Serotypes, antimicrobial resistance, and pathogenic characteristics of Streptococcus suis isolated from a slaughterhouse in an area of Zhejiang Province

Abstract:

[Objective] Streptococcus suis is a major swine pathogen and a zoonotic agent. Swine tonsil is a natural habitat of S. suis, and the tonsillar S. suis from healthy pigs is considered as an important source of infection for susceptible pigs and humans. Thus, the epidemiological investigation of S. suis from healthy pigs in slaughterhouses is of great significance for public health. [Methods] We collected healthy pigs' tonsils from a slaughterhouse in Zhejiang in 2020-2021, isolated and identified S. suis, and serotyped the strains by the serotype-specific PCR assay. Through the resistance gene detection, antimicrobial susceptibility testing, and zebrafish infection experiment, we examined the antimicrobial resistance and pathogenic characteristics of these isolates. [Results] The positive rate of S. suis in 131 tonsil samples of healthy pigs was 62.59% (82/131), and we isolated a total of 68 strains. The strains were dominated by serotype 16 (16.18%, 11/68), followed by serotype 31 (11.76%, 8/68), serotype 9 (7.35%, 5/68), and serotype 3 (7.35%, 5/68). In addition, tonsil samples containing 2 or more serotypes of S. suis accounted for 15.85% (13/82). The antimicrobial susceptibility testing showed that the isolates were mainly resistant to lincosamides (100%, 68/68), macrolides (98.53%, 67/68), and tetracyclines (100%, 68/68), and all isolates were multidrug-resistant. It is worth noting that 18 isolates were resistant to penicillin, 3 resistant to cefotaxime, 2 resistant to rifampicin, and 11 resistant to linezolid. The detection rate of both macrolides/lincosamides resistance genes and tetracycline resistance genes was 82.35% (56/68), which was the main reason why these isolates were resistant to the above antimicrobials. We selected 25 representative strains for the zebrafish infection experiment, and the results showed that 5 strains were highly virulent with mortality of 80%-100% when challenged with the dose of 3×10⁶ CFU/fish. [Conclusion] Healthy pigs in this area had a high detection rate of S. suis. All isolates were multidrug-resistant, and some were highly virulent. These results contribute to understanding the incidence of S. suis and formulating relevant prevention and control strategies in this area.