New and emerging SXT/R391 integrative conjugative elements as vehicles for stable mobile element transfer and spread of antibiotic resistance in both human and animals

The integrative conjugative elements, ICE’s SXT and R391 are the prototypes of a group of gram negative integrative elements known as the SXT/R391 group. R391 was identified in a clinical isolate of Providencia in the late 1960’s in South Africa, while SXT was initially isolated in 1992 in a clinical isolate of Vibrio cholerae O139 and variants have since been isolated in pandemic strains throughout the world. Subsequent sequencing of both elements demonstrated a high degree of structural similarity leading to the group being classified as the SXT/R391 group. The SXT/R391 ICE elements are characterised as integrating into a specific chromosomal site within gram –ve hosts, being extremely stable and promiscuous and possessing a number of element hotspots for integration of heterologous DNA including increasingly, antibiotic resistance determinants. This makes such ICE’s highly adapted for antibiotic spread. New evidence emerging indicates that SXT/R391-like ICE’s are increasingly being identified worldwide particularly in Asia not only from Vibrio species, where they have been found widely in human clinical isolates, but from other gram -ve associated infections of domestic animals and fish. Evidence of more such elements may emerge in the future as a new trapping vector pIceCap has been developed to capture them in a circular form, aiding characterisation. The types of the novel ICE’s now emerging, their comparison with prototype elements and the antibiotic resistances associated with them are important given their promiscuous nature and stability.