Quantitative susceptibility of Streptococcus suis strains isolated from diseased pigs in seven European countries to antimicrobial agents licenced in veterinary medicine

Susceptibility of Actinobacillus pleuropneumoniae, Pasteurella multocida and Streptococcus suis Isolated from Pigs in Hungary between 2018 and 2021

Porcine respiratory disease complex (PRDC) has been a major animal health, welfare, and economic problem in Hungary; therefore, great emphasis should be put on both the prevention and control of this complex disease. As antibacterial agents are effective tools for control, antibiotic susceptibility testing is indispensable for the proper implementation of antibacterial therapy and to prevent the spread of resistance. The best method for this is to determine the minimum inhibitory concentration (MIC) by the broth microdilution method. In our study, we measured the MIC values of 164 Actinobacillus pleuropneumoniae, 65 Pasteurella multocida, and 118 Streptococcus suis isolates isolated from clinical cases against the following antibacterial agents: amoxicillin, ceftiofur, cefquinome, oxytetracycline, doxycycline, tylosin, tilmicosin, tylvalosin, tulathromycin, lincomycin, tiamulin, florfenicol, colistin, enrofloxacin, and sulfamethoxazole-trimethoprim. Outstanding efficacy against A. pleuropneumoniae isolates was observed with ceftiofur (100%) and tulathromycin (100%), while high levels of resistance were observed against cefquinome (92.7%) and sulfamethoxazole-trimethoprim (90.8%). Ceftiofur (98.4%), enrofloxacin (100%), florfenicol (100%), and tulathromycin (100%) were found to be highly effective against P. multocida isolates, while 100% resistance was detected against the sulfamethoxazole-trimethoprim combination. For the S. suis isolates, only ceftiofur (100%) was not found to be resistant, while the highest rate of resistance was observed against the sulfamethoxazole-trimethoprim combination (94.3%). An increasing number of studies report multi-resistant strains of all three pathogens, making their monitoring a high priority for animal and public health.

Antimicrobial susceptibility among respiratory tract pathogens isolated from diseased cattle and pigs from different parts of Europe

AIMS

To survey antibiotic susceptibility of bacteria causing cattle and pig respiratory infections in 10 European countries.

METHODS AND RESULTS

Non-replicate nasopharyngeal/nasal or lung swabs were collected from animals with acute respiratory signs during 2015-2016. Pasteurella multocida, Mannheimia haemolytica, Histophilus somni from cattle (n = 281), and P. multocida, Actinobacillus pleuropneumoniae, Glaesserella parasuis, Bordetella bronchiseptica, and Streptococcus suis from pigs (n = 593) were isolated. MICs were assessed following CLSI standards and interpreted using veterinary breakpoints where available. Histophilus somni isolates were fully antibiotic susceptible. Bovine P. multocida and M. haemolytica were susceptible to all antibiotics, except tetracycline (11.6%-17.6% resistance). Low macrolide and spectinomycin resistance was observed for P. multocida and M. haemolytica (1.3%-8.8%). Similar susceptibility was observed in pigs, where breakpoints are available. Resistance in P. multocida, A. pleuropneumoniae, and S. suis to ceftiofur, enrofloxacin, and florfenicol was absent or <5%. Tetracycline resistance varied from 10.6% to 21.3%, but was 82.4% in S. suis. Overall multidrug-resistance was low. Antibiotic resistance in 2015-2016 remained similar as in 2009-2012.

CONCLUSIONS

Low antibiotic resistance was observed among respiratory tract pathogens, except for tetracycline.

Characterization of integrative and conjugative elements carrying antibiotic resistance genes of Streptococcus suis isolated in China

Streptococcus suis, an emerging zoonotic pathogen, is important reservoirs of antibiotic resistance genes that play critical roles in the horizontal transfer of corresponding resistances. In the present study, 656 antibiotic resistance (AR) genes were detected in 154 of 155 genomes of S. suis strains isolated from the nasopharynx of slaughtered pigs and the lungs of diseased pigs in China. The AR genes were clustered into 11 categories, consisting of tetracycline, macrolides, lincosamide, streptogramin, aminoglycoside, trimethoprim, amphenicols, nucleoside, quinupristin/dalfopristin, glycopeptide, and oxazolidinones resistance genes. In order to investigate the transmission patterns of the AR genes, AR genes-associated the mobile genetic elements (MGEs) were extracted and investigated. Twenty ICEs, one defective ICE, one tandem ICE, and ten prophages were found, which mainly carried tetracycline, macrolides/lincosamides/streptogramin (MLS), and aminoglycosides resistance genes. Three types of DNA cargo with AR genes were integrated into specific sites of ICEs: integrative mobilizable elements (IMEs), cis-IMEs (CIMEs), and transposon Tn916. Obvious differences in AR gene categories were found among the three cargo types. IMEs mainly harbored tetracycline and MLS resistance genes. CIMEs mainly carried aminoglycoside resistance genes, while transposon Tn916 carried only the tet (M) gene. Nearly all AR genes in ICEs were carried by IMEs and CIMEs. IMEs were prevalent and were also detected in additional 29 S. suis genomes. The horizontal transfer of IMEs and CIMEs may play critical role in ICE evolution and AR gene transmission in the S. suis population. Our findings provide novel insights into the transmission patterns of AR genes and the evolutionary mechanisms of ICEs in S. suis.

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.

Resistance of Streptococcus suis Isolates from the Czech Republic during 2018-2022

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.

Virulence gene profile and antimicrobial resistance of non-typeable Streptococcus suis isolated from clinically healthy and diseased pigs from North East India

The present study was conducted to investigate the virulence gene profile and antimicrobial resistance of non-typeable Streptococcus suis isolates circulating in pigs of North East India. Fifty-two non-typeable S. suis isolates from clinically healthy and diseased pigs were screened by using PCR for the presence of the muramidase-released protein (mrp), extracellular factor (epf), hemolysin suilysin (sly), arginine deiminase (arcA), and glutamate dehydrogenase (gdh) genes. Five different virulence gene profiles were observed and the most predominant virulence gene profile found in healthy pigs was mrp^- + sly^- + arcA^- + gdh ⁺ + epf^- whereas the most predominant virulence gene profile recorded in diseased pigs was mrp⁺ + sly^- + arcA⁺ + gdh⁺ + epf^-. Significantly lower carrier rate of mrp⁺ + sly^- + arcA⁺ + gdh⁺ + epf^- virulence gene profile was observed among the isolates from healthy pigs compared to those from diseased pigs (P < 0.05). Antimicrobial resistance patterns of the S. suis isolates revealed fourteen resistance groups (R1 to R14) where 88.46% isolates showed multi-drug resistance. The most predominant resistance pattern observed was CD-COT-E-TE. This is perhaps the first study reporting virulence gene profile and antimicrobial resistance of non-typeable S. suis isolates from pigs in North East India. The occurrence of relatively high levels of resistance of S. suis to some antimicrobials (e.g. macrolides, tetracyclines, and sulphonamides) as observed in the present study may represent a human health concern.

