16S rRNA gene mutations associated with decreased susceptibility to tetracycline in Mycoplasma bovis

Antimicrobial treatment and resistance in sexually transmitted bacterial infections

Sexually transmitted infections (STIs) have been part of human life since ancient times, and their symptoms affect quality of life, and sequelae are common. Socioeconomic and behavioural trends affect the prevalence of STIs, but the discovery of antimicrobials gave hope for treatment, control of the spread of infection and lower rates of sequelae. This has to some extent been achieved, but increasing antimicrobial resistance and increasing transmission in high-risk sexual networks threaten this progress. For Neisseria gonorrhoeae, the only remaining first-line treatment (with ceftriaxone) is at risk of becoming ineffective, and for Mycoplasma genitalium, for which fewer alternative antimicrobial classes are available, incurable infections have already been reported. For Chlamydia trachomatis, in vitro resistance to first-line tetracyclines and macrolides has never been confirmed despite decades of treatment of this highly prevalent STI. Similarly, Treponema pallidum, the cause of syphilis, has remained susceptible to first-line penicillin.

The complete genome sequence of unculturable Mycoplasma faucium obtained through clinical metagenomic next-generation sequencing

Introduction

Diagnosing Mycoplasma faucium poses challenges, and it's unclear if its rare isolation is due to infrequent occurrence or its fastidious nutritional requirements.

Methods

This study analyzes the complete genome sequence of M. faucium, obtained directly from the pus of a sternum infection in a lung transplant patient using metagenomic sequencing.

Results

Genome analysis revealed limited therapeutic options for the M. faucium infection, primarily susceptibility to tetracyclines. Three classes of mobile genetic elements were identified: two new insertion sequences, a new prophage (phiUMCG-1), and a species-specific variant of a mycoplasma integrative and conjugative element (MICE). Additionally, a Type I Restriction-Modification system was identified, featuring 5'-terminally truncated hsdS pseudogenes with overlapping repeats, indicating the potential for forming alternative hsdS variants through recombination.

Conclusion

This study represents the first-ever acquisition of a complete circularized bacterial genome directly from a patient sample obtained from invasive infection of a primary sterile site using culture-independent, PCR-free clinical metagenomics.

Mycoplasma bovis mastitis in dairy cattle

Mycoplasma bovis has recently been identified increasingly in dairy cows causing huge economic losses to the dairy industry. M. bovis is a causative agent for mastitis, pneumonia, endometritis, endocarditis, arthritis, otitis media, and many other clinical symptoms in cattle. However, some infected cows are asymptomatic or may not shed the pathogen for weeks to years. This characteristic of M. bovis, along with the lack of adequate testing and identification methods in many parts of the world until recently, has allowed the M. bovis to be largely undetected despite its increased prevalence in dairy farms. Due to growing levels of antimicrobial resistance among wild-type M. bovis isolates and lack of cell walls in mycoplasmas that enable them to be intrinsically resistant to beta-lactam antibiotics that are widely used in dairy farms, there is no effective treatment for M. bovis mastitis. Similarly, there is no commercially available effective vaccine for M. bovis mastitis. The major constraint to developing effective intervention tools is limited knowledge of the virulence factors and mechanisms of the pathogenesis of M. bovis mastitis. There is lack of quick and reliable diagnostic methods with high specificity and sensitivity for M. bovis. This review is a summary of the current state of knowledge of the virulence factors, pathogenesis, clinical manifestations, diagnosis, and control of M. bovis mastitis in dairy cows.

Antimicrobial susceptibility and molecular characteristics of Mycoplasma bovis isolated from cases of bovine respiratory disease in Australian feedlot cattle

To date, antimicrobial susceptibility has not been reported for Australian Mycoplasma bovis isolates. This study determined minimal inhibitory concentrations (MICs) for 12 different antimicrobials against Australian M. bovis isolates and used whole genome sequencing to screen those showing high macrolide MICs for point mutations in target genes. Most lung tissue/swab samples from bovine respiratory disease cases (61/76, 80.3%) tested positive for M. bovis. A set of 50 representative isolates (50/61, 82.0%) that showed adequate growth, was used for MIC testing. Uniformly, low MIC values were confirmed for enrofloxacin (≤ 4 μg/mL), florfenicol (≤ 8 μg/mL), gamithromycin (≤ 2 μg/mL), spectinomycin (≤ 4 μg/mL), tetracycline (≤ 8 μg/mL), tiamulin (≤ 4 μg/mL), and tulathromycin (≤ 0.5 μg/mL). A small proportion (10%) of isolates exhibited high MICs (≥ 32 μg/mL) for tildipirosin, tilmicosin, tylosin, and lincomycin, which were above the epidemiological cut-off values for each antimicrobial (≥ 4 μg/mL). These isolates, originating from three Australian states, underwent whole genome sequencing/multilocus sequencing typing and were compared with the reference strain PG45 to investigate mutations that might be linked with the high macrolide/lincosamide MICs. All five belonged to ST52 and two macrolide associated mutations were identified within the 23 S rRNA gene (A2058G in two sequenced isolates and G748A in all sequenced isolates). Four additional 23 S rRNA gene mutations did not appear to be linked to macrolide resistance. Whilst the majority of Australian M. bovis isolates appear susceptible to the tested antimicrobials, emerging macrolide resistance was detected in three Australian states and requires continued monitoring.

