Inhibition of protein synthesis on the ribosome by tildipirosin compared with other veterinary macrolides

Structural and computational insights into the substrate specificity of acyltransferase domains from modular polyketide synthases

Polyketides are natural products synthesized by polyketide synthases (PKSs), where acyltransferase (AT) domains play a crucial role in selection of extender units. Engineering of AT domains enables the site-specific incorporation of non-natural extender units, leading to generation of novel derivatives. Here, we determined the crystal structures of AT domains from the fifth module of tylosin PKS (TylAT5) derived from Streptomyces fradiae and the eighth module of spinosad PKS (SpnAT8) derived from Saccharopolyspora spinosa, and combined them with molecular dynamics simulations and enzyme kinetic studies to elucidate the molecular basis of substrate selection. The ethylmalonyl-CoA-specific conserved motif TAGH of TylAT5 and the MMCoA-specific conserved motif YASH of SpnAT8 were identified within the substrate-binding pocket, and several key residues close to the substrate acyl moiety were located. Through site-directed mutagenesis of four residues near the active site, we successfully reprogrammed the specificity of these two AT domains toward malonyl-CoA. Mutations in TylAT5 enhanced its catalytic activity 2.6-fold toward malonyl-CoA, and mutations in SpnAT8 eliminated the substrate promiscuity. These results extend our understanding of AT substrate specificity and would benefit the engineering of PKSs.

Comparison of tet(X4)-containing contigs assembled from metagenomic sequencing data with plasmid sequences of isolates from a cohort of healthy subjects

Recently discovered tet(X) gene variants have provided new insights into microbial antibiotic resistance mechanisms and their potential consequences for public health. This study focused on detection, analysis, and characterization of Tet(X4)-positive Enterobacterales from the gut microbiota of a healthy cohort of individuals in Singapore using cultivation-dependent and cultivation-independent approaches. Twelve Tet(X4)-positive Enterobacterales strains that were previously obtained from the cohort were fully genome-sequenced and comparatively analyzed. A metagenomic sequencing (MS) data set of the same samples was mined for contigs that harbored the tet(X4) resistance gene. The sequences of tet(X4)-containing contigs and plasmids sequences were compared. The presence of the resistance genes floR and estT (previously annotated as catD) was detected in the same cassette in 10 and 12 out of the 12 tet(X4)-carrying plasmids, respectively. MS detected tet(X4)-containing contigs in 2 out of the 109 subjects, while cultivation-dependent analysis previously reported a prevalence of 10.1%. The tet(X4)-containing sequences assembled from MS data are relatively short (~14 to 33 kb) but show high similarity to the respective plasmid sequences of the isolates. Our findings show that MS can complement efforts in the surveillance of antibiotic resistance genes for clinical samples, while it has a lower sensitivity than a cultivation-based method when the target organism has a low abundance. Further optimization is required if MS is to be utilized in antibiotic resistance surveillance.IMPORTANCEThe global rise in antibiotic resistance makes it necessary to develop and apply new approaches to detect and monitor the prevalence of antibiotic resistance genes in human populations. In this regard, of particular interest are resistances against last-resort antibiotics, such as tigecycline. In this study, we show that metagenomic sequencing can help to detect high abundance of the tigecycline resistance gene tet(X4) in fecal samples from a cohort of healthy human subjects. However, cultivation-based approaches currently remain the most reliable and cost-effective method for detection of antibiotic-resistant bacteria.

A three-level regulatory mechanism of the aldo-keto reductase subfamily AKR12D

Modulation of protein function through allosteric regulation is central in biology, but biomacromolecular systems involving multiple subunits and ligands may exhibit complex regulatory mechanisms at different levels, which remain poorly understood. Here, we discover an aldo-keto reductase termed AKRtyl and present its three-level regulatory mechanism. Specifically, by combining steady-state and transient kinetics, X-ray crystallography and molecular dynamics simulation, we demonstrate that AKRtyl exhibits a positive synergy mediated by an unusual Monod-Wyman-Changeux (MWC) paradigm of allosteric regulation at low concentrations of the cofactor NADPH, but an inhibitory effect at high concentrations is observed. While the substrate tylosin binds at a remote allosteric site with positive cooperativity. We further reveal that these regulatory mechanisms are conserved in AKR12D subfamily, and that substrate cooperativity is common in AKRs across three kingdoms of life. This work provides an intriguing example for understanding complex allosteric regulatory networks.

