Horizontal gene transfer (HGT) plays a major role in bacterial microevolution as evident from the rapid emergence and spread of antimicrobial drug resistance. Few studies have however addressed the population dynamics of newly imported genetic elements after HGT. Here, we show that newly acquired class-1 integrons from Salmonella enterica serovar Typhimurium and Acinetobacter baumannii, free of associated transposable elements, strongly reduce host fitness in Acinetobacter baylyi. Insertional inactivation of the integron intI1 restored fitness, demonstrating that the observed fitness costs were due to the presence of an active integrase. The biological cost of harboring class-1 integrons was rapidly reduced during serial transfers due to intI1 frameshift mutations leading to inactivated integrases. We use a mathematical model to explore the conditions where integrons with functional integrases are maintained and conclude that environmental fluctuations and episodic selection is necessary for the maintenance of functional integrases. Taken together, the presented data suggest a trade-off between the ability to capture gene cassettes and long-term stability of integrons and provide an explanation for the frequent observation of inactive integron-integrases in bacterial populations.

Gentamicin is important in synergistic bactericidal therapy with cell wall agents for severe enterococcal infections. During 2003-2008, a 10-fold increase in the prevalence of high-level gentamicin resistance (HLGR), to above 50%, in blood culture isolates of Enterococcus faecium, was reported by the Norwegian Surveillance System for Antimicrobial Resistance. A representative national collection of invasive E. faecium isolates (n = 99) from 2008 was examined by a multilevel approach. Genotyping revealed a polyclonal population dominated by major hospital-associated lineages (mainly ST203, ST17, ST18, ST202 and ST192). The presence of aac(6')-Ie-aph(2″)-Ia, encoding the bi-functional aminoglycoside-modifying enzyme, was found in 98% of HLGR isolates (56/57). Furthermore, a significantly higher prevalence of potential virulence genes, toxin-antitoxin loci as well as pRE25 and pRUM type replicons was demonstrated in isolates belonging to major hospital-associated lineages compared to other sequence types. Megaplasmids of pLG1 replicon type (200-330 kb) were present in 90% of the isolates. Co-hybridization analyses revealed genetic linkage of aac(6')-Ie-aph(2″)-Ia to this replicon type. Transfer of HLGR-encoding plasmids was restricted to E. faecium. In conclusion, the increased prevalence of HLGR in invasive E. faecium in Norway is associated with hospital-adapted genetic lineages carrying aac(6')-Ie-aph(2″)-Ia-encoding transferable megaplasmids of the pLG1 replicon type.

Background: The Tn916-Tn1545 family of Integrative Conjugative Elements (ICE) are mobile genetic elements (MGEs) that play a role in the spread of antibiotic resistance genes. The Tn916 harbors the tetracycline resistance gene tet(M) and it has been reported in various bacterial species. The increase in the levels of tetracycline resistance among oral streptococci is of great concern primarily due to the abundance of these species in the oral cavity and their ability to act as reservoirs for antibiotic resistance genes. Methods: In the current study, we screened 100 Norwegian clinical oral streptococcal isolates for the presence and diversity of the Tn916-Tn1545 family. In addition, we investigated the transferability the elements, and the associated transfer frequencies. Results: We observed that 21 isolates harboured the Tn916-Tn1545 family and that two of these elements were the novel Tn6815 and Tn6816. The most prevalent member of the Tn916-Tn1545 family observed in the Norwegian clinical oral streptococcal isolates was the wild type Tn916. Conclusion: The detection of other members of this family of ICE and varying transfer frequencies suggests high versatility of the Tn916 element in oral streptococci in Norway.

Background: Tn916 and Tn1545 are paradigms of a large family of related, broad host range, conjugative transposons that are widely distributed in bacteria and contribute to the spread of antibiotic resistance genes (ARGs). Variation in the copy number (CN) of Tn916-Tn1545 elements and the circularization ratio (CR) may play an important role in propagation of ARGs carried by these elements.Objectives and Design: In this study, the CN and CR of Tn916-Tn1545 elements in oral streptococci were determined using droplet digital PCR (ddPCR). In addition, we investigated the influence of tetracycline on the CR of Tn916-Tn1545 elements.Results: The ddPCR assay designed in this study is a reliable way to rapidly determine CN and CR of Tn916-Tn1545 elements.Conclusions: Our data also suggest that Tn916-Tn1545 elements are generally stable without selective pressure in the clinical oral Streptococcus strains investigated in this study.

Streptococcus species are the most abundant genera in the oral microbiota. The prevalence of antibiotic resistant oral streptococci is on the increase and the most reported antibiotic resistance profile in oral streptococci include, among several antibiotics, resistance to β-lactams antibiotics, macrolides and tetracycline. Several studies have shown that majority of the antibiotic resistance genes that have been isolated in oral streptococci are usually associated with mobile genetic elements such as plasmids and conjugative transposons. In the current study, we used molecular based techniques to screen a total of 100 clinical oral streptococci strains isolated in Norway for the presence of Tn916/Tn916-like elements. We also investigated the diversity of these elements in oral streptococci by Restriction Fragment Length Polymorphism (RFLP) and DNA sequencing. It was found that a total of 34 strains were positive for tetracycline resistance gene, tet(M), and integrase gene (int) belonging to the Tn916-like family. The minimum inhibitory concentration (MIC) of these tetracycline-resistant oral streptococci ranged between 2-64 µg/mL. Most of the positive strains for tet(M) and int genes harbor the wild type Tn916 and a few harbor other Tn916-like elements suggesting high versatility of the Tn916 element in oral streptococci in Norway.