A novel lysin Ply1228 provides efficient protection against Streptococcus suis type 2 infection in a murine bacteremia model

Streptococcus suis is an important zoonotic pathogen that is difficult to control with antibiotics due to the widespread development of multidrug-resistant strains. Phage lysin is considered a potential therapeutic agent to combat S. suis. In this study, the novel lysin Ply1228 derived from the prophage of S. suis type 12 was identified. Bioinformatics analysis showed that Ply1228 contains a CHAP catalytic domain, which is a binding domain composed of a CW-7 binding motif and an amidase-2 catalytic domain. The CHAP catalytic domain is essential for the bactericidal function of lysin Ply1228 and does not depend on the presence of Ca2+. C34 and H99 of the CHAP domain were identified as the key active sites. The CW-7 binding motif plays a key binding role in Ply1228. Ply1228 can specifically lyse S. suis, including types 2, 3, 7, 9, 10, 12, 14, and 27. Within 10 min, Ply1228 killed 4 log of the S. suis population, which had a starting concentration of approximately 107 CFU/mL. In addition, Ply1228 showed favourable thermal and pH stability. The therapeutic effect of Ply1228 was further investigated in a mouse model of S. suis bacteremia. The administration of the lysin Ply1228 (200 μg/mouse) 1 h after the intraperitoneal injection of 2 × MLD of SS2 strain SC225 was sufficient to protect the mice (P < 0.0001) and significantly reduced the bacterial loads in the blood and organs (livers, spleens, lungs and kidneys). The levels of inflammation and histopathological damage in infected mice were effectively relieved after the Ply1228 treatment. These results indicate that Ply1228 might represent a new enzybiotic candidate for S. suis infection.

Preparation of Aditoprim Injection against Streptococcus suis in Pigs and a Dose Regimen Based on Pharmacokinetic-Pharmacodynamic Modeling

In order to effectively treat the infection of Streptococcus suis and reduce the emergence of drug-resistant bacteria, an aditoprim (ADP) injection was developed in this study. The pharmaceutical property investigation results demonstrated that ADP injection was a clear yellow liquid with 10 g ADP distributing in every 100 mL solution uniformly. Its pH value and drug content were around 6.20 and 99.35~100.40%, respectively. And quality assessment preliminarily indicated its reliable quality and stability. Additionally, the bronchoalveolar lavage fluid method was first applied to evaluate accurate ADP concentration at infection site in this study. Through pharmacodynamic assay, the MIC, MBC and MPC of ADP against Streptococcus suis CVCC 607 was 2 μg/mL, 4 μg/mL and 12.8 μg/mL, respectively. The bacteria growth inhibition curves showed that ADP was a concentration-dependent antibacterial drug, and the PK-PD model parameter of AUC/MIC was selected. The pharmacokinetic parameters of alveolar fluid evaluated by WinNonlin software revealed similar pharmacokinetic process of ADP in healthy pigs and infected pigs. Combined with pharmacokinetics-pharmacodynamics (PK-PD) modeling, the dosage regimen of 3~5 days with an interval of 12 h at 4.10 mg/kg or 5.91 mg/kg could be adopted to treat the infection of Streptococcus suis. Consequently, this ADP injection with a multi-dose protocol would be a promising antimicrobial product for efficient treatment of S. suis infection of pigs.

Genomic and pathogenic investigations of Streptococcus suis serotype 7 population derived from a human patient and pigs

Streptococcus suis is one of the important emerging zoonotic pathogens. Serotype 2 is most prevalent in patients worldwide. In the present study, we first isolated one S. suis serotype 7 strain GX69 from the blood culture of a patient with septicemia complicated with pneumonia in China. In order to deepen the understanding of S. suis serotype 7 population characteristics, we investigated the phylogenetic structure, genomic features, and virulence of S. suis serotype 7 population, including 35 strains and 79 genomes. Significant diversities were revealed in S. suis serotype 7 population, which were clustered into 22 sequence types (STs), five minimum core genome (MCG) groups, and six lineages. Lineages 1, 3a, and 6 were mainly constituted by genomes from Asia. Genomes of Lineages 2, 3b, and 5a were mainly from Northern America. Most of genomes from Europe (41/48) were clustered into Lineage 5b. In addition to strain GX69, 13 of 21 S. suis serotype 7 representative strains were classified as virulent strains using the C57BL/6 mouse model. Virulence-associated genes preferentially present in highly pathogenic S. suis serotype 2 strains were not suitable as virulence indicators for S. suis serotype 7 strains. Integrative mobilizable elements were widespread and may play a critical role in disseminating antibiotic resistance genes of S. suis serotype 7 strains. Our study confirmed S. suis serotype 7 is a non-negligible pathotype and deepened the understanding of the population structure of S. suis serotype 7, which provided valuable information for the improved surveillance of this serotype.

Serotypes, Virulence-Associated Factors, and Antimicrobial Resistance of Streptococcus suis Isolates Recovered From Sick and Healthy Pigs Determined by Whole-Genome Sequencing

Streptococcus suis is ubiquitous in swine, and yet, only a small percentage of pigs become clinically ill. The objective of this study was to describe the distribution of serotypes, virulence-associated factor (VAF), and antimicrobial resistance (AMR) genes in S. suis isolates recovered from systemic (blood, meninges, spleen, and lymph node) and non-systemic (tonsil, nasal cavities, ileum, and rectum) sites of sick and healthy pigs using whole-genome sequencing. In total, 273 S. suis isolates recovered from 112 pigs (47 isolates from systemic and 136 from non-systemic sites of 65 sick pigs; 90 isolates from non-systemic sites of 47 healthy pigs) on 17 Ontario farms were subjected to whole-genome sequencing. Using in silico typing, 21 serotypes were identified with serotypes 9 (13.9%) and 2 (8.4%) as the most frequent serotypes, whereas 53 (19.4%) isolates remained untypable. The relative frequency of VAF genes in isolates from systemic (Kruskal–Wallis, p < 0.001) and non-systemic (Kruskal–Wallis, p < 0.001) sites in sick pigs was higher compared with isolates from non-systemic sites in healthy pigs. Although many VAF genes were abundant in all isolates, three genes, including dltA [Fisher's test (FT), p < 0.001], luxS (FT, p = 0.01), and troA (FT, p = 0.02), were more prevalent in isolates recovered from systemic sites compared with non-systemic sites of pigs. Among the isolates, 98% had at least one AMR gene, and 79% had genes associated with at least four drug classes. The most frequently detected AMR genes were tetO conferring resistance to tetracycline and ermB conferring resistance to macrolide, lincosamide, and streptogramin. The wide distribution of VAFs genes in S. suis isolates in this study suggests that other host and environmental factors may contribute to S. suis disease development.

Disposition of Cefquinome in Turkeys (Meleagris gallopavo) Following Intravenous and Intramuscular Administration

The bioavailability and pharmacokinetics in turkeys of cefquinome (CFQ), a broad-spectrum 4th-generation cephalosporin antibiotic, were explored after a single injection of 2 mg/kg body weight by intravenous (IV) and intramuscular (IM) routes. In a crossover design and 3-weeks washout interval, seven turkeys were assigned for this objective. Blood samples were collected prior to and at various time intervals following each administration. The concentration of CFQ in plasma was measured using HPLC with a UV detector set at 266 nm. For pharmacokinetic analysis, non-compartmental methods have been applied. Following IV administration, the elimination half-life (t_1/2ʎz), distribution volume at steady state (Vd_ss), and total body clearance (Cl_tot) of CFQ were 1.55 h, 0.54 L/kg, and 0.32 L/h/kg, respectively. Following the IM administration, CFQ was speedily absorbed with an absorption half-life (t_1/2ab) of 0.25 h, a maximum plasma concentration (C_max) of 2.71 μg/mL, attained (T_max) at 0.56 h. The bioavailability (F) and in vitro plasma protein binding of CFQ were 95.56% and 11.5%, respectively. Results indicated that CFQ was speedily absorbed with a considerable bioavailability after IM administration. In conclusion, CFQ has a favorable disposition in turkeys that can guide to estimate optimum dosage regimes and eventually lead to its usage to eradicate turkey's susceptible bacterial infections.

