Antibiotic Resistance in Oral Streptococci: The prevalence, diversity, stability, and fitness cost of Tn916 -Tn1545 family in oral streptococcal isolates

Abstract

Oral streptococci are important members of the oral microbiome that are gathering more attention due to the presence of antimicrobial resistance determinants and their association with mobile genetic elements. Mobile genetic elements such as the ICE, Tn916 are abundant in bacterial populations and have been implicated in the spread of antibiotic resistance. Despite their abundance, little is known regarding the presence, diversity, and evolutionary dynamics of these ICEs in oral streptococci. The work presented in this thesis addresses the prevalence, diversity, stability, and fitness cost of the Tn916 family in oral streptococcal isolates. We report that the wild-type Tn916 is the most prevalent member of this family detected was wild-type Tn916. In terms of diversity, we detected two additional members of this family of ICEs, besides Tn916, namely Tn6815 and Tn6816. As the conjugative transfer of these Tn916-Tn1545 family elements was high, we assessed the effect these elements have on the new host (Streptococcus oralis) in the absence of selective pressure. We, therefore, determined the relative fitness cost associated with the acquisition of these elements and their stability in the absence of selective pressure. The findings of this work showed that upon acquisition Tn916, imposes a reduction in relative fitness ranging from 6% to 25% at time zero (T0). Amelioration of the observed fitness cost was observed within 500 generations, and the ICEs were stable in the absence of selection for up to 1000 generations. Analysis of the number of copies of Tn916 in the transconjugants with both digital droplet PCR (ddPCR) copy number analysis and whole genome sequencing (WGS) indicated no changes in the copy number, elements sequence and insertion sites in the evolution experiment. Taken together, the findings of this work reaffirm the role that Tn916 elements play in the spread of antibiotic resistance among oral streptococci.