A novel integrative conjugative element mediates transfer of multi-drug resistance between Streptococcus suis strains of different serotypes

Streptococcus suis represents a key antibiotic resistance gene reservoir and an important pathogen for humans and animals. Resistance can be spread through horizontal gene transfer of chromosome-borne mobile genetic elements; however, the exact mechanism by which this occurs remains poorly understood. In the present study, we identified and characterized a novel 82-kb integrative conjugative element (ICE) named ICESsuCZ130302 from the virulent S. suis strain CZ130302. It carries genes that provide resistance to multiple antibiotics, such as tetracycline, doxycycline, erythromycin, lincomycin, neomycin, and kanamycin. It also contains a nisin biosynthesis gene cluster, a toxin-antitoxin system, a type IV secretion system, and an integrase and excisase system. The mobile element can be excised from the chromosome, circulized, and transferred via conjugation from serotype Chz strain CZ130302 to serotype 2 strain P1/7, where it confers resistance to the aforementioned antimicrobial agents. The full length ICE, where multiple antimicrobial resistance genes accumulated, was further identified to be naturally transferred between different serotypes strains of S. suis. This finding illustrates how such elements represent a potential means by which antimicrobial resistance is introduced to a wide range of bacteria of veterinary and medical significance.