Marcelo Gottschalk

SUMMARY: Streptococcus suis is a swine pathogen causing significant economic losses and an important zoonotic agent in humans. Southeast Asia is a high-incidence region, where infection is strongly linked to consumption of raw or undercooked pork and occupational exposure to pigs or pork products. Thailand and Vietnam report the highest case numbers, with additional cases in Indonesia (Bali), Malaysia, Singapore, Lao PDR, and Cambodia. Serotype 2 predominates in both humans and pigs, followed by serotype 14, while other serotypes (1, 4, 5, 7, 9, 16, 24, and 31) are also reported in humans. In pigs, a broader serotype range is observed, including 1 or 14, 2, 4, 5, 6, 7, 8, 9, 15, 16, 18, 21, 29, 31, and nontypeable strains. Human infections involve multiple clonal complexes (CC1, CC16, CC25, CC28, CC94, CC104, CC233, CC221/234, and CC1688), several overlapping with porcine lineages. The region is also challenged by increasing antimicrobial resistance, with rising non-susceptibility to macrolides and tetracyclines and occasional reduced susceptibility to penicillin. Diagnostic limitations due to misidentification, limited molecular tools, and reliance on conventional methods further complicate surveillance. The economic burden is substantial, driven by swine losses, treatment costs, outbreaks, and severe human disease. Vaccination remains suboptimal due to limited protection against genetically and antigenically diverse strains across serotypes and CCs. An urgent One Health approach integrating animal health, food safety, and human health is needed to monitor S. suis dynamics. Control measures should target farms, slaughterhouses, and retail markets while promoting community-accepted behavioral changes to reduce transmission risk.

Background: Human Streptococcus suis infections occur worldwide but are most common in East and Southeast Asia. In Europe, most cases are linked to occupational contact with pigs and are usually caused by serotype 2 sequence type 1 (ST1) and ST20 strains, whereas other serotype 2 genotypes seldom cause human disease on the continent. Case Summary: A 55-year-old male pig farmer from Brittany, France, presented with acute confusion, fever, and nuchal rigidity consistent with meningitis. Cerebrospinal fluid cultures were negative, but blood cultures grew S. suis . Empirical treatment with cefotaxime and dexamethasone was initiated, followed by high-dose amoxicillin once antimicrobial susceptibility results confirmed β-lactam sensitivity. The patient required 5 days of intensive care and 7 additional days in a general ward and then completed 8 days of home intravenous ceftriaxone, fully recovering without hearing loss, a frequent sequela of S. suis meningitis. The isolate was typed as serotype 2. It belonged to the ST25 lineage and was genetically closely related to North American strains. Conclusion: This case serves as a reminder that S. suis should be considered in meningitis among swine-exposed individuals and expands the known range of ST25 human infections to Western Europe, providing useful insights for regional surveillance efforts.

Streptococcus suis is an important swine pathogen and zoonotic agent, with the capsular polysaccharide recognized as its major virulence factor. Recent genomic studies have identified several virulence-associated genes (VAGs), including SSU_RS09155 , SSU_RS09525 , and SSU_RS03100 , as potential markers of pathogenicity. The current study evaluated the direct contribution of these genes to S. suis virulence using isogenic deletion mutants of the serotype 2 reference strain P1/7. In a murine infection model, ΔSSU_RS09525 and ΔSSU_RS03100 mutants displayed virulence comparable to the parental strain, whereas the ΔSSU_RS09155 mutant was significantly attenuated, showing reduced mortality and bacteremia. In vitro, the ΔSSU_RS09155 mutant demonstrated increased susceptibility to killing in whole blood and enhanced phagocytosis by murine macrophages, while no significant defects in capsular polysaccharide (CPS) expression or growth in rich medium and mouse and porcine plasma were observed. Complementation restored the resistance to the whole-blood killing phenotype, confirming the specific role of SSU_RS09155 . Adhesion to and invasion of porcine tracheal epithelial cells were unaffected. In pigs, the natural host, the ΔSSU_RS09155 mutant was almost completely avirulent, with minimal to undetectable bacteremia. In silico analysis suggested that SSU_RS09155 encodes a hydrophobic, membrane-associated protein potentially involved in resistance to innate immune mechanisms, although the real function remains unknown. Together, these findings identify SSU_RS09155 as a critical virulence factor in S. suis serotype 2, whereas SSU_RS09525 and SSU_RS03100 appear dispensable under the experimental conditions tested.

Deoxynivalenol (DON) is a common mycotoxin in swine feed known for its immunomodulatory effects and potential to exacerbate infectious diseases. However, its impact on systemic bacterial infections remains unclear. This study investigated whether DON influences the interaction between porcine marrow-derived dendritic cells (DCs) and Streptococcus suis , a major swine and zoonotic pathogen, as well as the progression of systemic infection in weaned piglets. In vitro exposure of DCs to non-toxic concentrations of DON (0.5 and 1 µM) neither altered the phagocytic uptake or intracellular survival of S. suis , nor did it affect anti-inflammatory cytokine expression. However, a moderate downregulation of IL-6 and IL-8 mRNA expression was observed. In vivo, piglets fed diets naturally contaminated with DON (1 or 2 mg/kg) and challenged with S. suis serotype 2 showed no significant differences in clinical signs, survival rates, or weight gain compared to controls. Although transiently increased bacteremia was observed at early stages post-infection in DON-exposed groups, it did not translate into more severe disease. These results suggest that under the conditions tested, DON has limited impact on DC functionality and does not significantly enhance susceptibility to systemic S. suis infection. This work provides relevant data for swine health risk assessments related to feed contamination with DON.