The Resistance Mechanism of Mycoplasma bovis From Yaks in Tibet to Fluoroquinolones and Aminoglycosides

Mycoplasma bovis (M. bovis) is one of the important pathogens for yaks. Aminoglycosides and fluoroquinolones are frequently used medications for the treatment of M. bovis. Drug-resistant strains were inevitable with the abuse of antibiotics. The resistance of M. bovis to aminoglycosides was related to the base mutations in drug target genes. Amino acid mutations at the quinolone resistance-determining region (QRDR) in gyrA, gyrB, parC, and parE conferred resistance to fluoroquinolones. In order to investigate the resistance mechanism of M. bovis from yaks in Tibet to aminoglycosides and fluoroquinolones, six frequently used antibiotics and ten clinical M. bovis strains were administered for a drug sensitivity test for in vitro-induced highly resistant strains, a drug stable-resistance test, cross-resistance test, and analysis of target gene mutations. The results showed that the clinical strains of M. bovis from yaks in Tibet had varying degrees of resistance to fluoroquinolones and aminoglycosides. The mechanism of resistance to fluoroquinolones and aminoglycosides was identified preliminarily for M. bovis from yaks: the single-site base mutation mediated the resistance of M. bovis from yaks and both base mutations led to highly resistant strains (aminoglycosides: rrs3 and rrs4; fluoroquinolones: gyrA and parC). The active efflux system results of M. bovis showed that there was no active efflux system based on fluoroquinolones and aminoglycosides expressed in M. bovis from yaks. The research could provide a reference for clinical treatment of M. bovis.

Genome-Wide Association Study of Nucleotide Variants Associated with Resistance to Nine Antimicrobials in Mycoplasma bovis

Antimicrobial resistance (AMR) studies of Mycoplasma bovis have generally focused on specific loci versus using a genome-wide association study (GWAS) approach. A GWAS approach, using two different models, was applied to 194 Mycoplasma bovis genomes. Both a fixed effects linear model (FEM) and a linear mixed model (LMM) identified associations between nucleotide variants (NVs) and antimicrobial susceptibility testing (AST) phenotypes. The AMR phenotypes represented fluoroquinolones, tetracyclines, phenicols, and macrolides. Both models identified known and novel NVs associated (Bonferroni adjusted p < 0.05) with AMR. Fluoroquinolone resistance was associated with multiple NVs, including previously identified mutations in gyrA and parC. NVs in the 30S ribosomal protein 16S were associated with tetracycline resistance, whereas NVs in 5S rRNA, 23S rRNA, and 50S ribosomal proteins were associated with phenicol and macrolide resistance. For all antimicrobial classes, resistance was associated with NVs in genes coding for ABC transporters and other membrane proteins, tRNA-ligases, peptidases, and transposases, suggesting a NV-based multifactorial model of AMR in M. bovis. This study was the largest collection of North American M. bovis isolates used with a GWAS for the sole purpose of identifying novel and non-antimicrobial-target NVs associated with AMR.

Impact of 16S rRNA Single Nucleotide Polymorphisms on Mycoplasma genitalium Organism Load with Doxycycline Treatment

Doxycycline targets the 16S rRNA and is widely used for the treatment of sexually transmitted infections. While it is not highly effective at eradicating Mycoplasma genitalium infections, it can reduce organism load. The aim of this study was to investigate the association between single nucleotide polymorphisms (SNPs) in the 16S rRNA gene of M. genitalium and change in organism load. M. genitalium samples were collected from 56 men prior to commencing doxycycline and at a median of 13 of 14 doses. These were sequenced for the 16S rRNA, and the association between 16S rRNA SNPs and change in organism load was determined. 16S rRNA sequences were available for 52/56 (92.9%) M. genitalium-infected men, of which 20 (38.5%) had an undetectable load, 26 (50.0%) had a decrease in M. genitalium load (median change of 105-fold), and 6 (11.5%) had an increase in load (median change of 5-fold). The most common SNPs identified were A742G (10/52 [19.2%]), GG960-961TT/C (7/52 [13.5%]), and C1435T (28/52 [53.8%]) (M. genitalium numbering). None were associated with a change in organism load (P = 0.76, 0.16, and 0.98, respectively). Using pooled published data from 28 isolates, no clear relationship between the SNPs and doxycycline MIC was identified. In conclusion, the low efficacy of doxycycline against M. genitalium does not appear to be due to variation in the 16S rRNA gene.

Developing visible light responsive BN/NTCDA heterojunctions with a good degradation performance for tetracycline

In this paper, a series of BN/NTCDA photocatalysts have been prepared using a simple calcination method and their photocatalytic performance under visible light irradiation is studied with tetracycline (TC) as the target pollutant.

Genome-Wide Association Study Reveals Genetic Markers for Antimicrobial Resistance in Mycoplasma bovis

Mycoplasma bovis causes many health and welfare problems in cattle. Due to the absence of clear insights regarding transmission dynamics and the lack of a registered vaccine in Europe, control of an outbreak depends mainly on antimicrobial therapy. Unfortunately, antimicrobial susceptibility testing (AST) is usually not performed, because it is time-consuming and no standard protocol or clinical breakpoints are available. Fast identification of genetic markers associated with acquired resistance may at least partly resolve former issues. Therefore, the aims of this study were to implement a first genome-wide association study (GWAS) approach to identify genetic markers linked to antimicrobial resistance (AMR) in M. bovis using rapid long-read sequencing and to evaluate different epidemiological cutoff (ECOFF) thresholds. High-quality genomes of 100 M. bovis isolates were generated by Nanopore sequencing, and isolates were categorized as wild-type or non-wild-type isolates based on MIC testing results. Subsequently, a k-mer-based GWAS analysis was performed to link genotypes with phenotypes based on different ECOFF thresholds. This resulted in potential genetic markers for macrolides (gamithromycin and tylosin) (23S rRNA gene and 50S ribosomal unit) and enrofloxacin (GyrA and ParC). Also, for tilmicosin and the tetracyclines, previously described mutations in both 23S rRNA alleles and in one or both 16S rRNA alleles were observed. In addition, two new 16S rRNA mutations were possibly associated with gentamicin resistance. In conclusion, this study shows the potential of quick high-quality Nanopore sequencing and GWAS analysis in the evaluation of phenotypic ECOFF thresholds and the rapid identification of M. bovis strains with acquired resistance.

