Characterization of a novel composite ICE in Streptococcus agalactiae conferring resistance to macrolides [erm(TR)] and cadmium (cadA)

Background Macrolide resistance genes ( erm and mef families) and heavy metal resistance genes ( cadA ) are increasingly disseminated among streptococci via diverse mobile genetic elements. Methods Through whole-genome sequencing of 16 Streptococcus agalactiae isolates resistant to both erythromycin and clindamycin, we identified 19 integrative and conjugative elements (ICEs), a type of self-transfer genetic elements, conferring antibiotic resistance. Among these, a novel composite ICE designated ICE Sag 39 was identified in S. agalactiae Sag 39 through comparative analysis with the NCBI database. Results ICE Sag 39 measured 113,125 bp in length, and it featured a nested “Russian doll” structure comprising an ICE Sa 2603 family backbone integrated with an internal Tn 1806 -like ICE. The embedded Tn 1806 -like ICE contained four variable regions (VR1-VR4) that serve as insertion hotspots; among these, VR3 and VR4 carry erm (TR) from ICE Sp 2907 and the cadmium resistance gene cadA from an uncharacterized genetic element, respectively. Conjugation and excision assays confirmed that ICE Sag 39 transfers at a frequency of 8.2 × 10 −9 and co-transfers both resistance determinants. Under cadmium stress, transconjugants carrying ICE Sag 39 displayed enhanced growth relative to the recipient. Although the internal Tn 1806 -like ICE was also capable of independent transfer, its efficiency was significantly lower (< 10 −9 ), and its circular form is undetectable by PCR. Database screening identified 199 structurally similar ICEs (ICE Sag 39-like ICE), 62.8% (125/199) of which co-carried erm (TR) and cadA , underscoring the prevalence of this ICE and its associated resistance traits. Conclusions This study characterizes a new composite ICE and elucidates a modular mechanism that facilitates the assembly and dissemination of resistance genes, thereby promoting bacterial genome diversification.