Antimicrobial susceptibility testing of streptococcus suis isolates to common antibiotics used in pig farms

Abstract

The goal of this study was to determine antimicrobial susceptibility of 34 Streptococcus suis strains isolated from healthy and diseased pigs from two pig farms. Disk diffusion was the method used to asses antibacterial susceptibility of S. suis according to CLSI (Clinical and Laboratory Standards Institute) and EUCAST standard protocols. Overnight cultures of S. suis grown on Colombia 5% blood agar (HIMEDIA, India) were suspended in Todd Hewitt broth (Oxoid Limited, England) and suspension was adjusted to 0,5 McFarland standard. Using sterile swabs suspension was spreaded to Mueller Hinton agar supplemented with 5% defibrinated sheep blood, after which standadized discs containing antimicrobial substances were placed on the agar surface. Plates were incubated at 37 °C in aerobic conditions usually for 18 hours, but no more than 24 hours after which diameter of S. suis inhibition zones were measured. Among 34 isolated S. suis strains all were resistant to tetracycline (TET), lincomycin (L) and clindamycin (CC). Susceptibility was very low to sulfamethoxazole-trimethoprim (SXT) combination (94,1% resistance), 94,1% of strains were sensitive to penicillin G (P), amoxicillin (AMX), erythromycin (E), enrofloxacin (ENR) and azithromycin (AZM) while sensitivity to cephalosporins of the first (cephalexin- CN), third (cefotaxime – CTX, ceftriaxone - CRO) and fourth ( cefepime-FEP) generation, as well as the sensitivity to florfenicol (FFC) and vancomycin (VAN) was absolute (100%). Therefore we can conclude that cephalosporins are very reasonable choice in treatment of human cases of S. suis meningitis but also as a treatment of choice for the S .suis pig septicemia.