{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,3,9]],"date-time":"2026-03-09T07:37:01Z","timestamp":1773041821837,"version":"3.50.1"},"reference-count":37,"publisher":"Frontiers Media SA","license":[{"start":{"date-parts":[[2026,3,9]],"date-time":"2026-03-09T00:00:00Z","timestamp":1773014400000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":["frontiersin.org"],"crossmark-restriction":true},"short-container-title":["Front. Microbiol."],"abstract":"\n Background<\/jats:title>\n \n Macrolide resistance genes (\n erm<\/jats:italic>\n and\n mef<\/jats:italic>\n families) and heavy metal resistance genes (\n cadA<\/jats:italic>\n ) are increasingly disseminated among streptococci via diverse mobile genetic elements.\n <\/jats:p>\n <\/jats:sec>\n \n Methods<\/jats:title>\n \n Through whole-genome sequencing of 16\n Streptococcus agalactiae<\/jats:italic>\n 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\n Sag<\/jats:italic>\n 39 was identified in\n S. agalactiae Sag<\/jats:italic>\n 39 through comparative analysis with the NCBI database.\n <\/jats:p>\n <\/jats:sec>\n \n Results<\/jats:title>\n \n ICE\n Sag<\/jats:italic>\n 39 measured 113,125 bp in length, and it featured a nested \u201cRussian doll\u201d structure comprising an ICE\n Sa<\/jats:italic>\n 2603 family backbone integrated with an internal Tn\n 1806<\/jats:italic>\n -like ICE. The embedded Tn\n 1806<\/jats:italic>\n -like ICE contained four variable regions (VR1-VR4) that serve as insertion hotspots; among these, VR3 and VR4 carry\n erm<\/jats:italic>\n (TR) from ICE\n Sp<\/jats:italic>\n 2907 and the cadmium resistance gene\n cadA<\/jats:italic>\n from an uncharacterized genetic element, respectively. Conjugation and excision assays confirmed that ICE\n Sag<\/jats:italic>\n 39 transfers at a frequency of 8.2 \u00d7 10\n \u22129<\/jats:sup>\n and co-transfers both resistance determinants. Under cadmium stress, transconjugants carrying ICE\n Sag<\/jats:italic>\n 39 displayed enhanced growth relative to the recipient. Although the internal Tn\n 1806<\/jats:italic>\n -like ICE was also capable of independent transfer, its efficiency was significantly lower (&lt; 10\n \u22129<\/jats:sup>\n ), and its circular form is undetectable by PCR. Database screening identified 199 structurally similar ICEs (ICE\n Sag<\/jats:italic>\n 39-like ICE), 62.8% (125\/199) of which co-carried\n erm<\/jats:italic>\n (TR) and\n cadA<\/jats:italic>\n , underscoring the prevalence of this ICE and its associated resistance traits.\n <\/jats:p>\n <\/jats:sec>\n \n Conclusions<\/jats:title>\n 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.<\/jats:p>\n <\/jats:sec>","DOI":"10.3389\/fmicb.2026.1763839","type":"journal-article","created":{"date-parts":[[2026,3,9]],"date-time":"2026-03-09T06:46:50Z","timestamp":1773038810000},"update-policy":"https:\/\/doi.org\/10.3389\/crossmark-policy","source":"Crossref","is-referenced-by-count":0,"title":["Characterization of a novel composite ICE in Streptococcus agalactiae conferring resistance to macrolides [erm(TR)] and cadmium (cadA)"],"prefix":"10.3389","volume":"17","author":[{"given":"Sida","family":"Yi","sequence":"first","affiliation":[{"name":"Department of Molecular Biology, Shanghai Center for Clinical Laboratory","place":["Shanghai, China"]},{"name":"Institute of Antibiotics, Huashan Hospital, Fudan University","place":["Shanghai, China"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Chunli","family":"Shi","sequence":"additional","affiliation":[{"name":"Department of Molecular Biology, Shanghai Center for Clinical Laboratory","place":["Shanghai, China"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Liufan","family":"Yin","sequence":"additional","affiliation":[{"name":"Department of Molecular Biology, Shanghai Center for Clinical Laboratory","place":["Shanghai, China"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Xing","family":"Xu","sequence":"additional","affiliation":[{"name":"Department of Molecular Biology, Shanghai Center for Clinical Laboratory","place":["Shanghai, China"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Xueliang","family":"Wang","sequence":"additional","affiliation":[{"name":"Department of Molecular Biology, Shanghai Center for Clinical Laboratory","place":["Shanghai, China"]},{"name":"Department of Molecular Diagnostic Innovation Technology, Shanghai Academy of Experimental Medicine","place":["Shanghai, China"]}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1965","published-online":{"date-parts":[[2026,3,9]]},"reference":[{"key":"B1","doi-asserted-by":"publisher","first-page":"1483","DOI":"10.3389\/fmicb.2015.01483","article-title":"New insights into the classification and integration specificity of Streptococcus integrative conjugative elements through extensive genome exploration","volume":"6","author":"Ambroset","year":"2016","journal-title":"Front. 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