Brazilian Clinical Strains of Actinobacillus pleuropneumoniae and Pasteurella multocida: Capsular Diversity, Antimicrobial Susceptibility (In Vitro) and Proof of Concept for Prevention of Natural Colonization by Multi-Doses Protocol of Tildipirosin

One hundred Actinobacillus pleuropneumoniae (App) and sixty Pasteurella multocida subsp. multocida serogroup A (PmA) isolates were recovered from porcine pneumonic lungs collected from eight central or southern states of Brazil between 2014 and 2018 (App) or between 2017 and 2021 (PmA). A. pleuropneumoniae clinical isolates were typed by multiplex PCR and the most prevalent serovars were 8, 7 and 5 (43, 25% and 18%, respectively). In addition, three virulence genes were assessed in P. multocida isolates, all being positive to capA (PmA) and kmt1 genes, all negative to capD and toxA, and most of them (85%) negative to pfhA gene. The susceptibility of both pathogens to tildipirosin was investigated using a broth microdilution assay. The percentage of isolates susceptible to tildipirosin was 95% for App and 73.3% for PmA. The MIC50 values were 0.25 and 1 μg/mL and the MIC90 values were 4 and >64 μg/mL for App and PmA, respectively. Finally, a multiple-dose protocol of tildipirosin was tested in suckling piglets on a farm endemic for both pathogens. Tildipirosin was able to prevent the natural colonization of the tonsils by App and PmA and significantly (p < 0.0001) reduced the burden of Glaesserella parasuis in this tissue. In summary, our results demonstrate that: (i) tildipirosin can be included in the list of antibiotics to control outbreaks of lung disease caused by App regardless of the capsular type, and (ii) in the case of clinical strains of App and PmA that are sensitive to tildipirosin based on susceptibility testing, the use of this antibiotic in eradication programs for A. pleuropneumoniae and P. multocida can be strongly recommended.

Prevalence and antimicrobial susceptibility of Mycoplasma bovis from the upper and lower respiratory tracts of healthy feedlot cattle and those diagnosed with bovine respiratory disease

Mycoplasma bovis is an important respiratory pathogen of cattle. In this study, the prevalence and antimicrobial susceptibility of M. bovis were evaluated from two Cohorts of feedlot cattle spanning an 8-year period. In the first study conducted in 2008-2009, nasopharyngeal swabs from cattle sampled at feedlot entry and after 60 days on feed were collected (Cohort 1). In a second study conducted in 2015-2016, nasopharyngeal and trans-tracheal samples were collected from cattle diagnosed with bovine respiratory disease (BRD) and matching healthy controls (Cohort 2). For Cohort 1, the prevalence of M. bovis was lower in cattle at entry compared to when the same individuals were sampled ≥60 days later (P < 0.05). For Cohort 2, the prevalence of M. bovis was greater in both nasopharyngeal and tracheal samples from cattle diagnosed with BRD, compared to controls (P < 0.05). In both Cohorts, almost all isolates were resistant to tilmicosin. Compared to M. bovis from Cohort 1, isolates of Cohort 2 exhibited increased resistance to clindamycin, enrofloxacin, florfenicol, tylosin, and tulathromycin, with the latter showing resistance levels >90 %. These data suggest that antimicrobials used to prevent and treat BRD selected for resistance in M. bovis over the 8-year period. For macrolides, cross-resistance occurred and M. bovis can retain resistance even when antimicrobial selection pressure is removed. Within 9 years of commercial availability of tulathromycin, the majority of M. bovis displayed resistance. Therefore, longitudinal evaluation of resistance in respiratory pathogens is important to ensure efficacious treatment of BRD.