Synergistic use of florfenicol and Salvia miltiorrhiza polysaccharide can enhance immune responses in broilers

To explore the effect of florfenicol (FFC) combined with Salvia miltiorrhiza polysaccharide (SMPs) on immune function of Broilers. One hundred and twenty-one-day-old chicks were chosen and divided into 6 groups. The group A received standard basal diet only, the group B received a basal diet with FFC (0.15 g/L diet), and the group C, D, E received a basal diet with FFC (0.15 g/L diet) and SMPs (1.25 g/L, 2.5 g/L, 5 g/L diet)，the group F received a basal diet with SMPs (5 g/L diet). FFC can significantly inhibit the growth performance of broilers, but has no significant damage to the immune function of broilers. The combination of FFC and SMPs can improve the growth performance of broilers, increase the number of leukocyte subtypes in blood (P < 0.05), increase the number of Newcastle disease (ND) and avian influenza (AI) antibodies in blood, the number of immunoglobulins, and the content of cytokines (P < 0.05). In addition, it significantly improve the lymphocyte conversion rate of broiler peripheral blood (P < 0.05). So that, synergistic use of FFC and SMPs can enhance immune responses in Broilers.

Florfenicol induces oxidative stress and hepatocyte apoptosis in broilers via Nrf2 pathway

In order to explore the mechanism of liver injury induced by florfenicol (FFC) in broilers, one hundred and twenty broilers were randomly divided into six groups, twenty broilers in each group. Except for control group, the other five groups were given different doses of FFC (0.15 g/L, 0.3 g/L, 0.6 g/L, 1.2 g/L and 1.8 g/L) in drinking water. After five days of continuous use, blood was collected from the subpterional vein and the chickens' liver were obtained. Chicken weight gain and liver indices were calculated; blood routine analysis was performed; the oxidative stress and apoptosis of hepatocytes was detected. The results showed that compared with the control group, except for 0.15 g/L FFC, the other doses of FFC significantly decreased the weight gain, white blood cell (WBC) and platelet (PLT) contents in blood, 0.3 g/mL FFC and 1.8 g/L FFC significantly reduced the content of hemoglobin (RGB) (P < 0.05); all doses of FFC significant decreased red blood cell (RBC) increased Alanine aminotransferase (ALT) and Aspartate aminotransferase (AST) contents in serum of chickens (P < 0.05), and significantly decreased the contents of albumin (ALB) and total protein (TP) in serum (P < 0.05), but had no significant effect on alkaline phosphatase (ALP) contents(P > 0.05). FFC significantly increased malondialdehyde (MDA) content in serum and liver tissues, but decreased glutathione (GSH), Superoxide dismutase (SOD) and catalase (CAT) content (P < 0.05), and significantly inhibited the mRNA transcription and protein expression of antioxidant proteins nuclear factor-erythroid 2-related factor 2 (Nrf2), heme oxygenase-1 (HO-1) and NAD(P)H dehydrogenase quinone-1 (NQO-1)(P < 0.05). FFC also inhibited the content and the transcription level of cytochrome P4501A1(CYP1A1) and CYP2H1 in liver (P < 0.05). At the same time, FFC significantly promoted the apoptotic rate of hepatocytes and the mRNA transcription and protein expression of caspase-3 and caspase-6 (P < 0.05). With the increase of FFC concentration, liver injury became more and more serious, which affected liver function in chickens by inhibiting enzyme activity in Nrf2-ARE pathway to increase oxidative stress and promoting apoptotic protein expression to accelerate hepatocyte apoptosis.

Bactericidal Effect of Clove Oil against Multidrug-Resistant Streptococcus suis Isolated from Human Patients and Slaughtered Pigs

Streptococcus suis is a zoonotic pathogen that is currently considered an emerging multidrug-resistant (MDR). Increasing antibiotic resistance can lead to the unsuccessful treatment of S. suis infection. Recently, many investigations of medicinal plants were conducted for the treatment of infection as a result of the increase of antibiotic-resistant bacteria. The aims of this study were to determine the chemical composition of essential oil from Syzygium aromaticum (L.) Merr. & L.M. Perry and the antibacterial activities of clove oil on MDR S. suis. Using gas chromatography coupled to a mass spectrometer, eugenol (97.76%) was found to be the major active ingredient of clove oil. In vitro antibacterial activities of clove oil against MDR S. suis were evaluated. Using the agar disc diffusion test, the clove oil showed a maximum zone of inhibition at 15% (v/v) oil concentration. In a broth microdilution method, the minimum bactericidal concentration of clove oil against all MDR S. suis isolates was 0.1% (v/v). A time-kill analysis was performed, and the killing kinetics of clove oil showed that MDR S. suis was completely reduced after 15 min of exposure to clove oil. In addition, clove oil exhibited a strong antibacterial activity at all pH values applied following incubation of MDR S. suis in pH-adjusted media with clove oil. Moreover, scanning electron microscopy revealed the nonviable S. suis isolates clearly showed atypical form and cell membrane lysis after incubation with clove oil. This study confirms the efficacy of clove oil as a natural antimicrobial against MDR S. suis and suggests the possibility of employing it as a promising alternative product for control of infectious diseases caused by S. suis in animal and human patients.

Prevalence and time trend analysis of antimicrobial resistance in respiratory bacterial pathogens collected from diseased pigs in USA between 2006-2016

Swine respiratory disease complex (SRDC) causes massive economic losses to the swine industry and is a major animal welfare concern. Antimicrobials are mainstay in treatment and control of SRDC. However, there is a lack of data on the prevalence and trends in resistance to antimicrobials in bacterial pathogens associated with SRDC. The objective of this study was to estimate the prevalence and changes in resistance to 13 antimicrobials in swine bacterial pathogens (Streptococcus suis, Pasteurella multocida, Actinobacillus suis and Haemophilus parasuis) in the U.S.A using data collected at University of Minnesota Veterinary Diagnostic Laboratory between 2006 and 2016. For antimicrobials for which breakpoints were available, prevalence of resistance remained below 10% except for tetracycline in S. suis and P. multocida isolates, and these prevalence estimates remained consistently low over the years despite statistical significance (p < .05) in trend analysis. For antimicrobial-bacterial combinations without available breakpoints, the odds of isolates being resistant increased by >10% annually for 7 and 1 antimicrobials in H. parasuis and S. suis isolates respectively, and decreased >10% annually for 4 and 1 antimicrobials in A. suis and H. parasuis isolates, respectively, according to the ordinal regression models. Clinical implications of changes in AMR for A. suis and H. parasuis should be interpreted cautiously due to the lack of interpretive criteria and challenges in antimicrobial susceptibility tests in the case of H. parasuis. Future studies should focus on surveillance of antimicrobial resistance and establishment of standardized susceptibility testing methodologies and interpretive criteria for these animal pathogens of critical importance.