IMPORTANCEMycoplasma bovis is a leading cause of pneumonia but also causes other clinical signs in cattle. Since no effective vaccine is available, current M. bovis outbreak treatment relies primarily on the use of antimicrobials. However, M. bovis is naturally resistant to different antimicrobials, and acquired resistance against macrolides and fluoroquinolones is frequently described. Therefore, AST is important to provide appropriate and rapid antimicrobial treatment in the framework of AMR and to prevent the disease from spreading and/or becoming chronic. Unfortunately, phenotypic AST is time-consuming and, due to the lack of clinical breakpoints, the interpretation of AST in M. bovis is limited to the use of ECOFF values. Therefore, the objective of this study was to identify known and potentially new genetic markers linked to AMR phenotypes of M. bovis isolates, exploiting the power of a GWAS approach. For this, we used high-quality and complete Nanopore-sequenced M. bovis genomes of 100 isolates.

Integrating the Human and Animal Sides of Mycoplasmas Resistance to Antimicrobials

Mycoplasma infections are frequent in humans, as well as in a broad range of animals. However, antimicrobial treatment options are limited, partly due to the lack of a cell wall in these peculiar bacteria. Both veterinary and human medicines are facing increasing resistance prevalence for the most commonly used drugs, despite different usage practices. To date, very few reviews have integrated knowledge on resistance to antimicrobials in humans and animals, the latest dating back to 2014. To fill this gap, we examined, in parallel, antimicrobial usage, resistance mechanisms and either phenotype or genotype-based methods for antimicrobial susceptibility testing, as well as epidemiology of resistance of the most clinically relevant human and animal mycoplasma species. This review unveiled common features and differences that need to be taken into consideration in a "One Health" perspective. Lastly, two examples of critical cases of multiple drug resistance are highlighted, namely, the human M. genitalium and the animal M. bovis species, both of which can lead to the threat of untreatable infections.

Efflux Might Participate in Decreased Susceptibility to Oxytetracycline in Contagious Agalactia-Causative Mycoplasma spp

Contagious agalactia is associated with mastitis, keratoconjunctivitis, arthritis, pneumonia, and septicemia in small ruminants in countries with large dairy industries worldwide. The causative agents belong to four (sub)species of the Mycoplasma genus that have remained essentially susceptible to antimicrobials, including to the widely-used tetracycline family. However, some clinical isolates have been detected that show increased minimum inhibitory concentrations of tetracyclines, although they do not harbor the mutation in the 16SrRNA gene usually associated with resistance. The present work aimed to assess whether efflux pumps, infrequently described in mycoplasmas, could participate in the observed moderate loss of susceptibility. General efflux mechanisms were measured (i) using the fluorescence property of ethidium bromide when accumulated intracellularly and intercalated in the mycoplasma genomes, its active extrusion resulting in a temperature-dependent decrease in fluorescence and (ii) monitoring the growth inhibition of mycoplasmas by subinhibitory concentrations of tetracycline with or without reserpine, a known inhibitor of efflux in other bacteria. Both methods revealed non-specific efflux phenomena in most of the isolates tested, although their efficacy was difficult to quantify. This property could contribute to the acquisition of mutations conferring resistance by maintaining intracellular concentrations of tetracyclines at subinhibitory levels.

The Antibiotic Resistome: A Guide for the Discovery of Natural Products as Antimicrobial Agents

The use of life-saving antibiotics has long been plagued by the ability of pathogenic bacteria to acquire and develop an array of antibiotic resistance mechanisms. The sum of these resistance mechanisms, the antibiotic resistome, is a formidable threat to antibiotic discovery, development, and use. The study and understanding of the molecular mechanisms in the resistome provide the basis for traditional approaches to combat resistance, including semisynthetic modification of naturally occurring antibiotic scaffolds, the development of adjuvant therapies that overcome resistance mechanisms, and the total synthesis of new antibiotics and their analogues. Using two major classes of antibiotics, the aminoglycosides and tetracyclines as case studies, we review the success and limitations of these strategies when used to combat the many forms of resistance that have emerged toward natural product-based antibiotics specifically. Furthermore, we discuss the use of the resistome as a guide for the genomics-driven discovery of novel antimicrobials, which are essential to combat the growing number of emerging pathogens that are resistant to even the newest approved therapies.

A GC-Rich Prophage-Like Genomic Region of Mycoplasma bovirhinis HAZ141_2 Carries a Gene Cluster Encoding Resistance to Kanamycin and Neomycin

Recently, a complete genome sequence of Mycoplasma bovirhinis HAZ141_2 was published showing the presence of a 54-kB prophage-like region. Bioinformatic analysis revealed that this region has a more than 40% GC content and a chimeric organization with three structural elements-a prophage continuous region, a restriction-modification cassette, and a highly transmittable aadE-sat4-aphA-3 gene cluster found in both Gram-positive and Gram-negative bacteria. It is known that aadE confers resistance to streptomycin, sat4 governs resistance to streptothricin/nourseothricin, and aphA-3 is responsible for resistance to kanamycin and structurally related antibiotics. An aadE-like (aadE*) gene of strain HAZ141_2 encodes a 228-amino acid (aa) polypeptide whose carboxy-terminal domain (positions 44 to 206) is almost identical to that of a functional 302-aa AadE (positions 140 to 302). Transcription analysis of the aadE*-sat4-aphA-3 genes showed their cotranscription in M. bovirhinis HAZ141_2. Moreover, a common promoter for aadE*-sat4-aphA-3 was mapped upstream of aadE* using 5' rapid amplification of cDNA ends analysis. Determination of MICs to aminoglycosides and nourseothricin revealed that M. bovirhinis HAZ141_2 is highly resistant to kanamycin and neomycin (≥512 μg/ml). However, MICs to streptomycin (64 μg/ml) and nourseothricin (16 to 32 μg/ml) were similar to those identified in the prophageless M. bovirhinis type strain PG43 and Israeli field isolate 316981. We cloned the aadE*-sat4-aphA-3 genes into a low-copy-number vector and transferred them into antibiotic-sensitive Escherichia coli cells. While the obtained E. coli transformants were highly resistant to kanamycin, neomycin, and nourseothricin (MICs, ≥256 μg/ml), there were no changes in MICs to streptomycin, suggesting a functional defect of the aadE*.

