Genetic diversity of Streptococcus suis isolated from three pig farms of China obtained by acquiring antibiotic resistance genes

Conjugative transfer of streptococcal prophages harboring antibiotic resistance and virulence genes

Prophages play important roles in the transduction of various functional traits, including virulence factors, but remain debatable in harboring and transmitting antimicrobial resistance genes (ARGs). Herein we characterize a prevalent family of prophages in Streptococcus, designated SMphages, which harbor twenty-five ARGs that collectively confer resistance to ten antimicrobial classes, including vanG-type vancomycin resistance locus and oxazolidinone resistance gene optrA. SMphages integrate into four chromosome attachment sites by utilizing three types of integration modules and undergo excision in response to phage induction. Moreover, we characterize four subtypes of Alp-related surface proteins within SMphages, the lethal effects of which are extensively validated in cell and animal models. SMphages transfer via high-frequency conjugation that is facilitated by integrative and conjugative elements from either donors or recipients. Our findings explain the widespread of SMphages and the rapid dissemination of ARGs observed in members of the Streptococcus genus.

ICESpsuAH0906, a novel optrA-carrying element conferring resistance to phenicols and oxazolidinones from Streptococcus parasuis, is transferable to Streptococcus suis

Streptococcus parasuis is a potential opportunistic zoonotic pathogen which is a close relative to Streptococcus suis, which exhibit extensive genetic exchange. The occurrence and dissemination of oxazolidinone resistance poses a severe threat to public health. However, such knowledge about the optrA gene in S. parasuis is limited. Herein, we characterized an optrA-positive multi-resistant S. parasuis isolate AH0906, in which the capsular polysaccharide locus exhibited a hybrid structure of S. suis serotype 11 and S. parasuis serotype 26. The optrA and erm(B) genes were co-located on a novel ICE of the ICESsuYZDH1 family, designated ICESpsuAH0906. IS1216E-optrA-carrying translocatable unit could be formed when excised from ICESpsuAH0906. ICESpsuAH0906 was found to be transferable from isolate AH0906 to Streptococcus suis P1/7RF at a relative high frequency of ∼ 10^-5. Nonconservative integrations of ICESpsuAH0906 into the primary site SSU0877 and secondary site SSU1797 with 2-/4-nt imperfect direct repeats in recipient P1/7RF were observed. Upon transfer, the transconjugant displayed elevated MICs of the corresponding antimicrobial agents and performed a weak fitness cost when compared with the recipient strain. To our knowledge, it is the first description of the transfer of optrA in S. prarasuis and the first report of interspecies transfer of ICE with triplet serine integrases (of the ICESsuYZDH1 family). Considering the high transmission frequency of the ICEs and the extensive genetic exchange potential of S. parasuis with other streptococci, attention should be paid to the dissemination of the optrA gene from S. parasuis to clinically more important bacterial pathogens.

Development of Two Loop-Mediated Isothermal Amplification Assays for Rapid Detection of ermB and mefA Genes in Streptococcus suis

Streptococcus suis is an important zoonotic pathogen that poses a serious threat to the pig industry and human health. The massive use of macrolides has led to the emergence of resistance in S. suis, and S. suis is suspected to be a reservoir of antimicrobial resistance genes. The mechanism to macrolide resistance in S. suis is mainly due to ermB and mefA. In this study, loop-mediated isothermal amplification (LAMP) methods were developed to detect ermB and mefA genes in S. suis through turbidimetry detection. The sensitivity and specificity of the LAMP reactions were determined. All results of LAMP and polymerase chain reaction (PCR) assay were compared to determine whether LAMP method was accurate and reliable. The results showed that all 100 nonstreptococcus clinical isolates tested negative, indicating the high specificity of LAMP assays. The detection limit of LAMP assay was 1 fg per reaction, and 10²-10⁴-fold lower than those of conventional PCR methods. Evaluation of the performance of the LAMP assay in S. suis clinical strains revealed a good consistency between LAMP and PCR assays. In conclusion, LAMP assays are specific, sensitive, and rapid methods to detect ermB and mefA in S. suis.

