[Bacterial SXT/R391 family from integrating conjugative elements--a review]

SXT/R391 family has the most abundant types and members in integrating conjugative elements (ICE). SXT/ R391 elements are comprised of conservative core genes and genes in variable regions. The functions of conservative core genes of SXT/R391 include integration and excision, self-transfer through conjugation, and regulation of its expression. The genes in the variable regions often encode for drug and heavy metal resistances, forming of biofilm, adjustment of bacterial motility, and toxin-antitoxin systems that prevent SXT/R391 deletion from hosts. Some genes in variable region of SXT/R391 also encode for restriction-modification system, helicase, and endonuclease. The activity of SXT/R391 is positively regulated by activator SetCD, and negatively regulated by repressor SetR. SXT/R391 cannot be easily deleted from the primary donors in the process of transfer. SXT/R391 prevent the acquirement of closely related and homogeneous elements but cannot prevent the acquirement of heterogenetic ICE, which leads to the generation of hybrid ICE under the action of recombination system encoded by SXT/R391 themselves. SXT/R391 have high transferable frequency and wide host range, and until now more than 40 different SXT/R391 elements have been discovered in various bacteria, especially in Vibrio species, which mainly distribute in coastal areas in Asia and Africa. It suggests that marine environments are likely the main reservoir for SXT/R391 and these elements probably spread from marine environmental strains to clinical strains, under increasing selective pressure. Due to the hazard caused by the prevalence and the transfer of SXT/R391, medical microbiologist and health departments should be fully alert to the spread of the elements.