The bacterial strains and plasmids used in this study are listed in Table S1 . S. suis strains were grown in Todd-Hewitt broth (THB) (Difco Laboratories, Detroit, MI, USA) supplemented with 2% yeast extract (THY), and E. coli strains were cultured in Luria-Bertani (LB) medium. Agar (1.5%) was included when solid medium was desired. If required, antibiotics were added at the following concentrations: spectinomycin, 100 mg ml -1 for both S. suis and E. coli; chloramphenicol, 5 mg ml -1 for S. suis and 10 mg ml -1 for E. coli; erythromycin, 1 mg ml -1 for S. suis and 250 mg ml -1 for E. coli; ampicillin, 100 mg ml -1 for E. coli; kanamycin, 50 mg ml -1 for E. coli.