Changes in antimicrobial resistance phenotypes and genotypes in Streptococcus suis strains isolated from pigs in the Tokai area of Japan

Streptococcus suis strains isolated from porcine endocarditis and tonsils in the Tokai area of Japan during 2004-2007 and 2014-2016 (n=114) were tested for antimicrobial susceptibility and distribution of selected resistance genes. No strains showed resistance to penicillin, ampicillin, cefotaxime, meropenem, vancomycin, and levofloxacin. High resistance to tetracycline (80.7%), clindamycin (65.8%), erythromycin (56.1%), and clarithromycin (56.1%) was observed. In chloramphenicol and sulfamethoxazole-trimethoprim, there was a trend towards increased resistance between the first (2004-2007) and second (2014-2016) periods. tet(O) and erm(B) genes were the most frequently detected, and tet(M) and mef(A/E) genes were only detected in strains isolated during 2014-2016. These results indicate that chloramphenicol and sulfamethoxazole-trimethoprim resistance, and tet(M) and mef(A/E) genes emerged in S. suis of this area after 2014.

Antimicrobial Resistance in Streptococcus spp

The genus Streptococcus includes Gram-positive organisms shaped in cocci and organized in chains. They are commensals, pathogens, and opportunistic pathogens for humans and animals. Most Streptococcus species of veterinary relevance have a specific ecological niche, such as S. uberis, which is almost exclusively an environmental pathogen causing bovine mastitis. In contrast, S. suis can be considered as a true zoonotic pathogen, causing specific diseases in humans after contact with infected animals or derived food products. Finally, Streptococcus species such as S. agalactiae can be sporadically zoonotic, even though they are pathogens of both humans and animals independently. For clarification, a short taxonomical overview will be given here to highlight the diversity of streptococci that infect animals. Several families of antibiotics are used to treat animals for streptococcal infections. First-line treatments are penicillins (alone or in combination with aminoglycosides), macrolides and lincosamides, fluoroquinolones, and tetracyclines. Because of the selecting role of antibiotics, resistance phenotypes have been reported in streptococci isolated from animals worldwide. Globally, the dynamic of resistance acquisition in streptococci is slower than what is experienced in Enterobacteriaceae, probably due to the much more limited horizontal spread of resistance genes. Nonetheless, transposons or integrative and conjugative elements can disseminate resistance determinants among streptococci. Besides providing key elements on the prevalence of resistance in streptococci from animals, this article will also largely consider the mechanisms and molecular epidemiology of the major types of resistance to antimicrobials encountered in the most important streptococcal species in veterinary medicine.

Relevance of Breast Cancer Resistance Protein to Pharmacokinetics of Florfenicol in Chickens: A Perspective from In Vivo and In Vitro Studies

Florfenicol (FFC) is a valuable synthetic fluorinated derivative of thiamphenicol widely used to treat infectious diseases in food animals. The aims of the study were to investigate whether FFC is a substrate for the breast cancer resistance protein (BCRP) and whether the transporter influences oral availability of FFC. In vitro transport assays using MDCK-chAbcg2 cells were conducted to assess chicken BCRP-mediated transport of FFC, while in vivo pharmacokinetic experiments with single or combined BCRP inhibitor gefitinib were employed to study the role of BCRP in oral FFC disposition. According to U.S. Food and Drug Administration (FDA) criteria, FFC was found to be a potential BCRP substrate due to the net efflux ratio being over 2.0 (2.37) in MDCK cells stably transfected with chicken BCRP and the efflux completely reversed by a BCRP inhibitor (Gefitinib). The molecular docking results indicated that florfenicol can form favorable interactions with the binding pocket of homology modeled chicken BCRP. Pharmacokinetic studies of FFC in different aged broilers with different expression levels of BCRP showed that higher BCRP expression would cause a lower Area Under Curve (AUC) and a higher clearance of FFC. In addition, more extensive absorption of florfenicol after the co-administration with gefitinib (a BCRP inhibitor) was observed. The overall results demonstrated that florfenicol is a substrate of the chicken breast cancer resistant protein which in turn affects its pharmacokinetic behavior.

Examination of Australian Streptococcus suis isolates from clinically affected pigs in a global context and the genomic characterisation of ST1 as a predictor of virulence

Streptococcus suis is a major zoonotic pathogen that causes severe disease in both humans and pigs. Australia's pig herd has been quarantined for over 30 years, however S. suis remains a significant cause of disease. In this study, we investigated S. suis from 148 cases of clinical disease in pigs from 46 pig herds over a period of seven years, to determine the level of genetic difference from international isolates that may have arisen over the 30 years of separation. Isolates underwent whole genome sequencing, genome analysis and antimicrobial susceptibility testing. Data was compared at the core genome level to clinical isolates from overseas. Results demonstrated five predominant multi-locus sequence types and two major cps gene types (cps2 and 3). At the core genome level Australian isolates clustered predominantly within one large clade consisting of isolates from the UK, Canada and North America. A small proportion of Australian swine isolates (5%) were phylogenetically associated with south-east Asian and UK isolates, many of which were classified as causing systemic disease, and derived from cases of human and swine disease. Based on this dataset we provide a comprehensive outline of the current S. suis clones associated with disease in Australian pigs and their global context, with the main finding being that, despite three decades of separation, Australian S. suis are genomically similar to overseas strains. In addition, we show that ST1 clones carry a constellation of putative virulence genes not present in other Australian STs.

The Two-Component Signaling System VraSRss Is Critical for Multidrug Resistance and Full Virulence in Streptococcus suis Serotype 2

Streptococcus suis has received increasing attention for its involvement in severe human infections worldwide as well as in multidrug resistance. Two-component signaling systems (TCSSs) play important roles in bacterial adaptation to various environmental stimuli. In this study, we identified a novel TCSS located in S. suis serotype 2 (SS2), designated VraSR_SS, which is involved in bacterial pathogenicity and susceptibility to antimicrobials. Our data demonstrated that the yvqF_SS gene, located upstream of vraSR_SS , shared the same promoter with the TCSS genes, which was directly regulated by VraSR_SS, as shown in electrophoretic mobility shift assays. Notably, YvqF_SS and VraSR_SS constitute a novel multidrug resistance module of SS2 that participates in resistance to certain groups of antimicrobials. Further analyses showed that VraSR_SS inactivation significantly attenuated bacterial virulence in animal models, which, coupled with the significant activation of VraSR_SS expression observed in host blood, strongly suggested that VraSR_SS is an important regulator of SS2 pathogenicity. Indeed, RNA-sequencing analyses identified 106 genes that were differentially expressed between the wild-type and ΔvraSR_SS strains, including genes involved in capsular polysaccharide (CPS) biosynthesis. Subsequent studies confirmed that VraSR_SS indirectly regulated the transcription of CPS gene clusters and, thus, controlled the CPS thickness shown by transmission electron microscopy. Decreased CPS biosynthesis caused by vraSR_SS deletion subsequently increased bacterial adhesion to epithelial cells and attenuated antiphagocytosis against macrophages, which partially clarified the pathogenic mechanism mediated by VraSR_SS Taken together, our data suggest that the novel TCSS, VraSR_SS, plays critical roles for multidrug resistance and full virulence in SS2.