Streptococcus dysgalactiae subsp. equisimilis (SDSE) has historically been recognized as a human pathogen, yet β-hemolytic streptococci consistent with SDSE have been documented in pigs for nearly a century. To investigate the population structure of porcine SDSE and the phylogenetic relationships between swine and human strains, we characterized 41 isolates recovered from diseased pigs in Quebec, Canada (2019–2022). Infected animals spanned all major production stages and frequently presented with invasive disease, including arthritis, endocarditis, and sudden death. Core-genome phylogenetics resolved two heterogeneous porcine clades separated by long internal branches and clearly distinct from dominant human SDSE lineages. Most porcine isolates were emm-negative or contained structurally altered emm regions compared with human strains. Analysis of Lancefield antigen loci identified a predominant group C lineage and a minority group L lineage, recapitulating historical serogroup distributions described since the early-20th century. Phenotypic testing showed susceptibility to β-lactams and florfenicol but high levels of resistance to tetracycline, macrolides and lincosamides. Detected antimicrobial resistance (AMR) genes correlated well with phenotypes, and multidrug resistance was frequent. Hybrid genome assemblies revealed integrative and mobilizable elements carrying AMR determinants. Collectively, our data indicate that porcine SDSE represents a long-standing, genetically structured, host-adapted population with notable AMR potential, underscoring the need for continued swine SDSE genomic surveillance.

Actinobacillus pleuropneumoniae is the causative agent of porcine pleuropneumonia, a disease of major economic impact. Serotype 12 is generally considered to be of low virulence and is typically associated with subclinical infections. However, we describe four atypical serotype 12 field strains recovered from severe clinical outbreaks in Chile. These strains exhibited an unusual toxin gene profile (apxIICA, apxIIICA, apxIBD, apxIIIBD), suggesting the ability to produce both ApxII and ApxIII toxins. Comparative genomic analyses revealed that these atypical strains carry the capsule genes of serotype 12 but the LPS biosynthesis genes of serotype 15, indicating a hybrid genomic structure. Phylogenetic analysis confirmed that they cluster separately from classical serotype 12 strains and are closely related to other atypical strains from Japan, USA, and Canada. Experimental infection in pigs demonstrated significantly increased virulence of the atypical Chilean strain compared to the reference strain of serotype 12, with higher clinical scores, severe lung lesions, and atypical serological responses against serotypes 3/6/8/15/17. These findings challenge the traditional view of serotype 12 as low-virulence and highlight the need for improved diagnostic approaches that incorporate both capsule and LPS profiling. The existence of these atypical strains has important implications for disease surveillance, diagnostics, and vaccine development in swine health management.

Actinobacillus pleuropneumoniae is a major swine pathogen, classified into 19 serotypes based on capsular polysaccharide (CPS) loci. This study aimed to improve the diagnostic method to differentiate between serotypes 9 and 11, which are challenging to distinguish using conventional serological and molecular methods. A novel qPCR assay based on locked nucleic acid (LNA) probes was developed and validated using a collection of reference strains representing all known 19 serotypes. The assay demonstrated specificity in detecting the nucleotide variation characteristic of the serotype 9 reference strain. However, the analysis of a clinical isolate collection identified discrepancies between LNA-qPCR and serological results, prompting further investigation of the cps and O-Ag loci. Subsequent nanopore sequencing and whole-genome sequencing of a collection of 31 European clinical isolates, previously identified as serotype 9, 11, or undifferentiated 9/11, revealed significant genetic variations in the cps and O-Ag loci. Ten isolates had a cpsF sequence identical to that of the serotype 11 reference strain, while six isolates had single-nucleotide polymorphisms that were unlikely to cause significant coding changes. In contrast, 15 isolates had interruptions in the cpsF gene, distinct from that found in the serotype 9 reference strain, potentially leading to a serotype 9 CPS structure. In the O-Ag loci, differences between serotypes 9 and 11 were minimal, although some isolates had mutations potentially affecting O-Ag expression. Overall, these findings suggest that multiple genetic events can lead to the formation of a serotype 9 CPS structure, hindering the development of a single qPCR assay capable of detecting all cpsF gene mutations. Our results suggest that, currently, a comprehensive analysis of the cpsF gene is necessary to accurately determine whether the capsule of an isolate corresponds to serotype 9 or 11. Although such analyses are feasible with the advent of third-generation sequencing technologies, their accessibility, cost, and time to result limit their use in routine diagnostic applications. Under these circumstances, the designation of the hybrid serovar 9/11 remains a valid approach.

Actinobacillus pleuropneumoniae is the causative agent of porcine pleuropneumonia, a challenging respiratory disease for the global swine industry. Variations in the serotypes associated with clinical disease have been observed in different regions worldwide. This study aimed to provide an updated epidemiological assessment of A. pleuropneumoniae serotypes in Spain, incorporating bacterial characterization through serotyping and toxinotyping. Serotypes 9/11, 2, 4, 5, 17, and 13 were frequently identified in diseased animals. Furthermore, qPCR of lung samples from an outbreak, even when samples were pooled, emerged as a robust diagnostic tool, enabling the rapid detection of A. pleuropneumoniae and their serotypes without the need for microbiological isolation. This technology also facilitates serotype monitoring of apparently healthy herds through the testing of oral fluids. The study revealed the frequent simultaneous presence of diverse serotypes within a farm. Serotypes 1, 7, 10, 12, 18, and 19 were frequently found in subclinically infected animals but were rarely detected in acute pleuropneumonia outbreaks in the current study. These results provide valuable information for interpreting the potential virulence of the different serotypes in Spain. However, other predisposing factors and the immune status of the herds such as type of vaccines used when appropriate, should be carefully considered before drawing definitive conclusions. Nevertheless, the study offers valuable insights that underscore the necessity for detailed regional data to contribute toward a comprehensive understanding of the disease dynamics and toward formulating effective control measures for porcine pleuropneumonia.