How Streptococcus suis escapes antibiotic treatments

Streptococcus suis is a zoonotic agent that causes sepsis and meningitis in pigs and humans. S. suis infections are responsible for large economic losses in pig production. The lack of effective vaccines to prevent the disease has promoted the extensive use of antibiotics worldwide. This has been followed by the emergence of resistance against different classes of antibiotics. The rates of resistance to tetracyclines, lincosamides, and macrolides are extremely high, and resistance has spread worldwide. The genetic origin of S. suis resistance is multiple and includes the production of target-modifying and antibiotic-inactivating enzymes and mutations in antibiotic targets. S. suis genomes contain traits of horizontal gene transfer. Many mobile genetic elements carry a variety of genes that confer resistance to antibiotics as well as genes for autonomous DNA transfer and, thus, S. suis can rapidly acquire multiresistance. In addition, S. suis forms microcolonies on host tissues, which are associations of microorganisms that generate tolerance to antibiotics through a variety of mechanisms and favor the exchange of genetic material. Thus, alternatives to currently used antibiotics are highly demanded. A deep understanding of the mechanisms by which S. suis becomes resistant or tolerant to antibiotics may help to develop novel molecules or combinations of antimicrobials to fight these infections. Meanwhile, phage therapy and vaccination are promising alternative strategies, which could alleviate disease pressure and, thereby, antibiotic use.

Genome analysis provides insight into hyper-virulence of Streptococcus suis LSM178, a human strain with a novel sequence type 1005

Streptococcus suis has been well-recognized as a zoonotic pathogen worldwide, and the diversity and unpredictable adaptive potential of sporadic human strains represent a great risk to the public health. In this study, S. suis LSM178, isolated from a patient in contact with pigs and raw pork, was assessed as a hyper-virulent strain and interpreted for the virulence based on its genetic information. The strain was more invasive for Caco-2 cells than two other S. suis strains, SC19 and P1/7. Sequence analysis designated LSM178 with serotype 2 and a novel sequence type 1005. Phylogenetic analysis showed that LSM178 clustered with highly virulent strains including all human strains and epidemic strains. Compared with other strains, these S. suis have the most and the same virulent factors and a type I-89 K pathogenicity island. Further, groups of genes were identified to distinguish these highly virulent strains from other generally virulent strains, emphasizing the key roles of genes modeling transcription, cell barrier, replication, recombination and repair on virulence regulation. Additionally, LSM178 contains a novel prophage conducive potentially to pathogenicity.

Virulence associated gene profiling and antimicrobial resistance pattern of Streptococcus suis isolated from clinically healthy pigs from North East India

This study revealed the prevalence of Streptococcus suis in 20·39% clinically healthy pigs from North East India. All these isolates were screened for the presence of virulence- associated genes such as suilysin (sly), muramidase released protein (mrp), extracellular protein factor (epf) and arginine deiminase (arcA). Of these 62 isolates, 29 isolates carried mrp gene, 17 isolates carried sly gene, 57 isolates carried arcA gene, whereas all isolates were negative for epf gene. The most prevalent genotype was mrp^- sly^- epf^- arcA⁺ (45·16%) followed by genotypes mrp⁺ sly^- epf^- arcA⁺ (27·41%), mrp⁺ sly⁺ epf^- arcA⁺ (19·35%) and mrp^- sly⁺ epf^- arcA^- (8·06%). High frequency of resistance was observed for antimicrobials such as tetracycline (93·54%), clindamycin (91·93%), co-trimoxazole (88·70%) and erythromycin (85·48%). Antimicrobial resistance patterns of the S. suis isolates revealed 16 resistance groups (R1 to R16), where 93·54% isolates showed multi-drug resistance (≥3 antimicrobial agents). It has also been observed that 57 (91·93%) isolates were resistant to at least four antimicrobials. The most predominant resistance pattern observed was CD-COT-E-TE, which accounted for 38·70% of the isolates. The occurrence of relatively high levels of resistance of S. suis to some antimicrobials (e.g., macrolides, tetracyclines, and sulphonamides) as observed in this study may represent a human health concern. In addition, a relatively higher percentage of S. suis isolated from clinically healthy pigs indicates a carrier status with risk of dissemination to other pigs in the herd as well as to humans.

