Antibiotics and UV Radiation Induce Competence for Natural Transformation inLegionella pneumophila

Abstract

Natural transformation by competence is a major mechanism of horizontal gene transfer in bacteria.Competence is defined as the genetically programmed physiological state that enables bacteria to actively take up DNA from the environment. The conditions that signal competence development are multiple and elusive, complicating the understanding of its evolutionary significance. We used expression of the competence gene comEA as a reporter of competence development and screened several hundred molecules for their ability to induce competence in the freshwater living pathogen Legionella pneumophila. We found that comEA expression is induced by chronic exposure to genotoxic molecules such as mitomycin C and antibiotics of the fluoroquinolone family. These results indicated that, in L. pneumophila, competence may be a response to genotoxic stress. Sunlight-emitted UV light represents a major source of genotoxic stress in the environment and we found that exposure to UV radiation effectively induces competence development. For the first time, we show that genetic exchanges by natural transformation occur within an UV-stressed population. Genotoxic stress induces the RecA-dependent SOS response in many bacteria. However, genetic and phenotypic evidence suggest that L. pneumophila lacks a prototypic SOS response and competence development in response to genotoxic stress is RecA independent. Our results strengthen the hypothesis that competence may have evolved as a DNA damage response in SOS-deficient bacteria. This parasexual response to DNA damage may have enabled L. pneumophila to acquire and propagate foreign genes, contributing to the emergence of this human pathogen.Bacterial competence is a genetically programmed physiological state that confers the ability to take up DNA from the environment and allows subsequent genetic and phenotypic transformation. The competent state involves expression of a dedicated, well-conserved DNA uptake machinery required to actively import exogenous DNA (9). Bacterial competence allows for uptake and recombination of DNA released by bacteria and can promote genetic exchange among individuals within a population. Most bacteria possess competence genes but the conditions or signals triggering competence are multiple and often species specific (25). Expression of competence is influenced by the growth phase, cell density, metabolic activity, and nutritional stress, and in the well-studied Grampositive organisms Bacillus subtilis and Streptococcus pneumoniae it is believed to be a general response to stress (10). In S. pneumoniae, competence is induced by antibiotic stress, including DNA-damaging antibiotics (36). Competence development in Gram-negative bacteria is less understood and is induced by different conditions, for example, by starvation for Haemophilus influenzae (21) or by growth on a specific carbon source (chitin) for Vibrio cholerae (31). For a specific organism it is therefore difficult to predict and identify the conditions that lead to competence development. Legionella ...

Silencing of natural transformation by an RNA chaperone and a multitarget small RNA

“…Total RNA from bacterial cultures was extracted (22), and Northern blot analysis was performed as previously described (12). RNA-seq analysis was performed on DNase-treated RNA samples from bacterial cultures grown to an OD 600 of 0.8 at 37°C.…”

“…However, a number of species that are naturally transformable lack these known competence activators. One such species is the Gram-negative pathogen Legionella pneumophila, which was found to develop competence under microaerophilic growth, exposure to DNA-damaging agents, or suboptimal growth temperature (11)(12)(13). Possible competence regulatory elements in L. pneumophila were revealed by a genetic screen that identified a gene annotated "proQ, activator of osmoprotectant ProP" as a repressor of natural transformation (14).…”

Key experimental evidence of chromosomal DNA transfer among selected tuberculosis-causing mycobacteria

“…However, neither the use of different mating temperatures (30 or 37°C), nor different concentrations of the DNA cross-linker Mitomycin C (1.7 ng/mL, 30 ng/mL, and 100 ng/mL) yielded recombinants (SI Appendix, Table S4). Mitomycin C was used because it is known to induce competence for transformation in some bacteria, including Bacillus subtilis and Legionella pneumophila by inducing DNA damage (26). However, in another series of experiments, mating assays were performed in the presence of low concentrations of either oxygen or nitrogen radicals (hydrogen peroxide, H 2 O 2 or nitrogen monoxide, NO), or slightly acidic pH (pH 6.1) to mimic putative stimuli encountered by pathogenic mycobacteria inside host cells.…”

Legionella, légionellose

“…environnementales à risque. Ainsi, par la génomique fonctionnelle (étude des gènes régulant les fonctions de virulence) [8,9], l'étude de la réponse de Legionella aux stress génotoxiques a révélé que, contrairement à la plupart des bactéries, celle-ci ne développe pas une réponse de type SOS (réparation de l'ADN et mutagenèse) [44], mais développe une compétence (état physiologique génétiquement programmé conférant la capacité de recueillir de l'ADN provenant de l'environnement) [10]. Cette particularité favorise la diversité génétique, l'acquisition de nouveaux gènes et augmente la virulence de la bactérie.…”