Antimicrobial drug resistance mechanisms among Mollicutes

Representatives of the Mollicutes class are the smallest, wall-less bacteria capable of independent reproduction. They are widespread in nature, most are commensals, and some are pathogens of humans, animals and plants. They are also the main contaminants of cell cultures and vaccine preparations. Despite limited biosynthetic capabilities, they are highly adaptable and capable of surviving under various stress and extreme conditions, including antimicrobial selective pressure. This review describes current understanding of antibiotic resistance (ABR) mechanisms in Mollicutes. Protective mechanisms in these bacteria include point mutations, which may include non-target genes, and unique gene exchange mechanisms, contributing to transfer of ABR genes. Better understanding of the mechanisms of emergence and dissemination of ABR in Mollicutes is crucial to control these hypermutable bacteria and prevent the occurrence of highly ABR strains.

Population structure and antimicrobial susceptibility of Mycoplasma ovipneumoniae isolates in France

Chronic non-progressive pneumonia in small ruminants caused by Mycoplasma (M.) ovipneumoniae is mainly controlled by chemotherapy. In France, during the last decade, a rise in M. ovipneumoniae cases was recorded in both sheep and goats, suggesting a possible emergence. Whether this rise is associated with antimicrobial resistance, as observed in other ruminant Mycoplasma species, has yet to be examined. The aim of the study was to characterize the diversity of M. ovipneumoniae strains circulating in France and assess their antimicrobial resistance, together with the underlying mechanisms, to help find an explanation for the increase in reported cases. The genetic diversity of 56 strains isolated between 2007 and 2018 from sheep and goats was assessed using different subtyping methods. Their susceptibility to six antimicrobial classes was profiled by estimating Minimum Inhibitory Concentrations (MICs) using an optimised agar dilution method. Resistance mechanisms were explored by sequence analysis of rRNA targets. A high genetic diversity of strains was evidenced, with consistent, marked animal-host clustering in the Hsp70 gene and whole genome sequence phylogeny. No clonal evolution could thus account for putative emergence. Apart from florfenicol, MICs were low except for a few isolates with increased values for tetracyclines, macrolides and lincosamides. Hotspot mutations in the target ribosomal gene could explain increased tetracycline MICs. Other mechanisms are suspected for macrolide-lincosamide and florfenicol resistance. The emergence of M. ovipneumoniae is thus not related to any increase in resistance or to a clonal spread. Explanations may lie in breeding practices.

Mutations potentially associated with decreased susceptibility to fluoroquinolones, macrolides and lincomycin in Mycoplasma synoviae

Mycoplasma synoviae is one of the economically most significant avian Mycoplasma species. It can cause great financial losses to the poultry industry by inducing respiratory diseases, infectious synovitis, or eggshell apex abnormalities. There are different approaches to control M. synoviae infection. Although antimicrobial therapy cannot replace long-term solutions, like eradication and vaccination, this strategy can be effective in the short term, as adequate antibiotic treatment can relieve economic losses through the attenuation of clinical signs and reduction of transmission. Using broth microdilution method, minimal inhibitory concentration (MIC) values to fourteen antibiotics related to eight antimicrobial groups were determined in 96 M. synoviae strains. Whole genome sequencing and sequence analysis revealed mutations potentially associated with decreased susceptibility to fluoroquinolones, macrolides and lincomycin. Molecular markers responsible for the high MICs to fluoroquinolones were found in the gyrA, gyrB, parC and parE genes. Besides, single nucleotide polymorphisms identified in genes encoding the 23S rRNA were found to be responsible for high MICs to the 50S inhibitor macrolides and lincomycin, while amino acid change in the 50S ribosomal protein L22 could be associated with decreased susceptibility to macrolides. The revealed mutations can contribute to the extension of knowledge about the genetic background of antibiotic resistance in M. synoviae. Moreover, the explored potentially resistance-related mutations may serve as targets for molecular biological assays providing data of antibiotic susceptibility prior to the laborious and time-consuming isolation of M. synoviae strains.

Mycoplasma bovis in Spanish Cattle Herds: Two Groups of Multiresistant Isolates Predominate, with One Remaining Susceptible to Fluoroquinolones

Mycoplasma bovis is an important bovine pathogen causing pneumonia, mastitis, and arthritis and is responsible for major economic losses worldwide. In the absence of an efficient vaccine, control of M. bovis infections mainly relies on antimicrobial treatments, but resistance is reported in an increasing number of countries. To address the situation in Spain, M. bovis was searched in 436 samples collected from beef and dairy cattle (2016–2019) and 28% were positive. Single-locus typing using polC sequences further revealed that two subtypes ST2 and ST3, circulate in Spain both in beef and dairy cattle, regardless of the regions or the clinical signs. Monitoring of ST2 and ST3 isolates minimum inhibitory concentration (MIC) to a panel of antimicrobials revealed one major difference when using fluoroquinolones (FQL): ST2 is more susceptible than ST3. Accordingly, whole-genome sequencing (WGS) further identified mutations in the gyrA and parC regions, encoding quinolone resistance-determining regions (QRDR) only in ST3 isolates. This situation shows the capacity of ST3 to accumulate mutations in QRDR and might reflect the selective pressure imposed by the extensive use of these antimicrobials. MIC values and detection of mutations by WGS also showed that most Spanish isolates are resistant to macrolides, lincosamides, and tetracyclines. Valnemulin was the only one effective, at least in vitro, against both STs.