Discovery and Development of Antibacterial Agents: Fortuitous and Designed

Today, antibacterial drug resistance has turned into a significant public health issue. Repeated intake, suboptimal and/or unnecessary use of antibiotics, and, additionally, the transfer of resistance genes are the critical elements that make microorganisms resistant to conventional antibiotics. A substantial number of antibacterials that were successfully utilized earlier for prophylaxis and therapeutic purposes have been rendered inadequate due to this phenomenon. Therefore, the exploration of new molecules has become a continuous endeavour. Many such molecules are at various stages of investigation. A surprisingly high number of new molecules are currently in the stage of phase 3 clinical trials. A few new agents have been commercialized in the last decade. These include solithromycin, plazomicin, lefamulin, omadacycline, eravacycline, delafloxacin, zabofloxacin, finafloxacin, nemonoxacin, gepotidacin, zoliflodacin, cefiderocol, BAL30072, avycaz, zerbaxa, vabomere, relebactam, tedizolid, cadazolid, sutezolid, triclosan and afabiacin. This article aims to review the investigational and recently approved antibacterials with a focus on their structure, mechanisms of action/resistance, and spectrum of activity. Delving deep, their success or otherwise in various phases of clinical trials is also discussed while attributing the same to various causal factors.

Effect of treatment of pneumonia and otitis media with tildipirosin or florfenicol + flunixin meglumine on health and upper respiratory tract microbiota of preweaned Holstein dairy heifers

The objective of this randomized clinical study was to compare the effect of 2 antimicrobial interventions, tildipirosin or florfenicol + flunixin meglumine, used for treatment of pneumonia and extralabel treatment for otitis on health parameters and upper respiratory tract (URT) microbiota of preweaned Holstein calves. Housed preweaned Holstein heifers diagnosed with either otitis or pneumonia were assigned into 1 of 2 treatment groups, receiving a single subcutaneous injection of either 4 mg/kg of tildipirosin (TLD; n = 444) or 40 mg/kg of florfenicol combined with 2.2 mg/kg of a nonsteroidal anti-inflammatory, flunixin meglumine (FLF; n = 442). Calves were enrolled and treated on the day of diagnosis of the first case of pneumonia or otitis. If a calf had a recurrent case, the opposite drug was administered, respecting an interval of 5 d between drug injections. Blood samples for leukocyte counts were collected at 0, 2, 4, and 6 d after treatment, and rectal temperature was measured daily during the 5 d after treatment. Ear scores were observed from calves with otitis. Additionally, swabs of the URT were collected from a subset of 20 calves in each treatment group at d 0, 3, 6, 9, and 11 following enrollment for analysis of URT microbiota through next-generation sequencing of the 16S rRNA gene and quantitative PCR. Swabs were also collected from a comparative group of 20 healthy calves that did not receive any drug. No differences were observed between groups for recurrence risk of either pneumonia (TLD = 32.4%; FLF = 29.7%) or otitis (TLD = 72.7%; FLF = 73.6%). Similarly, no differences were observed for the total number of treatments for pneumonia (TLD = 1.45; FLF = 1.42) or otitis (TLD = 2.96; FLF = 3.07). On the other hand, both drugs reduced rectal temperature, ear scores, and leukocyte counts, with FLF calves having a greater reduction in rectal temperature within 4 d after treatment. Both TLD and FLF reduced the total bacterial load when compared with healthy untreated calves, but no differences were observed between treatment groups. Furthermore, compared with the untreated group, treated calves had lower mean relative abundances (MRA) of the genera Mannheimia, Moraxella, and Pasteurella within 11, 9, and 3 d after treatment, respectively; however, no significant differences were observed between TLD and FLF. On the other hand, MRA of Mycoplasma was not decreased by both treatments compared to untreated animals, and a higher MRA was observed in the TLD group during 11 d after treatment in comparison to FLF and untreated calves. Based on this data, we concluded that both drugs used in the study were effective in reducing rectal temperature, ear scores, leukocyte counts, and MRA of the genera Mannheimia, Pasteurella, and Moraxella in the URT, and calves treated with FLF had a greater reduction in rectal temperature.

