ICESag08: A new composite integrative and conjugative element from Streptococcus agalactiae mediating resistance to chloramphenicol and macrolides.

OBJECTIVE

Streptococcus agalactiae poses increasing clinical challenges owing to emerging antibiotic resistance, particularly to chloramphenicol, mediated by the conserved catQ-mef(I) (IQ module). To date, IQ module-carrying elements have been described: ICESpn529IQ (ICESa2603 family) from Streptococcus pneumoniae, ICESpy029IQ and ICESpy005IQ (Tn5253 family) from Streptococcus pyogenes, and composite ICESag236 and ICESag403 from Streptococcus agalactiae.

METHODS

This study characterized a novel composite integrative and conjugative element (ICE) carrying an IQ module by bioinformatics analysis of sequenced chloramphenicol-resistant S. agalactiae. Transferability was confirmed by conjugation assays. Fitness costs were assessed through growth and competition assays, and stability was evaluated in serial passage experiments.

RESULTS

Comparative analysis with known ICEs from GenBank revealed that ICESag08 is a 133-kb composite ICE that integrates four distinct elements: ICESpn529IQ, ICESan95_hsdM, ICESpy009, and IMESp2907. The catQ-mef(I) locus is located within the ICESpn529IQ, whereas erm(TR) is carried by IMESp2907, which is embedded within ICESan95_hsdM. Conjugation experiments and sequencing confirmed that ICESag08 is transferable to recipient strains at frequency of 1.05 × 10⁻⁹. Transconjugants exhibited impaired growth and reduced competitive ability compared with those of the recipient strain. Stability assays performed after successive passages showed that ICESag08 remained stable and conferred resistance.

CONCLUSIONS

These results highlight the role of composite ICEs in the horizontal transfer of multidrug resistance, including to chloramphenicol (catQ) and macrolides [mef(I), erm(TR)], and reveal an intricate interplay between different ICEs. These insights advance our understanding of how resistance disseminates and evolves and support the development of effective strategies to limit the spread of antimicrobial resistance.