Bacteriophage-Insensitive Mutants of Antimicrobial-Resistant Salmonella Enterica are Altered in their Tetracycline Resistance and Virulence in Caco-2 Intestinal Cells

Bacteriophages have shown promise as therapeutic alternatives to antibiotics for the control of infectious bacteria, including the human pathogen Salmonella. However, the development of effective phage-based applications requires the elucidation of key interactions between phages and target hosts, particularly since host resistance to phage is inevitable. Little is known about the alteration of host phenotypes following the development of resistance to phage. The aim of this study is to evaluate the antibiotic susceptibility and virulence of a Salmonella isolate following the development of resistance to bacteriophage SI1. We observed enhanced susceptibility to tetracycline and decreased invasion capacity in a differentiated Caco-2 intestinal cell line. Whole genome sequence analysis revealed an array of mutations, most notably, truncations in vgrG1_2, a core gene involved in Type VI secretion and mutations in the lipopolysaccharide, thereby indicating the plausible attachment site of phage SI1. These findings shed light on understanding the underlying mechanism for phage immunity within the host. Importantly, we reveal an associated genetic cost to the bacterial host with developing resistance to phages. Taken together, these results will aid in advancing strategies to delay or eliminate the development of host resistance when designing informed phage-based antimicrobials.

Upregulation of PD-L1 Expression by Prostaglandin E2 and the Enhancement of IFN-γ by Anti-PD-L1 Antibody Combined With a COX-2 Inhibitor in Mycoplasma bovis Infection

Bovine mycoplasmosis caused by Mycoplasma bovis results in pneumonia and mastitis in cattle. We previously demonstrated that the programmed death 1 (PD-1)/PD-ligand 1 (PD-L1) pathway is involved in immune dysfunction during M. bovis infection and that prostaglandin E₂ (PGE₂) suppressed immune responses and upregulated PD-L1 expression in Johne's disease, a bacterial infection in cattle. In this study, we investigated the role of PGE₂ in immune dysfunction and the relationship between PGE₂ and the PD-1/PD-L1 pathway in M. bovis infection. In vitro stimulation with M. bovis upregulated the expressions of PGE₂ and PD-L1 presumably via Toll-like receptor 2 in bovine peripheral blood mononuclear cells (PBMCs). PGE₂ levels of peripheral blood in infected cattle were significantly increased compared with those in uninfected cattle. Remarkably, plasma PGE₂ levels were positively correlated with the proportions of PD-L1⁺ monocytes in M. bovis-infected cattle. Additionally, plasma PGE₂ production in infected cattle was negatively correlated with M. bovis-specific interferon (IFN)-γ production from PBMCs. These results suggest that PGE₂ could be one of the inducers of PD-L1 expression and could be involved in immunosuppression during M. bovis infection. In vitro blockade assays using anti-bovine PD-L1 antibody and a cyclooxygenase 2 inhibitor significantly upregulated the M. bovis-specific IFN-γ response. Our study findings might contribute to the development of novel therapeutic strategies for bovine mycoplasmosis that target PGE₂ and the PD-1/PD-L1 pathway.

Molecular basis of antimicrobial resistance in Mycoplasma genitalium

Mycoplasma genitalium is a sexually transmitted urogenital pathogen, and infection can result in serious symptoms. As M. genitalium is rather difficult to culture, infections are usually detected by molecular methods. Unfortunately, there has recently been a significant increase in resistance to azithromycin and moxifloxacin used for the treatment of M. genitalium infections. The increased resistance to (often empirically prescribed) M. genitalium treatments has resulted in frequent therapy failures and stresses the need for routine detection of antimicrobial resistance. In M. genitalium, antimicrobial resistance is almost always the result of DNA mutations and thus can easily be detected by molecular techniques. Regrettably, many microbiology laboratories do not use molecular techniques for the detection of bacterial antimicrobial resistance. As molecular tests are becoming available for M. genitalium, both for the establishment of infection and the detection of antimicrobial resistance, it is now more important to ensure that knowledge on the resistance mechanisms is transferred from the laboratory to the clinician. This review will provide a brief summary of the current status of antimicrobial resistance, its molecular mechanisms and the impact on the current status of M. genitalium treatment.

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.

Microcolonies: a novel morphological form of pathogenic Mycoplasma spp

Introduction. The Mollicutes class unites cell wall lacking bacteria many of which are membrane parasites and opportunistic bacteria.Aim. This study describes a novel morphological form found in the five species belonging to the bacterial class Mollicutes, and referred to as microcolonies (MCs).Methodology. MCs were obtained as described below and characterized with bacteriological and immunological methods, and microscopy.Results. In contrast to typical colonies (TCs), MCs are characterized by tiny propeller-shaped colonies formed by rod-like cells tightly packed in parallel rows. These colonies were observed within routinely cultivated cultures of type strains 7-12 days post-plating. Rod-like cells were visualized using a scanning electron microscope within TCs with a 'fried-egg-like' appearance. MCs were not observed to revert to TCs. MCs were resistant to antibiotics and other treatments effective against TCs. Pure MC cultures were generated in vitro by treatment of Mycoplasma cultures with hyperimmune serum, antibiotics or argon non-thermal plasma. MCs of Mycoplasma hominis strain H-34 were characterized in detail to confirm that they belonged to that species. MCs tested positive via PCR with M. hominis-specific primers, direct fluorescence and epifluorescence tests, and Western blotting with the camel-derived nanobody aMh-FcG2a, which is specific to the MH3620 transporter protein. Meanwhile, MCs behaved differently in standard bacteriological tests. Pure MC cultures were also isolated directly from clinical samples of the serum, synovial liquid and urine of patients within flammatory urogenital tract diseases, asthma or arthritis. In total, 79 independent MC cultures were isolated from clinical samples including M. hominis (n=70), Mycoplasma pneumoniae (n=2), Mycoplasma fermentans (n=2) and Mycoplasma spp. (n=5).Conclusion. MCs play an unknown role in infection pathology and display prominent antibiotic resistance, making them a challenge for the future studies on Mollicutes.