Effect of metaphylactic administration of tildipirosin on the incidence of pneumonia and otitis and on the upper respiratory tract and fecal microbiome of preweaning Holstein calves

The objectives of this study were to evaluate the effect of the metaphylactic use of a semi-synthetic long-acting macrolide (tildipirosin) on the prevention of pneumonia and otitis in preweaning Holstein calves, as well as its effects on the microbiome of their upper respiratory tract (URT) and feces. Newborn healthy Holstein heifers, collectively housed, were randomly allocated to 1 of 2 treatment groups: treatment (TRT; n = 932) or control (CTR; n = 927). Calves in the TRT group received a single subcutaneous injection of 4 mg/kg tildipirosin (Zuprevo, Merck Animal Health) at 7 ± 7 d of life. Calves in the CTR group received no drug injection. All enrolled calves were evaluated from 1 to 63 ± 3 d of life (weaning age) and monitored daily for any adverse health events during this period. Daily physical examination was performed to diagnose pneumonia and otitis, and body weight was measured weekly in all animals. From a randomly selected subset of 217 calves, blood samples for biochemical variables analysis and swabs were collected weekly from the URT and rectum for analysis of the nasal and fecal microbiome, respectively, via next-generation sequencing of the 16S rRNA gene. Total bacterial load was evaluated using quantitative PCR. In addition, another subset of 26 calves was randomly selected and fecal swabs were collected in a more intensive sampling to investigate the short-term effect of tildipirosin administration on the fecal microbiome. We performed general mixed linear models and logistic regression to analyze continuous and binary outcomes, respectively. Tildipirosin metaphylaxis reduced the incidence of otitis (CTR = 47.03%; TRT = 37.55%) and tended to reduce the incidence of pneumonia (CTR = 20.71%; TRT = 17.38%) and the overall mortality risk (CTR = 6.69%; TRT = 4.94%). We observed no significant differences between groups for mortality due to pneumonia (CTR = 0.86%; TRT = 0.97%) or mortality due to otitis (CTR = 2.05%; TRT = 1.39%). Calves in the TRT group had a higher average daily gain than calves in the CTR group. Furthermore, metaphylaxis had no significant effects on the total bacterial load, genus, or phylum analysis of the fecal microbiome from the 2 subset groups. However, for the URT microbiota, we observed a significant decrease in total bacterial load for the TRT group compared to the CTR group 1 week after metaphylactic injection. Tildipirosin metaphylaxis decreased the mean relative abundance of the genera Mannheimia, Moraxella, and Pasteurella but significantly increased the mean relative abundance of Mycoplasma. Although tildipirosin had no positive effect on Mycoplasma, it reduced the mean relative abundance of important pathogenic bacteria in the URT and had positive effects for the control of otitis. The metaphylactic use of tildipirosin can be a suitable strategy for the control of otitis on farms with a high prevalence of this disease.

Aldsulfin, a novel unusual anti-mannheimiosis epithiodiketopiperazine antibiotic produced by Lasiodiplodia pseudotheobromae FKI-4499

An anti-mannheimiosis agent, aldsulfin, was isolated from a culture broth of the fungus Lasiodiplodia pseudotheobromae FKI-4499, together with a known compound, lasiodipline C, using bioassay-guided fractionation. Spectroscopic analysis of aldsulfin, using NMR, mass spectrometry, and CD analyses revealed it to be an epithiodiketopiperazine with an unstable and unusual hemithioaminal moiety. Aldsulfin showed antibacterial activity against Mannheimia haemolytica and Pasteurella multocida.

Antimicrobial susceptibility profiles of porcine mycoplasmas isolated from samples collected in southern Europe

BACKGROUND

Mycoplasma (M.) hyopneumoniae, M. hyorhinis and M. hyosynoviae are significant pathogens for the porcine industry worldwide. The aim of the present study was to determine the antimicrobial susceptibility of six key antimicrobials (tylosin, tilmicosin, tylvalosin, lincomycin, tiamulin and valnemulin) routinely used for treating infections caused by these pathogens. Twenty-seven M. hyopneumoniae, 48 M. hyorhinis and 40 M. hyosynoviae field strains isolated from clinical samples from different Southern European countries between 2013 and 2018 using broth microdilution method were evaluated.

RESULTS

Tylvalosin exhibited the highest in vitro activity among the macrolides assayed, with MIC90 values 4 to 5 two-fold dilutions lower than those of tylosin and tilmicosin. The pleuromutilin valnemulin showed one of the highest in vitro activities against the three mycoplasma species. On the contrary, lincomycin exhibited the highest MIC values of the antimicrobials tested.

