Characterization of neurologic disease-associated
Streptococcus suis
strains within the United States swine herd and use of diagnostic tools
Authors: Santos Streauslin Jessica , Nielsen Daniel W. , Schwartz Kent J. , Derscheid Rachel J. , Magstadt Drew R. , Burrough Eric R. , Gauger Phillip C. , Schumacher Loni L. , Rahe Michael C. , Michael Alyona , Sitthicharoenchai Panchan , Siepker Christopher L. , Matias Ferreyra Franco , Nunes de Almeida Marcelo , Main Rodger , Bradner Laura K. , Hu Xiao , Li Ganwu , Poeta Silva Ana Paula S. , Sahin Orhan , Arruda Bailey L. 
Published: October 2024

ABSTRACT

Streptococcus suis
negatively impacts swine health, posing diagnostic and preventative challenges.
S. suis
can induce disease and also quietly reside on mucosal surfaces. The limited use of diagnostic tools to identify disease-associated strains and rule out differential diagnoses, alongside the complex ecology of
S. suis
, poses significant challenges in comprehending this important pathogen and defining pathotypes. This study evaluated 2,379
S
.
suis
central nervous system (CNS) isolates from diagnostic submissions between 2015 and 2019. Isolates originating from submissions with histologic evidence of CNS infection (
n
= 1,032) were further characterized by standard and advanced diagnostic techniques. We identified 29
S
.
suis
serotypes and 4 reclassified serotypes as putative causes of CNS disease. Among these, serotypes 1 and 7 emerged as the predominant putative causes of CNS infection (32% of submissions). Furthermore, 51 sequence types (STs), of which 15 were novel, were detected with ST1 predominating. Through whole-genome sequencing of 145 isolates, we observed that five commonly used virulence-associated genes (VAGs;
epf
,
mrp
,
sly
,
ofs
, and
srtF
) were not present in most disease-associated isolates, and matrix-assisted laser desorption ionization–time of flight mass spectrometry (MALDI-TOF MS) yielded false-positive results in 7% of isolates. These data indicate that (i) clinical signs and site of isolation alone are insufficient for defining a pathotype, (ii)
S. suis
serotypes and STs associated with CNS infection are more diverse than previously reported, (iii) MALDI-TOF MS may need to be supplemented with additional diagnostic tools for precise
S. suis
identification, and (iv) VAGs remain an unreliable means for identifying isolates associated with CNS disease.

IMPORTANCE

Streptococcus suis
is an important and complex systemic bacterial pathogen of swine. Characterization of
S. suis
strains originating from pigs with histologic confirmation of neurologic disease is limited. Review of swine diagnostic submissions revealed that fewer than half of cases from which
S. suis
was isolated from the brain had histologic evidence of neurologic disease. This finding demonstrates that clinical signs and site of isolation alone are not sufficient for identifying a neurologic disease-associated strain. Characterization of strains originating from cases with evidence of disease using classic and advanced diagnostic techniques revealed that neurologic disease-associated strains are diverse and commonly lack genes previously associated with virulence.