Mobile Genetic Elements Driving the Spread of Antimicrobial Resistance in Streptococci

Streptococci are adaptable bacteria that persist as commensals or pathogens under selective pressures. This adaptability is influenced by mobile genetic elements (MGEs), which mediate horizontal gene transfer (HGT) and frequently carry antimicrobial resistance (AMR) and virulence-associated genes. This review provides an overview of MGEs involved in the acquisition and dissemination of AMR across Streptococcus species, including human, animal-associated, and zoonotic streptococci. Integrative and conjugative elements (ICEs), including members of the Tn916/Tn1545 and Tn5253 families, and TnGBS-related ICEs, are among the best-characterized vehicles for the dissemination of AMR genes (ARGs) because their modular organization supports integration, excision, regulation, and conjugative transfer. Integrative mobilizable elements (IMEs) promote the dissemination of resistance by exploiting conjugative machinery supplied by compatible coresident elements. Plasmids, although less prominent than chromosomally integrated MGEs in several streptococcal species, can carry resistance determinants together with replication and mobilization functions, contributing to HGT. Insertion sequences (ISs) and transposons (Tns) contribute to genome plasticity by promoting gene mobilization, recombination, and the generation of complex resistance regions. Resistance-associated elements such as Tn917, Tn3872, Tn6002, and Tn1207.1-related structures illustrate the diversity of genetic platforms involved in macrolide and tetracycline resistance. Prophages, including Φm46.1-like elements, contribute to genome diversification and can serve as determinants of resistance transfer. Genomic islands and cis-mobilizable elements (CIMEs) expand mobilome diversity through recombination and mobilization in association with compatible ICEs. Collectively, these interconnected MGEs drive the dissemination of AMR, genome plasticity, ecological adaptation, and evolutionary diversification in streptococci. Reducing the need for antimicrobial use in livestock through improved biosecurity, vaccination, hygiene, nutrition, husbandry, disease prevention, and timely diagnosis is therefore essential to reduce selection pressure and limit the persistence and spread of resistance. These measures, integrated with genomic surveillance, antimicrobial stewardship, and One Health approaches, can help limit the circulation of resistance-associated MGEs across human, animal, and environmental interfaces.