CONCLUSIONS

The data obtained in the present study supports the use of pleuromutilins and macrolides for the control of infections caused by porcine mycoplasmas. The use of lincomycin for the treatment of porcine mycoplasma infections should be carefully evaluated due to the presence of circulating field isolates with decreased susceptibility to this antimicrobial.

Pharmacokinetics and bioavailability of tildipirosin following intravenous and subcutaneous administration in sheep

Tildipirosin is a semi-synthetic macrolide antibiotic commonly used in cattle and swine to treat bacterial pneumonia. The objective of this study was to investigate the pharmacokinetic profile of tildipirosin after a single intravenous (i.v.) and subcutaneous (s.c.) administration in healthy lambs. Eighteen lambs were randomly divided into three groups (n = 6 each). Lambs received a single s.c. dose of tildipirosin at 4 and 6 mg/kg b.w. in group 1 and 2, respectively. Lambs in group 3 received a single i.v. dose of tildipirosin at 4 mg/kg b.w. Blood samples were collected at 0, 0.5, 0.75, 1.5, 2, 3, 4, 6, 8, 10, 24, 36, 48 hr, and every 24 hr to day 21, and thereafter at day 28 posttildipirosin administration. The plasma concentrations of tildipirosin were determined using high-performance liquid chromatography with tandem mass spectrometry detection (LC⁄MS⁄MS). All lambs appeared to tolerate both the intravenous and subcutaneous injection of tildipirosin. Following i.v. administration, the elimination half-life (T_1/2 ), mean residence time (MRT), volume of distribution (Vd/F), and total body clearance (Cl/F) were 119.6 ± 9.0 hr, 281.9 ± 25.7 hr, 521.1 ± 107.2 L, and 2.9 ± 0.5 L/hr, respectively. No significant differences in C_max (657.0 ± 142.8 and 754.6 ± 227.1 ng/ml), T_max (1.21 ± 0.38 and 1.35 ± 0.44 hr), T_1/2 (144 ± 17.5, 156.5 ± 33.4 hr), and MRT (262.0 ± 30.2 and 250.6 ± 54.5 hr) were found in tildipirosin after s.c. dosing at 4 and 6 mg/kg b.w., respectively. The absolute bioavailability (F) of tildipirosin was 71.5% and 75.3% after s.c. administration of 4 and 6 mg/kg b.w., respectively. In conclusion, tildipirosin was rapidly absorbed and slowly eliminated after a single s.c. administration in healthy lambs. Tildipirosin could be used for the treatment and prevention of respiratory bacterial infections in sheep. However, further in vitro and in vivo studies to determine the efficacy and safety are warranted. To our knowledge, this is the first study to determine the tildipirosin pharmacokinetic parameters in sheep plasma.

Investigation of Macrolide Resistance Genotypes in Mycoplasma bovis Isolates from Canadian Feedlot Cattle

Mycoplasma bovis is associated with bovine respiratory disease (BRD) and chronic pneumonia and polyarthritis syndrome (CPPS) in feedlot cattle. No efficacious vaccines for M. bovis exist; hence, macrolides are commonly used to control mycoplasmosis. Whole genome sequences of 126 M. bovis isolates, derived from 96 feedlot cattle over 12 production years, were determined. Antimicrobial susceptibility testing (AST) of five macrolides (gamithromycin, tildipirosin, tilmicosin, tulathromycin, tylosin) was conducted using a microbroth dilution method. The AST phenotypes were compared to the genotypes generated for 23S rRNA and the L4 and L22 ribosomal proteins. Mutations in domains II (nucleotide 748; E. coli numbering) and V (nucleotide 2059 and 2060) of the 23S rRNA (rrl) gene alleles were associated with resistance. All isolates with a single mutation at Δ748 were susceptible to tulathromycin, but resistant to tilmicosin and tildipirosin. Isolates with mutations in both domain II and V (Δ748Δ2059 or Δ748Δ2060) were resistant to all five macrolides. However, >99% of isolates were resistant to tildipirosin and tilmicosin, regardless of the number and positions of the mutations. Isolates with a Δ748 mutation in the 23S rRNA gene and mutations in L4 and L22 were resistant to all macrolides except for tulathromycin.