Genomic comparisons of Streptococcus suis serotype 9 strains recovered from diseased pigs in Spain and Canada

Streptococcus suis is one of the most important bacterial pathogens in the porcine industry and also a zoonotic agent. Serotype 9 is becoming one of the most prevalent serotypes within the S. suis population in certain European countries. In the present study, serotype 9 strains isolated from a country where infection due to this serotype is endemic (Spain), were compared to those recovered from Canada, where this serotype is rarely isolated from diseased pigs. For comparison purposes, strains from Brazil and the only strain isolated from a human case, in Thailand, were also incorporated. Firstly, sequence types (STs) were obtained followed by detection of putative virulence factors. Phylogenetic trees were constructed using the non-recombinant single nucleotide polymorphisms from core genomes of tested strains. Most Spanish strains were either ST123 or ST125, whereas Canadian strains were highly heterogeneous. However, the distribution of putative virulence factors was similar in both groups of strains. The fact that ST16 strains harbored more putative virulence genes and shared greater similarity with the genome of human serotype 2 strains suggests that they present a higher zoonotic and virulence potential than those from Canada and Spain. More than 80% of the strains included in this study carried genes associated with resistance to tetracycline, lincosamides and macrolides. Serotype 9 strains may be nearly 400 years old and have evolved in parallel into 2 lineages. The rapid population expansion of dominant lineage 1 occurred within the last 40 years probably due to the rapid development of the porcine industry.

Patterns of antimicrobial resistance in Streptococcus suis isolates from pigs with or without streptococcal disease in England between 2009 and 2014

Antimicrobial resistance in Streptococcus suis, a global zoonotic pathogen of pigs, has been mostly studied only in diseased animals using surveys that have not evaluated changes over time. We compared patterns of resistance between S. suis isolates from clinical cases of disease (CC) and non-clinical case (NCC) pigs in England, collected over two discrete periods, 2009-2011 and 2013-2014. Minimum inhibitory concentrations (MIC) of 17 antimicrobials (nine classes) were determined on 405 S. suis isolates categorised by sampling period and disease association to assess changes in resistance over time and association with disease. First, isolates were characterized as resistant or susceptible using published clinical breakpoints. Second, epidemiological cut-offs (ECOFF) were derived from MIC values, and isolates classified as wild type (WT) below the ECOFF and non-wild type (NWT) above the ECOFF. Finally, isolate subsets were analysed for shifts in MIC distribution. NCC isolates were more resistant than CC isolates to cephalosporins, penams, pleuromutilins, potentiated sulphonamides and tetracyclines in both study periods. Resistance levels among CC isolates increased in 2013-2014 relative to 2009-2011 for antimicrobials including aminoglycosides, cephalosporins, fluoroquinolones, pleuromutilins, potentiated sulphonamides and tetracyclines. The prevalence of isolates categorised as NWT for five or more classes of antimicrobials was greater among NCC than CC isolates for both time periods, and increased with time. This study used standardised methods to identify significant shifts in antimicrobial resistance phenotypes of S. suis isolated from pigs in England, not only over time but also between isolates from known clinical cases or disease-free pigs.

Prevalence of multiple drug resistant Streptococcus suis in and around Guwahati, India

Aim:

This study was conducted to determine the prevalence and antimicrobial susceptibility of Streptococcus suis and their resistance patterns isolated from both clinically healthy carriers and diseased pigs in and around Guwahati, Assam, India.

Materials and Methods:

A total of 497 samples were collected during October, 2012, to April, 2014, from clinically healthy (n=67) and diseased (n=230) pigs of varying age and either sex maintained under organized and unorganized farming systems. Samples were processed for isolation and identification of S. suis by biochemical characterization and polymerase chain reaction targeting the housekeeping gene glutamate dehydrogenase. In vitro antimicrobial susceptibility of the recovered isolates against nine antibiotic groups comprising 17 antimicrobial agents was studied by standard method.

Results:

Of the 497 samples examined, 7 (1.41%) isolates were confirmed to be S. suis of which 5 (1.87%) and 2 (0.87%) were derived from clinically healthy and diseased pigs, respectively. All the isolates were susceptible to gentamicin, amikacin, and erythromycin (100%) followed by the penicillin group and enrofloxacin (85.71%), ceftriaxone, doxycycline HCL, ofloxacin and chloramphenicol (71.43%), to kanamycin, clindamycin and co-trimoxazole (42.85%). The isolates showed least susceptibility to cefalexin, tetracycline and streptomycin (28.57%). All the five S. suis isolates from clinically healthy pigs were susceptible to penicillin G, amoxyclav, doxycycline HCl, gentamicin, amikacin and erythromycin, 80.00% isolates susceptible to ampicillin, enrofloxacin and ofloxacin, 60.00% to ceftriaxone, kanamycin and chloramphenicol, 40% to cefalexin, tetracycline, clindamycin and co-trimoxazole, respectively. Only 20.00% isolates were susceptible to streptomycin. Both the isolates recovered from diseased pigs were susceptible to ampicillin, ceftriaxone, gentamicin, amikacin, enrofloxacin, erythromycin, and clindamycin. On the other hand, both the isolates were resistant to cefalexin, tetracycline, doxycycline HCL, and kanamycin. Altogether five different resistance patterns (multi-drug resistance) were observed. Of the seven S. suis isolates, two isolates were susceptible to all the 17 antimicrobial agents, one isolate was resistant to four antimicrobial agents, two isolates to seven agents, one isolate to nine agents, and one isolate exhibited resistance to 14 antimicrobial agents.

Conclusion:

This study was conducted to determine the prevalence of S. suis in clinically healthy and diseased pigs and their antimicrobial susceptibility patterns. All the isolates were susceptible to gentamicin, amikacin and erythromycin, and most of them were resistant to cefalexin, tetracycline and streptomycin. Five different patterns of antimicrobial resistance (multi-drug resistance) were observed.

Characterization of Spectinomycin Resistance in Streptococcus suis Leads to Two Novel Insights into Drug Resistance Formation and Dissemination Mechanism

Spectinomycin is an aminocyclitol antibiotic used clinically to treat a variety of infections in animals. Here, we characterized drug resistance prevalence in clinical Streptococcus suis isolates and discovered a novel resistance mechanism in which the s5 mutation (Gly26Asp) results in high spectinomycin resistance. Additionally, a novel integrative and conjugative element encompassing a multidrug resistance spw_like-aadE-lnu(B)-lsa(E) cluster and a cadmium resistance operon were identified, suggesting a possible cause for the wide dissemination of spectinomycin resistance in S. suis.

Monitoring of antimicrobial susceptibility of Streptococcus suis in the Netherlands, 2013-2015

The objective of the present study was to analyse the in vitro antimicrobial susceptibility of Streptococcus suis isolates from post-mortem samples from pigs in the Netherlands. S. suis isolates originated from diagnostic submissions of pigs sent to the Pathology Department of GD Animal Health, from April 2013 till June 2015. Minimal inhibitory concentrations (MICs) of in total 15 antimicrobials were assessed by broth microdilution following CLSI recommendations. MIC₅₀ and MIC₉₀ values were determined and MICs were interpreted as susceptible, intermediate and resistant using CLSI veterinary breakpoints (when available). Emergence of resistance among S. suis (n=1163) derived from clinical submissions of pigs appeared to be limited. Resistance to ampicillin, ceftiofur, clindamycin, enrofloxacin, florfenicol, penicillin, trimethoprim/sulfamethoxazole and tetracycline was 0.3%, 0.5%, 48.1%, 0.6%, 0.1%, 0.5%, 3.0%, and 78.4%, respectively. Cross-resistance between penicillin and ampicillin appeared to be incomplete. MIC values of erythromycin, clindamycin, neomycin, penicillin and tilmicosin for isolates originating from grower/finisher pigs were significantly more often lower than the MIC values of isolates from suckling/weaned piglets. It has to be kept in mind that these results represent only part of the Dutch pig population and it can be discussed whether this is a representative sample. Interpretation of the MIC results of (clinically relevant) antimicrobials tested for treatment of S. suis infection is strongly hampered by the lack of CLSI-defined veterinary clinical breakpoints that are animal species- and body site-specific. Therefore, and to conduct a clinically reliable monitoring of antimicrobial susceptibility of veterinary pathogens, more species- and organ-specific veterinary breakpoints are urgently needed.

