Variation on a theme; an overview of the Tn916/Tn1545 family of mobile genetic elements in the oral and nasopharyngeal streptococci.

Francesco Santoro,Adam P Roberts,Morgana E Vianna

doi:10.3389/fmicb.2014.00535

Abstract

The oral and nasopharyngeal streptococci are a major part of the normal microbiota in humans. Most human associated streptococci are considered commensals, however, a small number of them are pathogenic, causing a wide range of diseases including oral infections such as dental caries and periodontitis and diseases at other body sites including sinusitis and endocarditis, and in the case of Streptococcus pneumoniae, meningitis. Both phenotypic and sequence based studies have shown that the human associated streptococci from the mouth and nasopharynx harbor a large number of antibiotic resistance genes and these are often located on mobile genetic elements (MGEs) known as conjugative transposons or integrative and conjugative elements of the Tn916/Tn1545 family. These MGEs are responsible for the spread of the resistance genes between streptococci and also between streptococci and other bacteria. In this review we describe the resistances conferred by, and the genetic variations between the many different Tn916-like elements found in recent studies of oral and nasopharyngeal streptococci and show that Tn916-like elements are important mediators of antibiotic resistance genes within this genus. We will also discuss the role of the oral environment and how this is conducive to the transfer of these elements and discuss the contribution of both transformation and conjugation on the transfer and evolution of these elements in different streptococci.

Highlights

The oral microbiota is one of the most diverse bacterial populations found in the human body (Topazian et al, 2002) with the total number of distinct taxonomic units found in the oral cavity in the 100s, if not the 1000s (Aas et al, 2005; Keijser et al, 2008; Wade, 2013), these are not all found in every mouth as individuals have distinct bacterial populations, and distinct bacterial populations are found in different habitats within each mouth (Eren et al, 2014 )
The fact that Tn916-family elements in S. pneumoniae tend to be either inserted in composite elements or at the same hotspots, suggests that their autonomous conjugation in vivo might not be frequent, even if there is evidence that this event is possible, and that they may rely on the conjugation machinery of larger elements such as ICESp23FST81 or Tn5253 which are able to transfer at frequencies about three orders of magnitude higher (Iannelli et al, 2014)
The authors showed the presence of multiple new structures of mobile genetic elements (MGEs) based on Tn916-like elements and, to both the previous study (Ciric et al, 2012), and the situation previously described for S. pneumoniae showed the co-carriage of multiple resistance genes, e.g., tet(M) and erm(B) on Tn916-like MGEs in different oral streptococci

Summary

Introduction

The oral microbiota is one of the most diverse bacterial populations found in the human body (Topazian et al, 2002) with the total number of distinct taxonomic units found in the oral cavity in the 100s, if not the 1000s (Aas et al, 2005; Keijser et al, 2008; Wade, 2013), these are not all found in every mouth as individuals have distinct bacterial populations, and distinct bacterial populations are found in different habitats within each mouth (Eren et al, 2014 ). The elements were detected in strains of different serotypes, suggesting horizontal gene transfer of the resistance determinants, Tn916-family element are widespread among multidrug resistant pneumococcal clinical isolates.

Results

Conclusion