New antimicrobial susceptibility data from monitoring of Mycoplasma bovis isolated in Europe

Mycoplasma bovis is an important respiratory pathogen of cattle across Europe and is included in the MycoPath pan-European antimicrobial susceptibility monitoring programme. M. bovis strains (232) were isolated from cattle, not recently treated with antimicrobials, at diverse geographical locations in France, Great Britain, Hungary, Italy and Spain during 2014 to 2016. Only one isolate per farm and per outbreak was retained. For each isolate, the MICs of ten antimicrobials were determined in a central laboratory using a broth microdilution method with modified Eaton's medium and incubation at 35 °C ± 1 °C for 24 ± 6 h. MIC₅₀/MIC₉₀ (mg/L) values for the 232 strains were: danofloxacin 0.25/1; enrofloxacin 0.5/8; marbofloxacin 1/4; gamithromycin >64/>64; spiramycin 8/16; tilmicosin >64/>64; tulathromycin >64/>64; tylosin 64/>64; florfenicol 4/8; oxytetracycline 8/32. Minor between-country differences in the MIC₉₀ values were observed for the fluoroquinolones, spiramycin and oxytetracycline, whilst the MIC values for the other compounds were similar. Spain and Italy had the higher MIC₉₀ values for the fluoroquinolones. Compared with the 2010-2012 study (156 isolates) results are similar, with an overall MIC₅₀ increase of at most one doubling dilution for enrofloxacin, spiramycin, tylosin, florfenicol and oxytetracycline. In contrast, the MIC₉₀ value for oxytetracycline decreased from >64 to 32 mg/L. Standardized laboratory methods and interpretive criteria for MIC testing of veterinary mycoplasmas are clearly needed; there are currently no clinical breakpoints available to facilitate data interpretation and correlation of MICs with in vivo efficacy.

Antimicrobial resistance in mollicutes: known and newly emerging mechanisms

This review is devoted to the mechanisms of antibiotic resistance in mollicutes (class Bacilli, subclass Mollicutes), the smallest self-replicating bacteria, that can cause diseases in plants, animals and humans, and also contaminate cell cultures and vaccine preparations. Research in this area has been mainly based on the ubiquitous mollicute and the main contaminant of cell cultures, Acholeplasma laidlawii. The omics technologies applied to this and other bacteria have yielded a complex picture of responses to antimicrobials, including their removal from the cell, the acquisition of antibiotic resistance genes and mutations that potentially allow global reprogramming of many cellular processes. This review provides a brief summary of well-known resistance mechanisms that have been demonstrated in several mollicutes species and, in more detail, novel mechanisms revealed in A. laidlawii, including the least explored vesicle-mediated transfer of short RNAs with a regulatory potency. We hope that this review highlights new avenues for further studies on antimicrobial resistance in these bacteria for both a basic science and an application perspective of infection control and management in clinical and research/production settings.

Relationship between Antimicrobial Susceptibility and Multilocus Sequence Type of Mycoplasma bovis Isolates and Development of a Method for Rapid Detection of Point Mutations Involved in Decreased Susceptibility to Macrolides, Lincosamides, Tetracyclines, and Spectinomycin

Mycoplasma bovis isolates belonging to the sequence type 5 (ST5) group, the dominant group in Japan since 1999, were low susceptible to 16-membered macrolides and tetracyclines and were confirmed to have a guanine-to-adenine transition mutation at position 748 in the 23S rRNA gene (rrl) and adenine-to-thymine transversion mutations at positions 965 and 967 in the 16S rRNA gene (rrs) (Escherichia coli numbering). Moreover, isolates of ST93 and ST155, members of the ST5 group, were low susceptible to lincosamides and azithromycin and showed an adenine-to-guanine transition mutation at position 2059 of rrl Isolates of ST93 were additionally low susceptible to spectinomycin and showed a cytosine-to-adenine transversion mutation at position 1192 of rrs Strains of the ST5 group seem to spread to Japan and Europe from North America with imported cows, while strains of ST93 and ST155 originated in Japan. Melting curve analysis using hybridization probes revealed the existence of point mutations involved in decreased susceptibility to macrolides, lincosamides, and spectinomycin, as demonstrated by changes in the melting curve shape and/or decreases in the melting peak temperature, so the susceptibility to these antimicrobials can be assessed on the same day. For decreased susceptibility to fluoroquinolones to exist, nonsynonymous mutations in the DNA gyrase gene (gyrA) and topoisomerase IV gene (parC) had to coexist. The combination of amino acid substitutions of serine at position 83 in gyrA and serine at position 80 in parC resulted in particularly low susceptibility to fluoroquinolones.IMPORTANCE Mycoplasma bovis is the main causal species of bovine mycoplasmal disease and leads to significant economic losses because of its severe symptoms, strong infectivity, and refractoriness. As for mastitis, culling cows with intramammary infections is a general countermeasure to prevent spreading. The conventional antimicrobial susceptibility test for mycoplasma is time-consuming and troublesome, but no quick and easy method for grasping the antimicrobial susceptibility of the causal strain exists at present. Treatment without antimicrobial susceptibility information may be one reason why M. bovis infection is refractory. Detecting a mutation involved in decreased susceptibility to antimicrobial agents of the causal strain makes it possible to easily select suitable antimicrobials for treatment, and this technique will help improve the cure rate and prevent the overuse of ineffective antimicrobial agents. In this study, we developed a technique to quickly and easily assess antimicrobial susceptibility based on the genetic characteristics of M. bovis strains in Japan.

