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Streptococcus suis is a serious pathogen in the pig industry with zoonotic potential. With respect to the current effort to reduce antibiotic use in animals, a prophylactic measure is needed to control the disease burden. Unfortunately, immunization against streptococcal pathogens is challenging due to nature of the interaction between the pathogen and the host immune system, but vaccines based on conjugates of capsular polysaccharide (CPS) and carrier protein were proved to be efficient. The main obstacle of these vaccines is manufacturing cost, limiting their use in animals. In this work, we tested an experimental vaccine against Streptococcus suis serotype 2 based on capsular polysaccharide conjugated to chicken ovalbumin (OVA) and compared its immunogenicity and protectivity with a vaccine based on CRM197 conjugate. Ovalbumin was selected as a cheap alternative to recombinant carrier proteins widely used in vaccines for human use. We found that the ovalbumin-based experimental vaccine successfully induced immune response in pigs, and the IgG antibody response was even higher than after immunization with capsular polysaccharide-CRM197 conjugate. Protectivity of vaccination against infection was evaluated in the challenge experiment and was found promising for both conjugates.

The ubiquitous occurrence and high heterogeneity of Streptococcus uberis strains cause difficulties in the development and implementation of effective control strategies in dairy herds. In this study, S. uberis strains from 74 farms, obtained predominantly from subclinical, acute, and chronic recurrent mastitis, as well as from udder surface swabs and milk from healthy udders, were analysed for their genetic diversity using multilocus sequence typing (MLST). Isolates were tested for the presence of the genes encoding the virulence factors using polymerase chain reaction. Antibiotic susceptibility testing was performed using a microdilution assay including 14 antimicrobials. The virulence profiles and antimicrobial (AMR) profiles of the isolates were assembled and the overall heterogeneity was evaluated. Among the 124 isolates, 89 MLST genotypes, 7 different virulence profiles, and 12 AMR profiles were identified. The large number of different MLST allelic profiles in this study points to the high heterogeneity of strains in dairy herds in the Czech Republic. Isolates of a certain MLST genotype may possess a different set of virulence factor genes. We detected up to three different resistance profiles within a single MLST genotype. The results of our study showed that fully susceptible isolates coexisted with resistant or even multiresistant isolates in the same herd. Multiple genotypes within a herd were detected on many farms (up to seven MLST genotypes and four AMR profiles in one herd). This heterogenic population structure might suggest that environmental transmission is the predominant route of infection in herds in the Czech Republic.

A determination of susceptibility/resistance to antimicrobials via serotype was carried out in 506 field isolates of Streptococcus suis, originating from pig farms in the Czech Republic in the period 2018–2022. A very high level of susceptibility of S. suis isolates was found to amoxicillin, in combination with clavulanic acid and sulfamethoxazole potentiated with trimethoprim. None of the tested isolates were resistant to these antimicrobial substances. Only two isolates were found to be intermediately resistant to enrofloxacin in 2020. With regard to ceftiofur, one isolate was intermediately resistant in 2020 and 2022, and two isolates were intermediately resistant in 2018 and 2021. A low level of resistance was detected to ampicillin (0.6% in 2021) and to florfenicol (1.15% in 2019; 1.3% in 2022). With regard to penicillin, a medium level of resistance was detected in 2018 (10.6%), but a low level of resistance was found in the following years (7.0% in 2019; 3.1% in 2020; 3.3% in 2021; 3.9% in 2022). On the contrary, a high or very high level of resistance was found to tetracycline (66.0% in 2018; 65.1% in 2019; 44.35% in 2020; 46.4% in 2021; 54.0% in 2022). Using molecular and serological methods, serotype 7 (16.4%) was determined to be predominant among S. suis isolates, followed by serotypes 1/2, 2, 9, 4, 3, 1, 29, 16, and 31 (10.7%; 8.5%; 5.7%; 5.5%; 4.5%; 4.3%; 3.6%; 3.4%; 3.4%, respectively). Other serotypes were identified among the investigated strains either rarely (up to 10 cases) or not at all. A relatively high percentage of isolates were detected as non-typeable (79 isolates; 15.6%). Dependence of resistance upon serotype assignment could not be proven in all but serotype 31, wherein all isolates (n = 17) were resistant or intermediately resistant to clindamycin, tilmycosin, tulathromycin, and tetracycline. The resistance to clindamycin and tetracycline may be related to the high consumption of these antibiotics on pig farms at present or in previous years. Macrolides (tilmicosin and tulathromycin) and tiamulin are not suitable for the treatment of streptococcal infections, but are used on pig farms to treat respiratory infections caused by gram-negative bacteria, so they were included in the study.

As in other countries, in the Czech Republic, Streptococcus suis infection in pigs is considered an economically significant disease for the pig industry, though little is known about its population structure. We collected S. suis isolates from 144 farms in the years 2018–2022. All samples were taken from animals suffering from symptoms indicating possible S. suis infection. Serotyping revealed the presence of 23 different serotypes, and 18.94% were non-typable strains. The most common was S7 (14.96%), while other serotypes had frequencies of less than 10%. Sequence typing identified 56 different sequence types, including 31 newly assigned sequence types together with 41 new alleles in genes in the MLST schema. A large portion of isolates (25.70%) were of unknown sequence type. The most common sequence types were ST29 (14.77%) and ST28 (10.04%); the other sequence types had frequencies of less than 10%. In total, 100 different combinations of serotypes and sequence types were identified. Among them, S7ST29 was found in 72 isolates, representing 13.63% of all isolates, and was significantly associated with the central nervous system. Many other isolates of particular serotype and sequence type combinations were found in a few cases, and a number of isolates were non-typable.

Streptococcus suis represents a primary health problem (such as meningitis, septicemia and arthritis in piglets and fatteners) in the swine industry worldwide and also an emerging zoonotic pathogen. In the Czech Republic, many pig farms repopulated their herds over the past decades to reduce morbidity and minimize treatment. The study analysed serotypes, sequence types and antimicrobial susceptibility in 39 S. suis isolates obtained from organs of diseased pigs from selected 16 repopulated farms with a history of S. suis-associated diseases and routine antimicrobial treatment with tulathromycin and/or amoxicillin. The analysis revealed diversity of collected isolates with regular occurrence of more than three serotypes per farm. The serotypes identified were 1/2 and 7, each in six isolates, followed by serotype 2 and 3 found in five isolates each, other serotypes were less frequent. Seven isolates were not typable by multiplex PCR and we also found sequence type of unknown type in thirteen isolates. The majority of S. suis isolates were resistant to clindamycin (n = 31), tetracycline (n = 29) and tilmicosin and tulathromycin (n = 28). On the other hand, with the exception of two isolates that were intermediately susceptible to penicillin and ampicillin, all isolates were susceptible to all three tested subgroups of beta-lactam antibiotics.