Population Structure and Antimicrobial Resistance Profiles of Streptococcus suis Serotype 2 Sequence Type 25 Strains

Strains of serotype 2 Streptococcus suis are responsible for swine and human infections. Different serotype 2 genetic backgrounds have been defined using multilocus sequence typing (MLST). However, little is known about the genetic diversity within each MLST sequence type (ST). Here, we used whole-genome sequencing to test the hypothesis that S. suis serotype 2 strains of the ST25 lineage are genetically heterogeneous. We evaluated 51 serotype 2 ST25 S. suis strains isolated from diseased pigs and humans in Canada, the United States of America, and Thailand. Whole-genome sequencing revealed numerous large-scale rearrangements in the ST25 genome, compared to the genomes of ST1 and ST28 S. suis strains, which result, among other changes, in disruption of a pilus island locus. We report that recombination and lateral gene transfer contribute to ST25 genetic diversity. Phylogenetic analysis identified two main and distinct Thai and North American clades grouping most strains investigated. These clades also possessed distinct patterns of antimicrobial resistance genes, which correlated with acquisition of different integrative and conjugative elements (ICEs). Some of these ICEs were found to be integrated at a recombination hot spot, previously identified as the site of integration of the 89K pathogenicity island in serotype 2 ST7 S. suis strains. Our results highlight the limitations of MLST for phylogenetic analysis of S. suis, and the importance of lateral gene transfer and recombination as drivers of diversity in this swine pathogen and zoonotic agent.

In Vivo Pharmacodynamics of Cefquinome in a Neutropenic Mouse Thigh Model of Streptococcus suis Serotype 2 at Varied Initial Inoculum Sizes

Streptococcus suis serotype 2 is an emerging zoonotic pathogen and causes severe disease in both pigs and human beings. Cefquinome (CEQ), a fourth-generation cephalosporin, exhibits broad-spectrum activity against Gram-positive bacteria such as S. suis. This study evaluated the in vitro and in vivo antimicrobial activities of CEQ against four strains of S. suis serotype 2 in a murine neutropenic thigh infection model. We investigated the effect of varied inoculum sizes (10(6) to 10(8) CFU/thigh) on the pharmacokinetic (PK)/pharmacodynamic (PD) indices and magnitudes of a particular PK/PD index or dose required for efficacy. Dose fractionation studies included total CEQ doses ranging from 0.625 to 640 mg/kg/24 h. Data were analyzed via a maximum effect (Emax) model using nonlinear regression. The PK/PD studies demonstrated that the percentage of time that serum drug levels were above the MIC of free drug (%ƒT>MIC) in a 24-h dosing interval was the primary index driving the efficacy of both inoculum sizes (R(2) = 91% and R(2) = 63%). CEQ doses of 2.5 and 40 mg/kg body weight produced prolonged postantibiotic effects (PAEs) of 2.45 to 8.55 h. Inoculum sizes had a significant influence on CEQ efficacy. Compared to the CEQ exposure and dosages in tests using standard inocula, a 4-fold dose (P = 0.006) and a 2-fold exposure time (P = 0.01) were required for a 1-log kill using large inocula of 10(8) CFU/thigh.

Prophage lysin Ply30 protects mice from Streptococcus suis and Streptococcus equi subsp. zooepidemicus infections

Streptococcus suis and Streptococcus equi subsp. zooepidemicus are capable of infecting humans and various animals, causing significant problems for the worldwide swine industry. As antibiotic resistance has increased, lysosomal enzymes encoded by phages have shown potential for use against pathogenic bacteria. In this study, a novel bacteriophage lysin, Ply30, encoded by the S. suis prophage phi30c, was recombinantly expressed and purified. Ply30 showed high bacteriolysis activity on S. suis and S. equi subsp. zooepidemicus in vitro. The ratio of the optical density at 600 nm (OD600) with treatment versus the OD600 with no treatment for most tested S. suis and S. equi subsp. zooepidemicus strains decreased from 1 to <0.3 and <0.5, respectively, within 1 h. The results of plate viability assays showed that treated bacteria suffered a 1- to 2-log decrease in CFU within 1 h. The optimal concentration of Ply30 was 50 μg/ml, and the optimal pH was 7. Moreover, Ply30 maintained high activity over a wide pH range (pH 6 to 10). The MICs of Ply30 against Streptococcus strains ranged from 16 to 512 μg/ml. In vivo, a 2-mg dose of Ply30 protected 90% (9/10 mice) of mice from infection with S. equi subsp. zooepidemicus and 80% (8/10 mice) of mice from infection with S. suis. Seven days after lysin Ply30 treatment, bacterial loads were significantly decreased in all tested organs and blood compared with those at 1 h postinfection without Ply30 treatment. Ply30 showed in vitro and in vivo antimicrobial efficiency and protected mice against two kinds of bacterial infections, indicating that Ply30 may be an effective therapeutic against streptococci.

Antimicrobial Resistance Profile and Genotypic Characteristics of Streptococcus suis Capsular Type 2 Isolated from Clinical Carrier Sows and Diseased Pigs in China

Streptococcus suis serotype 2 is an important zoonotic pathogen. Antimicrobial resistance phenotypes and genotypic characterizations of S. suis 2 from carrier sows and diseased pigs remain largely unknown. In this study, 96 swine S. suis type 2, 62 from healthy sows and 34 from diseased pigs, were analyzed. High frequency of tetracycline resistance was observed, followed by sulfonamides. The lowest resistance of S. suis 2 for β-lactams supports their use as the primary antibiotics to treat the infection of serotype 2. In contrast, 35 of 37 S. suis 2 with MLS_B phenotypes were isolated from healthy sows, mostly encoded by the ermB and/or the mefA genes. Significantly lower frequency of mrp+/epf+/sly+ was observed among serotype 2 from healthy sows compared to those from diseased pigs. Furthermore, isolates from diseased pigs showed more homogeneously genetic patterns, with most of them clustered in pulsotypes A and E. The data indicate the genetic complexity of S. suis 2 between herds and a close linkage among isolates from healthy sows and diseased pigs. Moreover, many factors, such as extensive use of tetracycline or diffusion of Tn916 with tetM, might have favored for the pathogenicity and widespread dissemination of S. suis serotype 2.

Molecular Basis of Resistance to Selected Antimicrobial Agents in the Emerging Zoonotic Pathogen Streptococcus suis

Characterization of 227 Streptococcus suis strains isolated from pigs during 2010 to 2013 showed high levels of resistance to clindamycin (95.6%), tilmicosin (94.7%), tylosin (93.8%), oxytetracycline (89.4%), chlortetracycline (86.8%), tiamulin (72.7%), neomycin (70.0%), enrofloxacin (56.4%), penicillin (56.4%), ceftiofur (55.9%), and gentamicin (55.1%). Resistance to tetracyclines, macrolides, aminoglycosides, and fluoroquinolone was attributed to the tet gene, erm(B), erm(C), mph(C), and mef(A) and/or mef(E) genes, aph(3')-IIIa and aac(6')-Ie-aph(2″)-Ia genes, and single point mutations in the quinolone resistance-determining region of ParC and GyrA, respectively.

