Antimicrobial Susceptibility, Tetracycline and Erythromycin Resistance Genes, and Multilocus Sequence Typing of Streptococcus suis Isolates from Diseased Pigs in China

Streptococcus suis (S. suis) is an emerging zoonotic pathogen causing significant economic losses in the swine industry. Here, we investigated the antimicrobial susceptibility, associated antibiotic-resistant determinants and sequence type (ST) of S. suis isolates from diseased pigs in China from 2008 to 2010. Serotype 2 was the most frequently observed strain (n=95) among the 106 S. sans strains collected, followed by serotypes 3 (n=3), 5 (n=3), 4 (n=2), 7 (n=1), 11 (n=1) and 28 (n=1). Multilocus sequence typing analysis revealed that ST1 (n=21) and ST7 (n=74) were the predominant STs, and serotype 2 was found to be significantly correlated with ST7 (P=0.017, Fisher's exact test) and CC1 (P=0.024, Fisher's exact test). The antimicrobial susceptibility results indicated that the antibiotic resistance rate was highest for tetracycline (99.1%), followed by azithromycin (68.9%), erythromycin (67.9%), clindamycin (67.9%), trimethoprim/sulfamethoxazole (16%), levofloxacin (2.8%), chloramphenicol (1.9%), cefaclor (0.9%) and ceftriaxone (0.9%). Antibiotic-resistant genes tet(M), tet(O), tet(O/W/32/O), tet(O/32/O), tet(S), tet(W), tet(L), tet(40), erm(B), mef(A/E) and msr(D) could be detected, and several tandem organizations of antibiotic resistance genes were also found in this study. In conclusion, S. suis strains isolated from diseased pigs in China were less diverse and multi-drug resistant.