Biochemical traits and antimicrobial susceptibility of Streptococcus suis isolated from slaughtered pigs

A total of 522 Streptococcus suis isolates were recovered from the tonsils of slaughtered pigs by culturing on media containing different specific antisera. The serogroup reactions were determined by coagglutination. The most frequently isolated serotype was serotype 4 (representing 28.7% of the isolates), followed by serotype 3 (24.9%) and serotype 2 (22.6%). Typable isolates were examined by growth in 6.5% NaCl, amylase activity, acetoin production, susceptibility to optochin and production of acid from mannitol, melibiose, raffinose, salicin, sorbitol and trehalose. Finally, all isolates were tested for antimicrobial susceptibility against ampicillin, cephalotin, lincomycin, neomycin, penicillin, spiramycin and trimethoprim-sulphamethoxazole using the disk-diffusion technique. All isolates were amylase-positive and acetoin-negative, and failed to grow in 6.5% NaCl broth. Nine isolates were optochin-susceptible. Moreover, 10 isolates produced acid from mannitol, 21 produced acid from sorbitol, and 36 and 37 did not produce acid from salicin and trehalose, respectively. Six salicin- and trehalose-negative isolates and two mannitol- and sorbitol-fermenting isolates were found. Biochemical differences between serotypes were not observed. The most active drugs against S. suis were the beta-lactam antibiotics, although only 83.3%, 70% and 78.5% of isolates were susceptible to ampicillin, cephalotin and penicillin, respectively. Half of the isolates were resistant to trimethoprim-sulphamethoxazole, and resistance to lincomycin, neomycin and spiramycin appeared to be very high.