Stability of ceftiofur sodium and cefquinome sulphate in intravenous solutions

Stability of ceftiofur sodium and cefquinome sulphate in intravenous solutions was studied. Chromatographic separation and quantitative determination were performed by using a high-performance liquid chromatography with UV-DAD detection. During the stability study, poly(vinylchloride) minibags were filled with a solution containing 5 mg of ceftiofur sodium or cefquinome sulphate and diluted to 0.2 mg/mL with suitable intravenous solution depending on the test conditions. The solutions for the study were protected from light and stored at room temperature (22°C), refrigerated (6°C), frozen (−20°C) for 30 days, and then thawed at room temperature. A comparison of results obtained at 22°C and 6°C for the same intravenous solutions showed that temperature as well as components of solutions and their concentration had an influence on the stability of ceftiofur sodium and cefquinome sulphate. It was found that ceftiofur sodium and cefquinome sulphate dissolved in intravenous solutions used in this study may be stored at room temperature and at 6°C for up to 48 h.

Clinical resistance and decreased susceptibility in Streptococcus suis isolates from clinically healthy fattening pigs

Streptococcus suis (S. suis) has often been reported as an important swine pathogen and is considered as a new emerging zoonotic agent. Consequently, it is important to be informed on its susceptibility to antimicrobial agents. In the current study, the Minimum Inhibitory Concentration (MIC) population distribution of nine antimicrobial agents has been determined for nasal S. suis strains, isolated from healthy pigs at the end of the fattening period from 50 closed or semiclosed pig herds. The aim of the study was to report resistance based on both clinical breakpoints (clinical resistance percentage) and epidemiological cutoff values (non-wild-type percentage). Non-wild-type percentages were high for tetracycline (98%), lincomycin (92%), tilmicosin (72%), erythromycin (70%), tylosin (66%), and low for florfenicol (0%) and enrofloxacin (0.3%). Clinical resistance percentages were high for tetracycline (95%), erythromycin (66%), tylosin (66%), and low for florfenicol (0.3%) and enrofloxacin (0.3%). For tiamulin, for which no clinical breakpoint is available, 57% of the isolates did not belong to the wild-type population. Clinical resistance and non-wild-type percentages differed substantially for penicillin. Only 1% of the tested S. suis strains was considered as clinically resistant, whereas 47% of the strains showed acquired resistance when epidemiological cutoff values were used. In conclusion, MIC values for penicillin are gradually increasing, compared to previous reports, although pigs infected with strains showing higher MICs may still respond to treatment with penicillin. The high rate of acquired resistance against tiamulin has not been reported before. Results from this study clearly demonstrate that the use of different interpretive criteria contributes to the extent of differences in reported antimicrobial resistance results. The early detection of small changes in the MIC population distribution of isolates, while clinical failure may not yet be observed, provides the opportunity to implement appropriate risk management steps.

Characterization of a Streptococcus suis tet(O/W/32/O)-carrying element transferable to major streptococcal pathogens

Mosaic tetracycline resistance determinants are a recently discovered class of hybrids of ribosomal protection tet genes. They may show different patterns of mosaicism, but their final size has remained unaltered. Initially thought to be confined to a small group of anaerobic bacteria, mosaic tet genes were then found to be widespread. In the genus Streptococcus, a mosaic tet gene [tet(O/W/32/O)] was first discovered in Streptococcus suis, an emerging drug-resistant pig and human pathogen. In this study, we report the molecular characterization of a tet(O/W/32/O) gene-carrying mobile element from an S. suis isolate. tet(O/W/32/O) was detected, in tandem with tet(40), in a circular 14,741-bp genetic element (39.1% G+C; 17 open reading frames [ORFs] identified). The novel element, which we designated 15K, also carried the macrolide resistance determinant erm(B) and an aminoglycoside resistance four-gene cluster including aadE (streptomycin) and aphA (kanamycin). 15K appeared to be an unstable genetic element that, in the absence of recombinases, is capable of undergoing spontaneous excision under standard growth conditions. In the integrated form, 15K was found inside a 54,879-bp integrative and conjugative element (ICE) (50.5% G+C; 55 ORFs), which we designated ICESsu32457. An ∼1.3-kb segment that apparently served as the att site for excision of the unstable 15K element was identified. The novel ICE was transferable at high frequency to recipients from pathogenic Streptococcus species (S. suis, Streptococcus pyogenes, Streptococcus pneumoniae, and Streptococcus agalactiae), suggesting that the multiresistance 15K element can successfully spread within streptococcal populations.

Streptococcus suis, an Emerging Drug-Resistant Animal and Human Pathogen

Streptococcus suis, a major porcine pathogen, has been receiving growing attention not only for its role in severe and increasingly reported infections in humans, but also for its involvement in drug resistance. Recent studies and the analysis of sequenced genomes have been providing important insights into the S. suis resistome, and have resulted in the identification of resistance determinants for tetracyclines, macrolides, aminoglycosides, chloramphenicol, antifolate drugs, streptothricin, and cadmium salts. Resistance gene-carrying genetic elements described so far include integrative and conjugative elements, transposons, genomic islands, phages, and chimeric elements. Some of these elements are similar to those reported in major streptococcal pathogens such as Streptococcus pyogenes, Streptococcus pneumoniae, and Streptococcus agalactiae and share the same chromosomal insertion sites. The available information strongly suggests that S. suis is an important antibiotic resistance reservoir that can contribute to the spread of resistance genes to the above-mentioned streptococci. S. suis is thus a paradigmatic example of possible intersections between animal and human resistomes.

Comparative genomics study of multi-drug-resistance mechanisms in the antibiotic-resistant Streptococcus suis R61 strain

BACKGROUND

Streptococcus suis infections are a serious problem for both humans and pigs worldwide. The emergence and increasing prevalence of antibiotic-resistant S. suis strains pose significant clinical and societal challenges.

RESULTS

In our study, we sequenced one multi-drug-resistant S. suis strain, R61, and one S. suis strain, A7, which is fully sensitive to all tested antibiotics. Comparative genomic analysis revealed that the R61 strain is phylogenetically distinct from other S. suis strains, and the genome of R61 exhibits extreme levels of evolutionary plasticity with high levels of gene gain and loss. Our results indicate that the multi-drug-resistant strain R61 has evolved three main categories of resistance.

CONCLUSIONS

Comparative genomic analysis of S. suis strains with diverse drug-resistant phenotypes provided evidence that horizontal gene transfer is an important evolutionary force in shaping the genome of multi-drug-resistant strain R61. In this study, we discovered novel and previously unexamined mutations that are strong candidates for conferring drug resistance. We believe that these mutations will provide crucial clues for designing new drugs against this pathogen. In addition, our work provides a clear demonstration that the use of drugs has driven the emergence of the multi-drug-resistant strain R61.