Antimicrobial Sensitivity Testing of Mycoplasma bovis Isolates Derived from Western Canadian Feedlot Cattle

Mycoplasma bovis is particularly adept at evading the immune system, resulting in chronic infections of the lungs and joints of feedlot cattle. The chronicity of the lesions results in prolonged antimicrobial therapy, possibly exacerbating antimicrobial resistance. This cross-sectional study generated in vitro antimicrobial susceptibility testing (AST) data on 211 M. bovis isolates recovered from 159 healthy, diseased, and dead cattle, spanning the period of 2006-2018. Nine antimicrobials commonly administered to western Canadian feedlot cattle were assessed. The data were analyzed with non-parametric statistical tests with a level of significance of p < 0.05 (two-tailed). Minimum inhibitory concentration (MIC) values tended to increase between the isolates from healthy versus dead cattle and over time (2006-2018). Isolates from dead versus healthy cattle were more likely to be resistant to tulathromycin, gamithromycin, tylosin and enrofloxacin. There was no difference in the distributions of the MICs generated from the isolates recovered from the lungs and joints (p ≥ 0.124) and the lungs and deep nasal passages (p ≥ 0.157) of the same animals.

Antimicrobial Resistance in Mycoplasma spp

Mycoplasmas are intrinsically resistant to antimicrobials targeting the cell wall (fosfomycin, glycopeptides, or β-lactam antibiotics) and to sulfonamides, first-generation quinolones, trimethoprim, polymixins, and rifampicin. The antibiotics most frequently used to control mycoplasmal infections in animals are macrolides and tetracyclines. Lincosamides, fluoroquinolones, pleuromutilins, phenicols, and aminoglycosides can also be active. Standardization of methods used for determination of susceptibility levels is difficult since no quality control strains are available and because of species-specific growth requirements. Reduced susceptibility levels or resistances to several families of antimicrobials have been reported in field isolates of pathogenic Mycoplasma species of major veterinary interest: M. gallisepticum and M. synoviae in poultry; M. hyopneumoniae, M. hyorhinis, and M. hyosynoviae in swine; M. bovis in cattle; and M. agalactiae in small ruminants. The highest resistances are observed for macrolides, followed by tetracyclines. Most strains remain susceptible to fluoroquinolones. Pleuromutilins are the most effective antibiotics in vitro. Resistance frequencies vary according to the Mycoplasma species but also according to the countries or groups of animals from which the samples were taken. Point mutations in the target genes of different antimicrobials have been identified in resistant field isolates, in vitro-selected mutants, or strains reisolated after an experimental infection followed by one or several treatments: DNA-gyrase and topoisomerase IV for fluoroquinolones; 23S rRNA for macrolides, lincosamides, pleuromutilins, and amphenicols; 16S rRNAs for tetracyclines and aminoglycosides. Further work should be carried out to determine and harmonize specific breakpoints for animal mycoplasmas so that in vitro information can be used to provide advice on selection of in vivo treatments.

Optimal Regimens and Cutoff Evaluation of Tildipirosin Against Pasteurella multocida

