Search

The rapidly emerging phenomenon of antibiotic resistance threatens to substantially reduce the efficacy of available antibacterial therapies. Dissemination of resistance, even between phylogenetically distant bacterial species, is mediated mainly by mobile genetic elements, considered to be natural vectors of horizontal gene transfer. Transposable elements (TEs) play a major role in this process—due to their highly recombinogenic nature they can mobilize adjacent genes and can introduce them into the pool of mobile DNA. Studies investigating this phenomenon usually focus on the genetic load of transposons and the molecular basis of their mobility. However, genes introduced into evolutionarily distant hosts are not necessarily expressed. As a result, bacterial genomes contain a reservoir of transcriptionally silent genetic information that can be activated by various transposon-related recombination events. The TEs themselves along with processes associated with their transposition can introduce promoters into random genomic locations. Thus, similarly to integrons, they have the potential to convert dormant genes into fully functional antibiotic resistance determinants. In this review, we describe the genetic basis of such events and by extension the mechanisms promoting the emergence of new drug-resistant bacterial strains. [ABSTRACT FROM AUTHOR]

Several trap plasmids (enabling positive selection of transposition events) were used to identify a pool of functional transposable elements (TEs) residing in bacteria of the genus Paracoccus (Alphaproteobacteria). Complex analysis of 25 strains representing 20 species of this genus led to the capture and characterization of (i) 37 insertion sequences (ISs) representing 9 IS families (IS3, IS5, IS6, IS21, IS66, IS256, IS1182, IS1380 and IS1634), (ii) a composite transposon Tn6097 generated by two copies of the ISPfe2 (IS1634 family) containing two predicted genetic modules, involved in the arginine deiminase pathway and daunorubicin/doxorubicin resistance, (iii) 3 non-composite transposons of the Tn3 family, including Tn5393 carrying streptomycin resistance and (iv) a transposable genomic island TnPpa1 (45 kb). Some of the elements (e.g. Tn5393, Tn6097 and ISs of the IS903 group of the IS5 family) were shown to contain strong promoters able to drive transcription of genes placed downstream of the target site of transposition. Through the application of trap plasmid pCM132TC, containing a promoterless tetracycline resistance reporter gene, we identified five ways in which transposition can supply promoters to transcriptionally silent genes. Besides highlighting the diversity and specific features of several TEs, the analyses performed in this study have provided novel and interesting information on (i) the dynamics of the process of transposition (e.g. the unusually high frequency of transposition of TnPpa1) and (ii) structural changes in DNA mediated by transposition (e.g. the generation of large deletions in the recipient molecule upon transposition of ISPve1 of the IS21 family). We also demonstrated the great potential of TEs and transposition in the generation of diverse phenotypes as well as in the natural amplification and dissemination of genetic information (of adaptative value) by horizontal gene transfer, which is considered the driving force of bacterial evolution. [ABSTRACT FROM AUTHOR]

Phenotypic and genotypic resistance to benzalkonium chloride (BC), cadmium and arsenic was tested (by susceptibility assays and molecular methods) in 287 Listeria monocytogenes strains isolated from fish and fish products, and food-producing factories in Poland. Overall, 40% of the isolates were resistant to BC, 56% to cadmium and 41% to arsenic (57% displayed resistance to more than one of the tested compounds). Among BC-resistant isolates, the most commonly detected resistance determinant was the qacH gene (83%). Three distinct types of cadA gene determining resistance to cadmium were detected, with the cadA1 variant predominant (88%), while most arsenic-resistant isolates (86%) harbored the arsA gene associated with a Tn 554- like transposon (one strain harbored two copies of arsA in different arsenic resistance cassettes). 53% of all tested isolates contained plasmids (from 4 kb to > 90 kb in size), which were classified into 11 groups (p1–p11) based on their restriction patterns. Interestingly, 12 isolates harbored the small mobilizable pLMST6-like plasmid pLIS3 encoding multidrug efflux pump EmrC. Clustering analysis of PFGE patterns revealed that these isolates represent several diverse bacterial populations, which strongly suggests mobility of the pLMST6-like plasmids among L. monocytogenes strains and their role in dissemination of BC resistance. • Among 287 Listeria monocytogenes isolates, 40% were resistant to benzalkonium chloride, 56% to cadmium and 41% to arsenic. • 83% of isolates resistant to benzalkonium chloride carried the qacH gene. • 88% of cadmium-resistant isolates carried the cadA1 gene. • 86% of isolates resistant to arsenic harbored the Tn 554 associated arsA gene. • 4% of isolates carried a mobilizable pLMST6-like plasmid pLIS3 encoding multidrug efflux pump EmrC. [ABSTRACT FROM AUTHOR]