TroA of Streptococcus suis is required for manganese acquisition and full virulence

Streptococcus suis causes infections in pigs and occasionally in humans, resulting in manifestations as meningitis, sepsis, arthritis, and septic shock. For survival within the host, S. suis requires numerous nutrients including trace metals. Little is known about the specific proteins involved in metal scavenging in S. suis. In this study we evaluated the role of the putative high-affinity metal binding lipoprotein TroA in metal acquisition and virulence. A mutant strain deficient in the expression of TroA (ΔtroA mutant) was constructed. Growth of the ΔtroA mutant in Todd-Hewitt broth was similar to wild-type growth; however, growth of the ΔtroA mutant in cation-deprived Todd-Hewitt broth and in porcine serum was strongly reduced compared to growth of wild-type bacteria. Supplementing the medium with extra manganese but not with magnesium, zinc, copper, nickel, or iron restored growth to wild-type levels, indicating that TroA is specifically required for growth in environments low in manganese. The ΔtroA mutant also showed increased susceptibility to H2O2, suggesting that TroA is involved in counteracting oxidative stress. Furthermore, the expression of the troA gene was subject to environmental regulation at the transcript level. In a murine S. suis infection model, the ΔtroA mutant displayed a nonvirulent phenotype. These data indicate that S. suis TroA is involved in manganese acquisition and is required for full virulence in mice.

Detection of florfenicol resistance genes in Riemerella anatipestifer isolated from ducks and geese

The cat gene, coding for chloramphenicol acetyltransferase has been reported for conferring the chloramphenicol resistance for Riemerella anatipestifer. Chloramphenicol acetyltransferases, however, are unable to inactivate florfenicol. In this study, 66 R. anatipestifer isolates were investigated for their susceptibility to chloramphenicol and florfenicol and the presence of floR gene. Results showed nine florfenicol intermediate or resistant R. anatipestifer isolates were all floR positive. The expression of floR gene in E. coli and inhibition studies with PAβN indicated that the floR gene was as an efflux pump conferring resistance to both chloramphenicol and florfenicol. Southern hybridization revealed the floR was located in the plasmid DNA of five isolates and in the chromosomal DNA of four isolates. Furthermore, two novel floR-carrying plasmids designated pRA0726 and pRA0846 were sequenced completely. pRA0726 was 11,704 bp in size with 10 putative open reading frames which included the floR, catB and bla(OXA-209) resistance genes. The most differences between sequences of pRA0846 and pRA0726 were the absence of a bla(OXA-209) gene and the deletion of 321 nucleotides of orf1 in pRA0846. Plasmid curing tests demonstrated that pRA0726 carried functional coding proteins for resistance to phenicol and β-lactam antimicrobials. To the best of our knowledge, this is the first report of presence of the floR and bla(OXA-209) resistance genes in R. anatipestifer.

Different genetic elements carrying the tet(W) gene in two human clinical isolates of Streptococcus suis

The genetic support for tet(W), an emerging tetracycline resistance determinant, was studied in two strains of Streptococcus suis, SsCA and SsUD, both isolated in Italy from patients with meningitis. Two completely different tet(W)-carrying genetic elements, sharing only a tet(W)-containing segment barely larger than the gene, were found in the two strains. The one from strain SsCA was nontransferable, and aside from an erm(B)-containing insertion, it closely resembled a genomic island recently described in an S. suis Chinese human isolate in sequence, organization, and chromosomal location. The tet(W)-carrying genetic element from strain SsUD was transferable (at a low frequency) and, though apparently noninducible following mitomycin C treatment, displayed a typical phage organization and was named ΦSsUD.1. Its full sequence was determined (60,711 bp), the highest BLASTN score being Streptococcus pyogenes Φm46.1. ΦSsUD.1 exhibited a unique combination of antibiotic and heavy metal resistance genes. Besides tet(W), it contained a MAS (macrolide-aminoglycoside-streptothricin) fragment with an erm(B) gene having a deleted leader peptide and a cadC/cadA cadmium efflux cassette. The MAS fragment closely resembled the one recently described in pneumococcal transposons Tn6003 and Tn1545. These resistance genes found in the ΦSsUD.1 phage scaffold differed from, but were in the same position as, cargo genes carried by other streptococcal phages. The chromosome integration site of ΦSsUD.1 was at the 3' end of a conserved tRNA uracil methyltransferase (rum) gene. This site, known to be an insertional hot spot for mobile elements in S. pyogenes, might play a similar role in S. suis.

Postantibiotic effects and postantibiotic sub-MIC effects of tilmicosin, erythromycin and tiamulin on erythromycin-resistant Streptococcus suis

The postantibiotic effects (PAEs) and postantibiotic sub-MIC effects (PA SMEs) of tilmicosin, erythromycin and tiamulin on erythromycin-susceptible and erythromycin-resistant strains of Streptococcus suis (M phenotype) were investigated in vitro. Tilmicosin and tiamulin induced significantly longer PAE and PA SME against both erythromycin-susceptible and erythromycin-resistant strains than did erythromycin. The durations of PAE and PA SMEs were proportional to the concentrations of drugs used for exposure. The PA SMEs were substantially longer than PAEs on S. suis (P<0.05) regardless of the antimicrobial used for exposure. The results indicated that the PAE and PA SME could help in the design of efficient control strategies for infection especially caused by erythromycin-resistant S. suis and that they may provide additional valuable information for the rational drug use in clinical practice.

Antimicrobial resistance in zoonotic bacteria: lessons learned from host-specific pathogens

The relative contribution of veterinary and human clinical treatments to the selection of antimicrobial resistance in zoonotic pathogens remains controversial. In this review, we consider bacterial pathogens that differ in host specificity and address their resistance profiles: pathogens that only occur in the human host, pathogens that are specific to particular food-producing animals and pathogens that occur in both host types. Compared with those pathogens restricted to a single animal host, pathogens found in both human and animal hosts appear to have higher incidences of resistance. However, the most urgent and severe resistance problems occur with pathogens exclusively infecting humans. Differences exist in the available genetic repertoire of a bacterial species and these are reflected in the observed resistance patterns; it is important to note that different bacterial species do not automatically result in similarly resistant populations when they undergo comparable selection in different host species. Thus, within a bacterial species, prevalence of resistance can differ between populations isolated from different hosts. For some species, fluctuations in dominant subpopulations, for instance particular serotypes, can be the most important factor determining resistance. The frequently expressed opinion that veterinary use of antimicrobials is at the heart of many resistance problems may be an oversimplification of the complex forces at play.

A steric block in translation caused by the antibiotic spectinomycin

The widely used antibiotic spectinomycin inhibits bacterial protein synthesis by blocking translocation of messenger RNA and transfer RNAs on the ribosome. Here, we show that in crystals of the Escherichia coli 70S ribosome spectinomycin binding traps a distinct swiveling state of the head domain of the small ribosomal subunit. Spectinomycin also alters the rate and completeness of reverse translocation in vitro. These structural and biochemical data indicate that in solution spectinomycin sterically blocks swiveling of the head domain of the small ribosomal subunit and thereby disrupts the translocation cycle.

A proposal of clinical breakpoints for amoxicillin applicable to porcine respiratory tract pathogens

In the present position paper, an attempt was made to establish clinical breakpoints of amoxicillin to classify porcine respiratory tract pathogens as susceptible, intermediate or resistant based on their minimum inhibitory concentrations of amoxicillin. For this, a thorough review of the published literature with regard to swine-specific pharmacological data (including dosages of amoxicillin applied and routes of administration used), clinical efficacy, and in vitro susceptibility of the target pathogens was performed. Based on the comparative analysis of the results, the working group "Antibiotic Resistance" of the German Veterinary Medical Society (DVG) proposed to classify porcine respiratory tract pathogens that show MIC values of amoxicillin of < or =0.5microg/ml as "susceptible", those with MICs of 1microg/ml as "intermediate", and those with MICs of > or =2microg/ml as "resistant".