This review discusses the various types of bacterial genomic islands that can be transferable by conjugation, their intra- and intercellular mobility, and the m

Integrative and conjugative elements (ICEs), also called conjugative transposons, are genomic islands that excise, self-transfer by conjugation, and integrate in the genome of the recipient bacterium. The current investigation shows the intraspecies conjugative transfer of the first described ICEs in Streptococcus thermophilus , ICE St1 and ICE St3 . Mitomycin C, a DNA-damaging agent, derepresses ICE St3 conjugative transfer almost 25-fold. The ICE St3 host range was determined using various members of the Firmicutes as recipients. Whereas numerous ICE St3 transconjugants of Streptococcus pyogenes and Enterococcus faecalis were recovered, only one transconjugant of Lactococcus lactis was obtained. The newly incoming ICEs, except the one from L. lactis , are site-specifically integrated into the 3′ end of the fda gene and are still able to excise in these transconjugants. Furthermore, ICE St3 was retransferred from E. faecalis to S. thermophilus . Recombinant plasmids carrying different parts of the ICE St1 recombination module were used to show that the integrase gene is required for the site-specific integration and excision of the ICEs, whereas the excisionase gene is required for the site-specific excision only.

Some antibiotics induce the dissemination of their own resistance genes by interfering with the regulation of specific mobile genetic elements. In Tn916, subinhibitory concentrations of tetracycline activate the transfer of the element through an anti-attenuation mechanism that relies on the Tet(M) resistance protein, itself encoded by the element. This work explores the effects of a broad range of antibiotics on the transfer of Tn916 and for which the element does not provide any selective advantage. A sensitive promoter–reporter fusion approach was developed to test the effects of full antibiotic concentration gradients on gene promoter expression. Sixty molecules, covering most classes of antibiotics, were screened for their ability to modulate the activity of promoter P_orf12 controlling the transfer of Tn916. Induction of Tn916 transfer was further demonstrated in mating assays with Enterococcus faecalis donors pre-exposed to subinhibitory concentrations of modulating antibiotics. Several antibiotics, other than tetracyclines, were identified as interfering with Tn916 regulation. Macrolides, lincosamides and streptogramins appeared to activate the transfer of Tn916 at unprecedented levels, in a Tet(M)-independent way that implies a yet undescribed regulatory mechanism for controlling the mobility of the element. These results demonstrate that some ribosome-targeting antibiotics can induce the transfer of a given mobile genetic element, here Tn916, although it does not provide any resistance determinant for most of the triggering drugs. This implies that specific antibiotic therapies can have dramatic impacts on the dissemination of unexpected and unlinked resistance genes, with the clear risk of reducing our therapeutic potential for later treatments.

Reducing the dissemination of antimicrobial resistance (AMR) is one of the biggest challenges for public health today. Containment of AMR is compromised by AMR determinants of anthropogenic origin that are continually released into the environment via human waste streams from animal husbandry, food production facilities and reclaimed or untreated wastewater in the form of resistant bacteria or as antimicrobial resistance genes (ARGs) encoded on free extracellular DNA (exDNA) or encapsulated in vesicles or bacteriophages. Although much effort has been devoted into the elucidation of cell-associated propagation of AMR substantial knowledge gaps on the role of cellfree DNA in horizontal gene transfer in wastewater and downstream environments remain. The contribution of cell-free DNA to the overall spread of AMR has been underestimated for a long period of time. The increasing water scarcity associated to climatic change requires developing innovative wastewater reuse practices and, concomitantly, a robust evaluation of AMR occurrence by implementing treatment technologies able to exert a stringent control on AMR propagation in downstream environments exposed to treated or non-treated wastewater. This necessarily implies understanding the fate of ARGs on various forms of cell-free DNA, and during treatment processes that are permissive to their formation. We propose that comprehensive approaches, investigating both the occurrence of ARGs and their compartmentalization in different forms of cellular or cell-free associated DNA should be established for each treatment technology. This should then allow