Anatomical Site, Typing, Virulence Gene Profiling, Antimicrobial Susceptibility and Resistance Genes of Streptococcus suis Isolates Recovered from Pigs in Spain

A set of 207 Streptococcus suis isolates were collected from ten autonomous communities from Spain in 2019 to 2020 from pigs with meningitis, pneumonic lungs, arthritic joints or other swollen viscera, to a lesser extent. Thirteen capsular types were detected being the most prevalent serotype 2 (21.7%), followed by serotypes 1 (21.3%), 9 (19.3%) and 3 (6.3%). Serotypes 2 and 9 were recovered mainly from the central nervous system (CNS), while serotype 1 was isolated mostly from swollen joints and serotype 3 from the lungs. Twenty-five isolates (12.1%) could not be typed. The most prevalent pathotype was epf + mrp + sly + luxS (49 isolates, 23.8%), and it was related mainly to serotypes 1 and 2. Serotypes 1–3 and 9 were significantly associated with anatomical sites of isolation and virulence factors, serotype 9 (CNS) and serotypes 3 and 9 (lungs) being associated with virulence profiles without the epf gene. S. suis isolates showed globally high antimicrobial resistances, but ampicillin followed by spectinomycin and tiamulin resulted in the highest activities, while the greatest resistances were detected for sulphadimethoxine, tetracyclines, neomycin, clindamycin and macrolides. A total of 87.4% isolates were positive to the tetO gene, 62.4% to the ermB gene and 25.2% to the fexA gene, while 14.6% were positive to all three genes simultaneously. A significative association between isolate resistances to tetracyclines and macrolides and the resistance genes tested was established, except for phenicol resistance and the fexA gene. A set of 14 multiresistance patterns were obtained according to the number of antimicrobials to which the isolates were resistant, the resistances to 12 or more agents being the most prevalent ones. A remarkable amount of multiresistance profiles could be seen among the S. suis serotype 9 isolates.

Horizontal Transfer of Different erm(B)-Carrying Mobile Elements Among Streptococcus suis Strains With Different Serotypes

Macrolide-resistant Streptococcus suis is highly prevalent worldwide. The acquisition of the erm(B) gene mediated by mobile genetic elements (MGEs) in particular integrative and conjugative elements (ICEs) is recognized as the main reason for the rapid spread of macrolide-resistant streptococcal strains. However, knowledge about different erm(B)-carrying elements responsible for the widespread of macrolide resistance and their transferability in S. suis remains poorly understood. In the present study, two erm(B)- and tet(O)-harboring putative ICEs, designated as ICESsuYSB17_rplL and ICESsuYSJ15_rplL, and a novel erm(B)- and aadE-spw-like-carrying genomic island (GI), named GISsuJHJ17_rpsI, were identified to be excised from the chromosome and transferred among S. suis strains with different serotypes. ICESsuYSB17_rplL and ICESsuYSJ15_rplL were integrated downstream the rplL gene, a conserve locus of the ICESa2603 family. GISsuJHJ17_rpsI, with no genes belonging to the conjugation module, was integrated into the site of rpsI. All transconjugants did not exhibit obvious fitness cost by growth curve and competition assays when compared with the recipient. The results demonstrate that different erm(B)-carrying elements were presented and highlight the role of these elements in the dissemination of macrolide resistance in S. suis.

Nonconservative integration and diversity of a new family of integrative and conjugative elements associated with antibiotic resistance in zoonotic pathogen Streptococcus suis

Macrolide and tetracycline resistance in streptococci is mainly caused by acquisition of integrative and conjugative elements (ICEs) of the ICESa2603 family carrying erm(B) and tet(O). But the characteristics about the transferability and physiological consequences of ICEs with triplet serine integrases are still rare. This study tested the transferability of ICESsuYZDH1_SSU0877, a novel erm(B)- and tet(O)-carrying ICESa2603 family-like ICE with triplet serine integrases, and evaluated the physiological consequences after ICE transferred and integrated into recipient. The prevalence of ICESsuYZDH1-like ICEs in S. suis was analyzed based on 1334 genomic sequences available in GenBank and examined in 330 clinical isolates in China. Nonconservative transfer was observed by integrating of ICESsuYZDH1 into SSU1797 gene besides the primary SSU0877 site. Imperfect direct repeats of 2-/4-nt (5'-TC-3'/5'-TCCC-3') and (5'-GC-3'/5'-TCCC-3') were observed at SSU0877 and SSU1797 sites, respectively. The transconjugant suffered a weak fitness cost with stunted growth and less competition with recipient strain. Successive passages indicate the ICESsuYZDH1 could be persist and endued stable resistant phenotype. Comprehensive analysis of the ICESsuYZDH1-like ICEs from both public genome database and our clinical isolates revealed the widespread and diversity of the ICEs by integration at the sites of SSU0877, SSU0468, SSU1262, and SSU1797. The ICESsuYZDH1-like ICEs could stably co-exist within the host chromosome at more than one attachment sites, which is probably mediated by the triplet serine integrases. Nonconservative integration and diversity of the ICESsuYZDH1 family of ICEs might have contributed to the evolution of ICEs and the dissemination of macrolide and tetracycline resistance in S. suis.

