfer, the impact of the latter on the subsequent evolution of bacterial genomes by point mutations remains poorly understood. Here, we studied the evolution of resistance to quinolones in thousands of Escherichia coli genomes. Resistance-conferring point mutations in the core genes are frequent and accumulate very quickly. We searched for gene gains and losses significantly associated with the subsequent acquisition of these resistance mutations. This revealed 60 groups of genes in genetic linkage whose gain or loss induced a change in the probability of subsequently becoming resistant to quinolones by point mutations in gyrA and parC. Although some of these chronologies may reflect epidemiological trends, most of these groups encoded functions that were previously associated with antibiotic resistance, tolerance, or persistence, often specifically under quinolone treatment. A lot of the largest groups were found in prophages or plasmids, and they usually increased the likelihood of subsequent resistance mutations. Conversely groups of lost genes were typically small and chromosomal. Quinolone resistance was among the first resistances acquired in the extant lineages of E. coli and its acquisition was associated with an increased likelihood of acquiring other types of resistances, including to aminoglycosides and beta-lactams. Our findings suggest that gene flow shapes the subsequent fixation rate of adaptive mutations in core genes. Given the substantial gene flow within bacterial genomes, interactions between horizontal transfer and point mutations in core genes may be key to the success of adaptation processes.

Natural transformation is the only mechanism of genetic exchange controlled by the recipient bacteria. We quantified its rates in 1282 strains of the human pathogens Legionella pneumophila (Lp) and Acinetobacter baumannii (Ab) and found that transformation rates evolve by large quick changes as a jump process across six orders of magnitude. Close to half of the strains are non-transformable in standard conditions. Transitions to non-transformability were frequent and recent, suggesting that they are deleterious and subsequently purged by natural selection. Accordingly, we find that transformation decreases genetic linkage in both species, which often accelerates adaptation. Intragenomic conflicts with chromosomal mobile genetic elements (MGEs) and plasmids could explain these transitions and a GWAS confirmed systematic negative associations between transformation and MGEs: plasmids and other conjugative elements in Lp, prophages in Ab, and transposable elements in both. In accordance with the modulation of transformation rates by genetic conflicts, transformable strains have fewer MGEs. Defense systems against the latter are associated with lower transformation except the adaptive CRISPR-Cas systems which show the inverse trend. The two species have different lifestyles and gene repertoires, but they exhibit very similar trends in terms of variation of transformation rates and its determinants, suggesting that genetic conflicts could drive the evolution of natural transformation in many bacteria.Competing Interest StatementThe authors have declared no competing interest.Footnotes* We improved the methods used for the detection of the recombination regions and the inference of recombination-free phylogenetic trees.