Pasteurella multocida (PM) can invade the upper respiratory tract of the body and cause death and high morbidity. Tildipirosin, a new 16-membered-ring macrolide antimicrobial, has been recommended for the treatment of respiratory diseases. The objective of this research was to improve the dose regimes of tildipirosin to PM for reducing the macrolides resistance development with the pharmacokinetic/pharmacodynamic (PK/PD) modeling approach and to establish an alternate cutoff for tildipirosin against PM. A single dose (4 mg/kg body weight) of tildipirosin was administered via intramuscular (i.m.) and intravenous (i.v.) injection to the pigs. The minimum inhibitory concentration (MIC) values of clinical isolates (112) were measured in the range of 0.0625–32 μg/ml, and the MIC₅₀ and MIC₉₀ values were 0.5 and 2 μg/ml, respectively. The MIC of the selected PM04 was 2 and 0.5 μg/ml in the tryptic soy broth (TSB) and serum, respectively. The main pharmacokinetic (PK) parameters including the area under the curve at 24 h (AUC_{24 h}), AUC, terminal half-life (T_1/2), the time to peak concentration (T_max), peak concentration (C_max), relative total systemic clearance (CL_b), and the last mean residence time (MRT_last) were calculated to be 7.10, 7.94 μg^∗h/ml, 24.02, NA h, NA μg/ml, 0.46 L/h^∗kg, 8.06 h and 3.94, 6.79 μg^∗h/ml, 44.04, 0.25 h, 0.98 μg/ml, 0.43 L/h^∗kg, 22.85 h after i.v. and i.m. induction, respectively. Moreover, the bioavailability of i.m. route was 85.5%, and the unbinding of tildipirosin to serum protein was 78%. The parameters AUC_{24 h}/MIC in serum for bacteriostatic, bactericidal, and elimination activities were calculated as 18.91, 29.13, and 34.03 h based on the inhibitory sigmoid E_max modeling. According to the Monte Carlo simulation, the optimum doses for bacteriostatic, bactericidal, and elimination activities were 6.10, 9.41, and 10.96 mg/kg for 50% target and 7.86, 12.17, and 14.57 mg/kg for 90% target, respectively. The epidemiological cutoff value (ECV) was calculated to be 4 μg/ml which could cover 95% wild-type clinical isolates distribution. The PK-PD cutoff (CO_PD) was analyzed to be 0.25 μg/ml in vitro for tildipirosin against PM based on the Monte Carlo simulation. Compared with these two cutoff values, the finial susceptible breakpoint was defined as 4 μg/ml. The data presented now provides the optimal regimens (12.17 mg/kg) and susceptible breakpoint (4 μg/ml) for clinical use, but these predicted data should be validated in the clinical practice.

Assessment of Draxxin® (tulathromycin) as an inhibitor of in vitro growth of Babesia bovis, Babesia bigemina and Theileria equi

Babesia bovis, Babesia bigemina and Theileria equi are worldwide tick-borne hemoprotozoan that cause diseases characterized by fever, anemia, weight loss and abortion. A common feature of these diseases are transition from acute to chronic phases, in which parasites may persist in the host for life, and becoming a reservoir for tick transmission. The live-attenuated vaccines for B. bovis and B. bigemina are not available for worldwide use due to legal restrictions and other concerns such as potential erythrocyte antigen and pathogen contamination, and a vaccine for T. equi is not available. The use of chemotherapeutics is essential to treat and control these diseases, but several studies have shown the development of drug-resistance by these parasites, and safe and effective alternative drugs are needed. Tulathromycin, a macrolide antibiotic, has proven to be effective against a vast range of bacteria and Plasmodium yoelli, a Babesia and Theileria related intra-erythrocytic apicomplexan. Draxxin^® (tulathromycin) is currently licensed to treat infections that cause respiratory diseases in cattle in several countries. In this study, the activity of Draxxin^® was tested in vitro on cultured B. bovis, B. bigemina and T. equi. Addition of the drug to in vitro cultures resulted in cessation of parasite replication of the three species tested, B. bovis, B. bigemina and T. equi, with estimated IC₅₀ of 16.7 ± 0.6 nM; 6.2 ± 0.2 nM and 2.4 ± 0.1 nM, respectively, at 72 h. Furthermore, neither parasites nor parasite DNA were detectable in cultures treated with IC₁₀₀, suggesting Draxxin^® is a highly effective anti-Babesia/Theileria drug. Importantly, the IC₅₀ calculated for Draxxin^® for the Babesia/Theileria parasites tested is lower that the IC₅₀ calculated for some drugs currently in use to control these parasites. Collectively, the data strongly support in vivo testing of Draxxin^® for the treatment of bovine babesiosis and equine piroplasmosis.

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Chemotherapeutics are critical to treat bovine babesiosis and equine theileriosis.

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Study of the activity of Draxxin^® in vitro B. bovis, B. bigemina, T. equi cultures.

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Addition of Draxxin^® to the cultures resulted in cessation of parasite replication.

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IC₅₀ (nM) of 16.7; 6.2 and 2.4 to B. bovis, B. bigemina and T. equi, respectively.

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Results suggests that Draxxin^® is a highly effective anti-Babesia/Theileria drug.

