#17-129

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Over the past few years, Streptococcus suis has become a reemerging bacteria causing significant economic losses for the swine industry. However, the reason for the reemergence is unclear. As both commensal and pathogenic strains of S. suis exist, numerous serotypes and multilocus sequence types (ST) have been identified and are used to differentiate S. suis strains. The goal of our study was to characterize historical strains of S. suis to identify any shift in the historical isolates that may possibly explain its reemergence.

A total of 114 historical isolates were selected for characterization from clinical diagnostic cases between 1999 and 2013 and partitioned into time frames consisting of 5 years to investigate trends. The serotype was identified for 93% of the isolates, with 14 serotypes identified, while 7% of the isolates were nontypable (NT). The predominant serotypes were 1/2 , 2, and 7. Within the predominant serotypes, serotype 2 slightly decreased while serotype 7 remained relatively constant over the 3 time periods. Serotype 1/2 nearly doubled between the first and last time period. The ST was identified in 68% of the isolates with 14 STs identified. ST28 was the predominant sequence type (24%) while 25% of the isolates had novel ST profiles. ST28 remained relatively constant over time. Interestingly, only a single ST1 was identified within the first 10 years while five isolates were identified within the last five years of the study. The number of novel ST and ST87 isolates decreased during the third time period.

In Europe, MRP, EF, and SLY have been associated with some pathogenic strains of S. suis. Only five and six isolates contained the MRP and EF gene, respectively, and most of the isolates were from 2009-2013. The gene SLY was identified in 61% of the isolates. Three genes were recently identified as indicators of pathogenicity in a contemporary set of S. suis strains. In this historical set, 95-100% of the isolates contained these three genes, indicating the new virulence genes have not changed overtime. AMR genes were investigated in the historical S. suis isolates, and 93% of the S. suis isolates contain an AMR profile of ErmB, tet32, and tetO or tet32 and tetO. The reduction of the ErmB gene occurred over time, which contributed to an increase of the tet32 and tetO profile over time. ErmB confers resistance to macrolides, lincosamides, & streptogramin while the tet32 and tetO genes confer resistance to tetracycline. Only two isolates had AMR genes to lincosamide and nucleoside antibiotics while a single isolate had AMR gene to lincosamides & pleuromutilins. Four isolates had AMR genes to aminoglycosides. AMR genes to beta-lactams were not identified in the isolates.

Recently, our research group published results on a contemporary set of pathogenic S. suis isolates (1). From the previous study, the predominant serotype was serotype 1/2, which is consistent with the increase of serotype 1/2 in this study. This historical study suggests an increase in serotype 1 and decrease in serotype 7, indicating a shift in S. suis isolates. In the contemporary set of S. suis isolates, ST28 was the dominant ST followed by ST1, ST94, and ST108. Therefore, ST28 was predominant in both studies. The increase of ST1 in the historical study may align with ST1 being one of the dominant STs in the contemporary set of S. suis isolates. In both the historical and contemporary S. suis isolate sets, MRP, EF, and SLY genes are not indicators of virulence while the new genes predicting pathogenesis were present in both sets of S. suis isolates. Interestingly, there was a reduction of the AMR gene ErmR in the historical set, indicating an increase of multi-resistant S. suis isolates was not occurring over time. The comparison between historical and contemporary S. suis isolates provides reference and highlights that other factors may be contributing to the reemergence of S. suis in the United States.

1. Estrada AA, Gottschalk M, Rossow S, Rendahl A, Gebhart C, Marthaler DG. Serotype and Genotype (Multilocus Sequence Type) of Streptococcus suis Isolates from the United States Serve as Predictors of Pathotype. J Clin Microbiol. 2019 Jun 26.