Streptococcus suis (S. suis) continues to be ranked as one of the most common diseases on pig farms on a global basis. Efforts to reduce antimicrobial usage as part of a more comprehensive plan to limit the development of antimicrobial resistance has spurred investment in S. suis research. Understanding the ecology of S. suis is important as disease prevention and control strategies are developed. There is a wide range of disease causing abilities within this species. S. suis is a bacterium that may be classified into pathogenic, opportunistic or commensal strains.
- Pathogenic strains of S. suis are associated with clinical cases in all ages of growing pigs and breeding stock and isolates are retrieved from pigs with meningitis, arthritis, endocarditis, epicarditis, polyserositis and septicemia.
- Possibly opportunistic strains of S. suis are isolated from the lung of individual pigs that are otherwise free of neurological or systemic disease.
- Commensal strains of S. suis are isolated from laryngeal, tonsil, or nasal samples from farms with no known history or current control methods for S. suis on that farm.
Serotyping and Multilocus Sequence Typing (MLST) are the primary diagnostic methods used to differentiate strains. Serotyping is the best known and the more traditional diagnostic technique. Multilocus sequence typing (MLST) is a common research diagnostic approach that is used in many species of bacterial pathogens. US, Canadian and Mexican researchers wanted to characterize the diversity of 208 S. suis isolates collected between 2014 and 2017 across North America by using serotyping and MLST. They also wanted to further investigate associations between subtype and pathotype classifications (pathogenic, possibly opportunistic, and commensal), based on clinical information and site of isolation.
The researchers found the following in these 208 North American isolates:
- 20 serotypes were identified with the predominant serotypes being 1/2 (n=54) and 7 (n=23).
- serotypes 1, 1/2, 2, 7, 14, and 23 were most commonly pathogenic
- 14 of those 20 serotypes ranged from 56‐100% that were classed as pathogenic
- 6 of those 20 serotypes had 0% that were classed as pathogenic
- serotypes 21 and 31 were most commonly commensals
- 58 sequence types (STs) were identified with the predominant STs being ST28 (n=52) and ST94 (n=18).
- ST1, ST13, ST25, ST28, ST29, ST94, ST108, ST117, ST225, ST373, ST961, and ST977 were most commonly classified as pathogenic
- ST750, and ST821 were most commonly classified as commensals
Take Home Message
- Streptococcus suis isolates were classified into 20 different serotypes and 58 different Sequence types (ST)s.
- Sequence Types (ST)s rather than the more traditional serotype appear to be a more effective predictor of disease causing ability for any particular isolate.
- This research supports the findings of previous studies that indicate that within serotypes there can be a variety of disease causing abilities. Some serotypes are rarely if ever associated with clinical disease.
- These findings are important in guiding how we think about diagnostic sample selection for S. suis autogenous vaccines.
Submitted by Clint Lichty, DVM
Ref: April A. Estrada, Marcelo Gottschalk, Stephanie Rossow, Aaron Rendahl, Connie Gebhart, Douglas G. Marthaler Serotype and Genotype (Multilocus Sequence Type) of Streptococcus suis Isolates from the United States Serve as Predictors of Pathotype Journal of Clinical Microbiology Aug 2019, 57 (9) e00377-19; DOI: 10.1128/JCM.00377-19