In this review an overview is given on antibiotic resistance (AR) mechanisms with special attentions to the AR genes described so far preceded by a short introduction on the discovery and mode of action of the different classes of antibiotics. As this review is only dealing with acquired resistance, attention is also paid to mobile genetic elements such as plasmids, transposons, and integrons, which are associated with AR genes, and involved in the dispersal of antimicrobial determinants between different bacteria.

Erythromycin resistance in Clostridium difficile strain 630 is conferred by a genetic element termed Tn5398 which contains two erm(B) genes: erm1(B) and erm2(B). An erythromycin-sensitive derivative of strain 630 (designated 630Δerm) was generated by spontaneous mutation after continuous subculture for 30 days. This strain had lost the erm2(B) gene from within Tn5398 but retained erm1(B). However, the strain could revert to erythromycin resistance at a frequency of 2.79 × 10⁻⁸, although it still contained the deletion of erm2(B). The availability of C. difficile 630Δerm allowed the behaviour of Tn916ΔE to be investigated in this strain. This element entered the genome at multiple sites indicating that it could be useful as an insertional mutagen.

In this review we explore the diversity of the Tn916 / Tn1545 family of mobile genetic elements in four major Gram positive pathogens and show that these elemen