Can Streptococcus suis genotype predict its pathogenicity?

PhD candidate April Estrada, working with Drs. Gebhart and Marthaler published an open-access article on Streptococcus suis in the Journal of Clinical Microbiology. Streptococcus suis is an older pathogen but its prevalence as well as its virulence have been rising lately. This study investigates if categorizing isolates into serotypes and genotypes could help predict the severity of the disease on the farm.

Objectives

• Study the diversity of Streptococcus suis strains in the US
• Investigate associations between subtypes and clinical presentation

Methods

208 isolates were obtained from the UMN and Kansas State Veterinary Diagnostic Laboratories and were classified as follows:

• pathogenic if the strain was isolated from neurologic or systemic tissues of a diseased pig
• opportunistic if the strain was isolated from lung tissue
• commensal if the strain came from a healthy pig

The isolates came from more than 20 states across the country and were classified by serotype and by genotype using Multilocus Sequence Typing. 35 serotypes are currently known for S. suis with serotypes 1/2, 2, 3, 7, and 8 being the most commonly found in the US. As a reference, 1,161 sequence type (ST) profiles have been registered for S. suis over the world. ST1, ST25, and ST28 are the most common in pigs.

Results

Between 80 and 100% of the strains identified as serotypes 1, 1/2, 2, 7, 14, and 23 were classified as pathogenic in this study. Additionally, 12 ST contained more than 75% of pathogenic strains. These were ST 1, 13, 25, 28, 29, 94, 108, 117, 225, 373, 961, and 977. The analysis of the association between pathotype, serotype, and ST showed that overall, STs were a better predictor of pathogenicity than serotypes.

The full article can be read on the journal’s website.

Abstract

Streptococcus suis is a significant cause of mortality in piglets and growing pigs worldwide. The species contains pathogenic and commensal strains with pathogenic strains causing meningitis, arthritis, endocarditis, polyserositis, and septicemia. Serotyping and multilocus sequence typing (MLST) are primary methods to differentiate strains, but information is limited for strains found in the United States. The objective of this study was to characterize the diversity of 208 S. suis isolates collected between 2014 to 2017 across North America (mainly the United States) by serotyping and MLST and to investigate associations between subtype and pathotype classifications (pathogenic, possibly opportunistic, and commensal), based on clinical information and site of isolation. Twenty serotypes were identified, and the predominant serotypes were 1/2 and 7. Fifty-eight sequence types (STs) were identified, and the predominant ST was ST28. Associations among serotypes, STs, and pathotypes were investigated using odds ratio and clustering analyses. Evaluation of serotype and ST with pathotype identified a majority of isolates of serotypes 1, 1/2, 2, 7, 14, and 23 and ST1, ST13, ST25, ST28, ST29, ST94, ST108, ST117, ST225, ST373, ST961, and ST977 as associated with the pathogenic pathotype. Serotypes 21 and 31, and ST750 and ST821 were associated with the commensal pathotype, which is composed of isolates from farms with no known history of S. suis-associated disease. Our study demonstrates the use of serotyping and MLST to differentiate pathogenic from commensal isolates and establish links between pathotype and subtype thus, increasing the knowledge about S. suis strains circulating in the United States.