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.

Changes in antimicrobial susceptibility profiles of Mycoplasma bovis over time

Mycoplasma bovis is a major cause of pneumonia, arthritis, and mastitis in cattle and can lead to significant economic losses. Antimicrobial resistance is a concern and further limits the already short list of drugs effective against mycoplasmas. The objective of this study was to examine changes in in vitro minimum inhibitory concentrations (MICs) of antimicrobials of aminoglycoside, fluoroquinolone, lincosamide, macrolide, pleuromutilin, phenicol, and tetracycline classes for 210 M. bovis isolates collected from 1978 to 2009. The MIC₅₀ values of the various antimicrobials were also compared. The MIC₅₀ levels for enrofloxacin and danofloxacin remained low (0.25 μg/mL) across all 3 decades. MIC₅₀ levels for tetracyclines, tilmicosin, and tylosin tartrate were low in the 1980s, then increased in the 1990s and remained high. In the 1980s, MIC₅₀ levels were low for clindamycin, spectinomycin, and tulathromycin, increased in the 1990s to 8 μg/mL (clindamycin) and 32 μg/mL (spectinomycin and tulathromycin), then decreased again in the 2000s. Members of the fluoroquinolone class of antimicrobials had the lowest MIC₅₀ levels across all 3 decades, which suggests in vitro susceptibility of M. bovis to this class of antimicrobials. Statistically significant associations were observed between MIC values for chlortetracycline, oxytetracycline, tylosin tartrate, and tilmicosin; between clindamycin, tulathromycin, spectinomycin, and tiamulin; and between tylosin tartrate and clindamycin. Changes in MIC levels of various antimicrobials over time show the importance of monitoring the susceptibility of mycoplasmas to antimicrobials. The number of antimicrobials that showed elevated MIC₅₀ levels, and therefore possibly reduced in vitro effectiveness against M. bovis, supports initiatives that promote prudent use of antimicrobials in agriculture.

Comprehensive screening of antimicrobials to control phytoplasma diseases using an in vitro plant-phytoplasma co-culture system

Phytoplasmas are plant-pathogenic bacteria that infect many important crops and cause serious economic losses worldwide. However, owing to an inability to culture phytoplasmas, screening of antimicrobials on media is difficult. The only antimicrobials being used to control phytoplasmas are tetracycline-class antibiotics. In this study, we developed an accurate and efficient screening method to evaluate the effects of antimicrobials using an in vitro plant-phytoplasma co-culture system. We tested 40 antimicrobials, in addition to tetracycline, and four of these (doxycycline, chloramphenicol, thiamphenicol and rifampicin) decreased the accumulation of 'Candidatus (Ca.) Phytoplasma asteris'. The phytoplasma was eliminated from infected plants by the application of both tetracycline and rifampicin. We also compared nucleotide sequences of rRNAs and amino acid sequences of proteins targeted by antimicrobials between phytoplasmas and other bacteria. Since antimicrobial target sequences were conserved among various phytoplasma species, the antimicrobials that decreased accumulation of 'Ca. P. asteris' may also have been effective against other phytoplasma species. These approaches will provide new strategies for phytoplasma disease management.

The moderate drift towards less tetracycline-susceptible isolates of contagious agalactia causative agents might result from different molecular mechanisms

Contagious agalactia is a mycoplasmosis that affects small ruminants, is associated with loss of milk production and high morbidity rates, and is highly deleterious to dairy industries. The etiological agents are four mycoplasma (sub)species, of which the relative importance depends on the countries and the animal host. Tetracyclines are non-expensive, broad-spectrum antimicrobials and are often used to control mastitis in dairy herds. However, the in vitro efficiency of tetracyclines against each of the etiological agents of contagious agalactia has been poorly assessed. The aims of this study were i) to compare the tetracycline susceptibilities of various field isolates, belonging to different mycoplasma (sub)species and subtypes, collected over the years from different clinical contexts in France or Spain, and ii) to investigate the molecular mechanisms behind the decreased susceptibility of some isolates to tetracyclines. The Minimum Inhibitory Concentrations (MICs) of tetracyclines were determined in vitro on a set of 120 isolates. Statistical analyses were run to define the significance of any observed differences in MICs distribution. As mutations in the genes encoding the tetracycline targets (rrs loci) are most often associated with increased tetracycline MICs in animal mycoplasmas, these genes were sequenced. The loss of susceptibility to tetracyclines after year 2010 is not significant and recent MICs are higher in M. agalactiae, especially isolates from mastitis cases, than in other etiological agents of contagious agalactia. The observed increases in MICs were not always associated with mutations in the rrs alleles which suggests the existence of other resistance mechanisms yet to be deciphered.