Comparative transcriptional profiling of tildipirosin-resistant and sensitive Haemophilus parasuis

Numerous studies have been conducted to examine the molecular mechanism of Haemophilus parasuis resistance to antibiotic, but rarely to tildipirosin. In the current study, transcriptional profiling was applied to analyse the variation in gene expression of JS0135 and tildipirosin-resistant JS32. The growth curves showed that JS32 had a higher growth rate but fewer bacteria than JS0135. The cell membranes of JS32 and a resistant clinical isolate (HB32) were observed to be smoother than those of JS0135. From the comparative gene expression profile 349 up- and 113 downregulated genes were observed, covering 37 GO and 63 KEGG pathways which are involved in biological processes (11), cellular components (17), molecular function (9), cellular processes (1), environmental information processing (4), genetic information processing (9) and metabolism (49) affected in JS32. In addition, the relative overexpression of genes of the metabolism pathway (HAPS_RS09315, HAPS_RS09320), ribosomes (HAPS_RS07815) and ABC transporters (HAPS_RS10945) was detected, particularly the metabolism pathway, and verified with RT-qPCR. Collectively, the gene expression profile in connection with tildipirosin resistance factors revealed unique and highly resistant determinants of H. parasuis to macrolides that warrant further attention due to the significant threat of bacterial resistance.

In vitro activity of tylvalosin against Spanish field strains of Mycoplasma hyopneumoniae

Mycoplasma hyopneumoniae is involved in the porcine enzootic pneumonia and respiratory disease complex; therefore, the search for new treatment options that contribute to the control of this organism is relevant. The minimal inhibitory concentrations (MICs) and minimal bactericidal concentrations of tylvalosin and 19 other antimicrobial agents against 20 Spanish field isolates of M. hyopneumoniae were determined using the broth microdilution method, with the type strain (J) as a control strain. Tylvalosin had MIC50 and MIC90 values of 0.016 and 0.06 µg/ml, respectively, and was the second-most effective of the assayed antibiotics, after valnemulin. Tiamulin, tylosin and lincomycin were also among the antibiotics with the lowest MIC50 and MIC90 values against the 20 field isolates (0.06-0.25 µg/ml). However, resistance to tylosin and spiramycin, which like tylvalosin, are 16-membered macrolides, was observed. The MIC50 and MIC90 values for ciprofloxacin and enrofloxacin ranged from 0.125 to 1 µg/ml; the corresponding values ranged from 2 to 4 µg/ml for oxytetracyline, which was the most active tetracycline. Furthermore, tylvalosin and valnemulin exhibited the highest bactericidal activities. In conclusion, the macrolide tylvalosin and the pleuromutilin valnemulin exhibited the highest in vitro antimicrobial activities against M. hyopneumoniae field isolates in comparison with the other tested antibiotics.

Effect of subtherapeutic vs. therapeutic administration of macrolides on antimicrobial resistance in Mannheimia haemolytica and enterococci isolated from beef cattle

Macrolides are the first-line treatment against bovine respiratory disease (BRD), and are also used to treat infections in humans. The macrolide, tylosin phosphate, is often included in the diet of cattle as a preventative for liver abscesses in many regions of the world outside of Europe. This study investigated the effects of administering macrolides to beef cattle either systemically through a single subcutaneous injection (therapeutic) or continuously in-feed (subtherapeutic), on the prevalence and antimicrobial resistance of Mannheimia haemolytica and Enterococcus spp. isolated from the nasopharynx and faeces, respectively. Nasopharyngeal and faecal samples were collected weekly over 28 days from untreated beef steers and from steers injected once with tilmicosin or tulathromycin or continuously fed tylosin phosphate at dosages recommended by manufacturers. Tilmicosin and tulathromycin were effective in lowering (P < 0.05) the prevalence of M. haemolytica, whereas subtherapeutic tylosin had no effect. M. haemolytica isolated from control- and macrolide-treated animals were susceptible to macrolides as well as to other antibiotics. Major bacteria co-isolated with M. haemolytica from the nasopharynx included Pasteurella multocida, Staphylococcus spp., Acinetobacter spp., Escherichia coli and Bacillus spp. With the exception of M. haemolytica and P. multocida, erythromycin resistance was frequently found in other isolated species. Both methods of macrolide administration increased (P < 0.05) the proportion of erythromycin resistant enterococci within the population, which was comprised almost exclusively of Enterococcus hirae. Injectable macrolides impacted both respiratory and enteric microbes, whereas orally administered macrolides only influenced enteric bacteria.