Rapid Detection of mrp, epf, and sly Genes by Loop-Mediated Isothermal Amplification in Streptococcus suis

Streptococcus suis remains a serious threat to the worldwide swine industry and human health. In this study, rapid assays for the detection of three common virulence-related factors (mrp, epf, and sly) were developed, evaluated, and applied. Loop-mediated isothermal amplification (LAMP) primers were designed using Primer Explorer V5 software. The sensitivity and specificity of the LAMP assays were determined based on sample turbidity. For all three genes, LAMP assays were performed at 62°C with a reaction time of 60 min. The detection limit of conventional polymerase chain reaction (PCR) was 1 ng/μL, 10 pg/μL, and 100 fg/μL for the epf, sly, and mrp genes, respectively. For the LAMP assays, the detection limits were 10 pg/μL, 10 fg/μL, and 100 fg/μL for epf, sly, and mrp, respectively, representing sensitivities 100-1000 times higher than those of the PCR assay. Furthermore, when the LAMP assays were applied to clinical strains, the results were consistent with those of the PCR assay, confirming the LAMP assays as rapid and reliable detection techniques. In conclusion, the LAMP assays described in this study have the potential to become standard methods to detect the virulence factors mrp, epf, and sly. To the best of our knowledge, this is the first study to report the application of LAMP to detect the mrp, epf, and sly genes.

Ethyl-N-dodecanoyl-l-arginate hydrochloride combats pathogens with low-resistance generation by membrane attack and modifies gut microbiota structure

Ethyl-N-dodecanoyl-l-arginate hydrochloride (LAE, ethyl lauroyl arginate HCl) is a cationic surfactant used as a food preservative with broad-spectrum antibacterial activities. However, its resistance development, influences on gut microbiome and molecular target are unclear. In this study, bacteria were stimulated by LAE for 30 days to test the bacterial resistance. Several infected animal models were used to evaluate the antibacterial effect of LAE in vivo. Mice were orally treated with LAE to test its effect on animal growth. The influence of LAE on mice gut microbiome was analysed by 16S rDNA sequencing. The results indicated that Escherichia coli did not develop resistance to LAE. LAE significantly combats bacterial infection in mice, ducklings and piglets. Moreover, LAE promotes mouse weight gain without changing body composition or reducing animal vitality, and induces lower hepatotoxicity than ampicillin. In the mouse gut microbiome assessment and characterization, LAE modifies host gut microbiota structure. Mechanistically, LAE specifically binds to acidic phospholipids including phosphatidylserine, depolarizes the membrane and disrupts the bacterial membrane followed by bacterial growth inhibition. This study investigates the molecular mechanism of LAE as well as its antibacterial functions in poultry and livestock. Our data suggest LAE is a potential antibacterial agent in animal health.

Bacitracin resistance and enhanced virulence of Streptococcus suis via a novel efflux pump

Background

Streptococcus suis is a prominent pathogen causing septicemia and meningitis in swine and humans. Bacitracin is used widely as a growth promoter in animal feed and to control the spread of necrotic enteritis in most developing countries. This study aimed to characterize a novel membrane transporter module Sst comprising SstE, SstF, and SstG for bacitracin resistance.

Results

Comparative genomics and protein homology analysis found a potential efflux pump SstFEG encoded upstream of well-known bacitracin-resistance genes bceAB and bceRS. A four-fold decrease in bacitracin susceptibility was observed in sstFEG deletion mutant comparing with S. suis wildtype strain CZ130302. Further studies indicated that the bacitracin tolerance mediated by SstFEG is not only independent of the BceAB transporter, but also regulated by the two-component system BceSR. Given that SstFEG are harbored by almost all virulent strains, but not in the avirulent strains, we managed to explore its potential role in bacterial pathogencity. Indeed, our results showed that SstFEG is involved in S. suis colonization and virulence in animal infection model by its potential competitive survival advantage against host bactericidal effect.

Conclusion

To our knowledge, this is the first study to functionally characterize the bacitracin efflux pump in S. suis to provide evidence regarding the important roles of the novel ABC transporter system SstFEG with respect to drug resistance and virulence.

