Capsular serotypes, antimicrobial susceptibility, and the presence of transferable oxazolidinone resistance genes in Streptococcus suis isolated from healthy pigs in China

Highlights

• The most prevalent serotype of S. suis isolated from healthy pigs in this study belongs to serotype 29.

• Multidrug resistance was observed in 98.7 % of S. suis isolates.

• The multidrug resistance gene optrA was detected on the plasmid of S. suis for the first time.

• The coexistence of pathogenicity island (PAI) and optrA in 3 S. suis isolates.

Abstract

Streptococcus suis is a pig pathogen and a vector of zoonotic diseases that can cause severe systemic infection in humans. S. suis can colonize the nasal cavity, tonsils, and upper respiratory, genital, and digestive tracts in healthy pigs. Here, to determine prevalence, serotype distribution, and antimicrobial susceptibility of S. suis in healthy pigs, we collected 1813 nasal cavity samples from healthy pigs raised on 17 independent farms in six Chinese provinces between 2016 and 2018. We obtained 223 S. suis isolates (12.3 %) and the antimicrobial susceptibility to a panel of 11 antimicrobial agents was measured by microbroth dilution. Most S. suis isolates (98.7 %) were resistant to at least three classes of antimicrobial agents. The optrA gene conferring resistance to oxazolidinones and phenicols was identified in the chromosome of 27 isolates and on a ∼40-kb plasmid in one isolate; to the best of our knowledge, this was the first report of plasmid-borne optrA gene in S. suis. The genetic environment of optrA showed substantial diversity and could be divided into eleven different types. Interestingly, some fragments of the 89 K pathogenicity island (PAI) were observed together with optrA in 3 isolates, which warrants further attention. Capsular serotypes of S. suis isolates were determined by multiplex PCR. Serotype 29 was the most prevalent, followed by serotype 7 and serotype 2. The presence of highly virulent serotype 2 strains may pose a threat to public health.

Introduction

Streptococcus suis is one of the major pathogens of swine, which causes swine arthritis, septicemia, meningitis endocarditis, abortions and sudden death. It represents also a serious threat to public health as an emerging zoonotic agent, with nearly 1600 human infections by S. suis having been reported across 30 countries (Gottschalk et al., 2010; Lun et al., 2007; Youjun et al., 2014). Notably, two large outbreaks of S. suis serotype 2 in humans occurred in China in 1998 and 2005, causing 14 and 38 deaths, respectively (Yu et al., 2006).

Based on the capsular polysaccharides, S. suis was traditionally divided into 35 serotypes (1–34 and 1/2). However, recent studies suggest six of those serotypes have been reclassified as Streptococcus orisratti (serotypes 32 and 34), Streptococcus parasuis (serotypes 20, 22, and 26), or Streptococcus ruminantium (serotype 33) (Gottschalk et al., 2010; Tohya et al., 2017). Generally, serotype 2 was considered to be the most virulent serotype, while serotype 3, 7 and 9 were also widespread in many countries and associated with S. suis infections (Oh et al., 2017).

Given that vaccines have limited availability and could only provide partial protection against certain serotypes of S. suis, antimicrobial agents offer the most effective way to treat and control S. suis infections (Zhang et al., 2008). Unfortunately, accumulating evidence points to decreased susceptibility of S. suis to commonly used antimicrobials (Huan et al., 2020; Peng et al., 2019). Oxazolidinones, including linezolid and tedizolid, represent last-resort antimicrobial agents against clinically important Gram-positive pathogens. These drugs could inhibit protein synthesis by binding to the peptidyl transferase core of bacterial 23S rRNA and had not been approved for veterinary applications worldwide. However, several studies had confirmed florfenicol, a widely used antimicrobial agent in veterinary medicine, could provide co-selection of oxazolidinones resistance (Fan et al., 2017; Wang et al., 2015b). The florfenicol and oxazolidinone resistance has been ascribed to mutations in the central loop of 23S rRNA and in genes encoding the 50S ribosomal proteins, as well as the acquisition of the transferable resistance genes cfr, cfr(B), cfr(C), optrA and poxtA (Antonelli et al., 2018; Fan et al., 2016). So far, only cfr and optrA have been detected in S. suis, but information about the molecular epidemiology of these genes was still limited (Du et al., 2019; Huang et al., 2019; Shang et al., 2019; Wang et al., 2013).

Healthy pigs can carry multiple serotypes of S. suis in their upper respiratory tract, particularly the tonsils and nasal cavity, and the alimentary and genital tracts (Gottschalk et al., 2010; Zhang et al., 2015). Although several publications reported isolates from asymptomatic pigs, a comprehensive analysis of prevalence, serotype distribution and resistance profiles of S. suis in China is still lacking. In this study, we investigated the prevalence of S. suis in healthy pigs from 17 farms in 6 provinces in China. We measured the susceptibility of S. suis isolates to a panel of antimicrobial agents commonly used in animal and clinical settings, characterized serotypes distribution of these isolates, and compared them to clinical isolates. Furthermore, the presence and transfer of the phenicols-oxazolidinones resistance genes in S. suis were also investigated.