Identification and pathogenicity of an XDR Streptococcus suis isolate that harbours the phenicol-oxazolidinone resistance genes optrA and cfr, and the bacitracin resistance locus bcrABDR

One hundred and seven Streptococcus suis isolates were collected from healthy pigs or asymptomatic carriers in Jiangsu, China in 2016-2017. Thirty-eight percent of the isolates were linezolid-resistant and all carried the optrA gene. Among them, one isolate, SFJ44, was resistant to all 20 of the antibiotics tested, except for ceftiofur, and thus exhibited an extensively-drug-resistant phenotype. This isolate carried the optrA gene and the bacitracin resistance locus bcrABDR on an antibiotic-resistance-associated genomic island (ARGI1), and harboured the resistance genes cfr, aadE, sat4, spw-like, aphA3, mef(A), msr(D), erm(A)-like, erm(B), tetAB(P)', tet(M) and catQ on ARGI2∼4. The IS1216E-bcrABDR-ISEnfa1 segment showed >99.9% sequence identity to corresponding sequences from other species. The cfr gene was located on ARGI4, and two IS6 family insertion sequences, IS1216E and ISTeha2, were found upstream and downstream of cfr-ΔISEnfa5, respectively. A circular intermediate of bcrABDR-ISEnfa1 was detected, suggesting the role of ISEnfa1 in dissemination of bcrABDR. Other antibiotic resistance genes might be acquired from different Gram-positive pathogens. Infection of zebrafish showed that SFJ44 exhibited a virulence level comparable to serotype 2 hypervirulent strain SC070731, highlighting the need for surveillance of the pathogenicity of multi-drug-resistant S. suis isolates. This is the first report of the co-existence of optrA and cfr, and of the bcrABDR locus in streptococci. As it has been suggested that S. suis may act as an antibiotic resistance reservoir contributing to the spread of resistance genes to major streptococcal pathogens, the potential dissemination of these resistance genes among Gram-positive bacteria is of concern and routine surveillance should be strengthened.

Antimicrobial susceptibility of Streptococcus suis isolated from diseased pigs, asymptomatic pigs, and human patients in Thailand

Background

Prophylaxis and treatment of emerging zoonotic Streptococcus suis infection in agricultural and healthcare settings mainly rely on antibiotics. However, continued use of antibiotics contributing to emergence and widespread of antibiotic resistant S. suis becomes a significant challenge in many endemic countries, including Thailand. Meanwhile, the knowledge of antibiotic susceptibility patterns of bacterial pathogens is required for overcoming the antimicrobial resistance problem, the information of antibiotic susceptibility of S. suis strains isolated in Thailand remains limited. This study aims to assess the susceptibility of Thai-isolated S. suis strains to different antibiotic classes in order to gain an insight into the distribution of antibiotic-resistant patterns of S. suis strains in different regions of Thailand.

Results

This study revealed the antimicrobial resistance and multidrug resistance of 262 S. suis strains isolated in different regions of Thailand. Susceptibility testing indicated widespread resistance to macrolides and tetracyclines of S. suis strains in the country. Beta-lactam antibiotic drugs (including cefotaxime and ceftiofur), vancomycin, chloramphenicol, as well as florfenicol were potentially the most effective therapeutic drugs for the treatment of S. suis infection in both pigs and humans. High prevalence of intermediate susceptibility of S. suis isolated from asymptomatic pigs for penicillin G, gentamicin, enrofloxacin, and norfloxacin could be the premise of the emergence of S. suis antibiotic resistance. Resistance was also found in S. suis strains isolated from asymptomatic pigs indicating that they could act as reservoirs of antibiotic resistance genes.

Conclusions

To the best of our knowledge, this is the first report on antimicrobial resistance of a large collection of S. suis strains isolated from pigs and humans in Thailand. It revealed the multidrug resistance of S. suis strains in pigs and humans. The information gained from this study raises an awareness and encourage best practices of appropriate antibiotic drug prescribing and use among human health and agriculture sectors.

Electronic supplementary material

The online version of this article (10.1186/s12917-018-1732-5) contains supplementary material, which is available to authorized users.

Combination Therapy Strategies Against Multiple-Resistant Streptococcus Suis

Streptococcus suis is a major swine pathogen, an emerging zoonotic agent responsible for meningitis, endocarditis and septicaemia followed by deafness in humans. The development of antimicrobial resistance in S. suis increases the risk for therapeutic failure in both animals and humans. In this study, we report the synergism of combination therapy against multi-resistant S. suis isolates from swine. Twelve antibiotic profiles were determined against 11 S. suis strains. To investigate their synergistic/antagonistic activity, checkerboard assay was performed for all the possible combinations. In-vitro killing curves and in-vivo treatment trials were used to confirm the synergistic activity of special combinations against S. suis dominant clones. In this study, 11 S. suis isolates were highly resistant to erythromycin, clindamycin, trimethoprim/sulfamethoxazole, and tetracycline with ratios of 80–100%, and the resistance percentages to enrofloxacin, florfenicol, and spectinomycin were ~50%. The checkerboard data identified two combination regimens, ampicillin plus apramycin and tiamulin plus spectinomycin which gave the greatest level of synergism against the S. suis strains. In-vitro kill-curves showed a bacterial reduction of over 3-logCFU with the use of combination treatments, whilst the application of mono-therapies achieve less than a 2-logCFU cell killing. In-vivo models confirm that administration of these two combinations significantly reduced the number of bacterial cells after 24 h of treatment. In conclusions, the combinations of ampicillin plus apramycin and tiamulin plus spectinomycin showed the greatest synergism and may be potential strategies for treatment of multi-resistant S. suis in animal.

Effects of Environmental and Management-Associated Factors on Prevalence and Diversity of Streptococcus suis in Clinically Healthy Pig Herds in China and the United Kingdom

Streptococcus suis, a global zoonosis of pigs, shows regional differences in the prevalence of human-associated disease for Asian and non-Asian countries. The isolation rates and diversities of S. suis on tonsils of healthy slaughter pigs in China and the United Kingdom were studied for effects of geography, temperature, pig age, and farm type. Isolates underwent analysis of molecular serotype and multilocus sequence type and virulence-associated genotyping. Although we found no significant difference in positive isolation rates between Chinese and UK farms, the prevalences of serotypes previously associated with human disease were significantly greater in the Chinese collection (P = 0.003). A significant effect of temperature was found on the positive isolation rate of the Chinese samples and the prevalence of human disease-associated serotypes in the UK S. suis population (China, P = 0.004; United Kingdom, P = 0.024) and on the prevalence of isolates carrying key virulence genes in China (P = 0.044). Finally, we found marked diversity among S. suis isolates, with statistically significant temperature effects on detection of multiple strain types within individual pigs. This study highlighted the high carriage prevalence and diversity of S. suis among clinically healthy pig herds of China and the United Kingdom. The significant effect of temperature on prevalence of isolation, human disease-associated serotypes, and diversity carried by individual pigs may shed new light on geographic variations in human S. suis-associated disease.IMPORTANCEStreptococcus suis is a global zoonotic pathogen and also a normal colonizer mainly carried on the tonsil of pigs. Thus, it is important to study the effect of environmental and management-associated factors on the S. suis populations in clinically healthy pigs. In this research, we investigated the similarities and differences between the S. suis populations obtained from different pig ages, seasons, and farm management systems and discovered the relationship between high climatic temperature and the prevalence of S. suis.

Use of Antibiotics and Antimicrobial Resistance in Veterinary Medicine as Exemplified by the Swine Pathogen Streptococcus suis

Use of antimicrobial agents in veterinary medicine is essential to control infectious diseases, thereby keeping animals healthy and animal products safe for the consumer. On the other hand, development and spread of antimicrobial resistance is of major concern for public health. Streptococcus (S.) suis reflects a typical bacterial pathogen in modern swine production due to its facultative pathogenic nature and wide spread in the pig population. Thus, in the present review we focus on certain current aspects and problems related to antimicrobial use and resistance in S. suis as a paradigm for a bacterial pathogen affecting swine husbandry worldwide. The review includes (i) general aspects of antimicrobial use and resistance in veterinary medicine with emphasis on swine, (ii) genetic resistance mechanisms of S. suis known to contribute to bacterial survival under antibiotic selection pressure, and (iii) possible other factors which may contribute to problems in antimicrobial therapy of S. suis infections, such as bacterial persister cell formation, biofilm production, and co-infections. The latter shows that we hardly understand the complexity of factors affecting the success of antimicrobial treatment of (porcine) infectious diseases and underlines the need for further research in this field.