Development of molecular methods for the rapid detection of antibiotic susceptibility of Mycoplasma bovis

Determining the antibiotic susceptibility profile of Mycoplasma bovis isolates in vitro provides the basis for the appropriate choice of antibiotics in the therapy. Traditionally, the antibiotic susceptibility examination of mycoplasmas is technically demanding, time-consuming and rarely performed in diagnostic laboratories. The aim of the present study was to develop rapid molecular assays to determine mutations responsible for elevated minimal inhibitory concentrations (MICs) to fluoroquinolones, tetracyclines, aminocyclitols, macrolides, lincosamides, phenicols and pleuromutilins in M. bovis. The nine mismatch amplification mutation assays (MAMA) and seven high resolution melt (HRM) tests designed in the present study enable the simultaneous detection of these genetic markers. The sensitivity of the assays varied between 10²-10⁵ copy numbers/reaction. Cross-reactions with other mycoplasmas occurring in cattle were detected in assays targeting universal regions (e.g. 16S rRNA). Nevertheless, results of the novel method were in accordance with sequence and MICs data of the M. bovis pure cultures. Also, the tests of clinical samples containing high amount of M. bovis DNA were congruent even in the presence of other Mycoplasma spp. The presented method is highly cost-effective and can provide an antibiogram to 12 antibiotics in approximately 3-4 days when previous isolation of M. bovis is applied. In order to assure the proper identification of the genetic markers at issue, the regions examined by the MAMA and HRM tests are overlapping. In conclusion, the developed assays have potential to be used in routine diagnostics for the detection of antibiotic susceptibility in M. bovis.

Mutations Associated with Decreased Susceptibility to Seven Antimicrobial Families in Field and Laboratory-Derived Mycoplasma bovis Strains

The molecular mechanisms of resistance to fluoroquinolones, tetracyclines, an aminocyclitol, macrolides, a lincosamide, a phenicol, and pleuromutilins were investigated in Mycoplasma bovis For the identification of mutations responsible for the high MICs of certain antibiotics, whole-genome sequencing of 35 M. bovis field isolates and 36 laboratory-derived antibiotic-resistant mutants was performed. In vitro resistant mutants were selected by serial passages of M. bovis in broth medium containing subinhibitory concentrations of the antibiotics. Mutations associated with high fluoroquinolones MICs were found at positions 244 to 260 and at positions 232 to 250 (according to Escherichia coli numbering) of the quinolone resistance-determining regions of the gyrA and parC genes, respectively. Alterations related to elevated tetracycline MICs were described at positions 962 to 967, 1058, 1195, 1196, and 1199 of genes encoding the 16S rRNA and forming the primary tetracycline binding site. Single transversion at position 1192 of the rrs1 gene resulted in a spectinomycin MIC of 256 μg/ml. Mutations responsible for high macrolide, lincomycin, florfenicol, and pleuromutilin antibiotic MICs were identified in genes encoding 23S rRNA. Understanding antibiotic resistance mechanisms is an important tool for future developments of genetic-based diagnostic assays for the rapid detection of resistant M. bovis strains.

High Prevalence of Diverse Insertion Sequences within the rRNA Operons of Mycoplasma bovis

Insertion sequences (ISs) are widespread in the genome of Mycoplasma bovis strain PG45, but no ISs were identified within its two tandemly positioned rRNA operons (rrn1 and rrn2). However, characterization of the rrn locus in 70 M. bovis isolates revealed the presence of ISs related to the ISMbov1 (IS30 family) and ISMbov4 (IS4 family) isomers in 35 isolates. ISs were inserted into intergenic region 1 (IGR-1) or IGR-3, which are the putative promoter regions of rrn1 and rrn2, respectively, and into IGR-5, located downstream of the rrl2 gene. Seven different configurations (A to G) of the rrn locus with respect to ISs were identified, including those in five annotated genomes. The transcriptional start site for the single rrn operon in M. bovis strain 88127 was mapped within IGR-1, 60 bp upstream of the rrs gene. Notably, only 1 nucleotide separated the direct repeat (DR) for ISMbov1 and the promoter -35 element in configuration D, while in configuration F, the -35 motif was a part of the ISMbov1 DR. Relative quantitative real-time (qRT) PCR analysis and growth rate comparisons detected a significant increase (P < 0.05) in the expression of the rrs genes and in the number of viable cells during log phase growth (8, 12, and 16 h) in the strains with configuration F in comparison to strains with one or two rrn operons that did not have ISs. A high prevalence of IS elements within or close to the M. bovis rrn operon-promoter region may reflect their important role in regulation of both ribosome synthesis and function.

IMPORTANCE

Data presented in this study show a high prevalence of diverse ISs within the M. bovis rrn locus resulting in intraspecies variability and diversity. Such abundance of IS elements near or within the rrn locus may offer a selective advantage to M. bovis Moreover, the fact that expression of the rrs genes as well as the number of viable cells increased in the group of strains with IS element insertion within a putative promoter -35 sequence (configuration F) in comparison to that in strains with one or two rrn operons that do not have ISs may serve as a basis for understanding the possible role of M. bovis IS elements in fundamental biological processes such as regulation of ribosome synthesis and function.

The Novel Aminomethylcycline Omadacycline Has High Specificity for the Primary Tetracycline-Binding Site on the Bacterial Ribosome

Omadacycline is an aminomethylcycline antibiotic with potent activity against many Gram-positive and Gram-negative pathogens, including strains carrying the major efflux and ribosome protection resistance determinants. This makes it a promising candidate for therapy of severe infectious diseases. Omadacycline inhibits bacterial protein biosynthesis and competes with tetracycline for binding to the ribosome. Its interactions with the 70S ribosome were, therefore, analyzed in great detail and compared with tigecycline and tetracycline. All three antibiotics are inhibited by mutations in the 16S rRNA that mediate resistance to tetracycline in Brachyspira hyodysenteriae, Helicobacter pylori, Mycoplasma hominis, and Propionibacterium acnes. Chemical probing with dimethyl sulfate and Fenton cleavage with iron(II)-complexes of the tetracycline derivatives revealed that each antibiotic interacts in an idiosyncratic manner with the ribosome. X-ray crystallography had previously revealed one primary binding site for tetracycline on the ribosome and up to five secondary sites. All tetracyclines analyzed here interact with the primary site and tetracycline also with two secondary sites. In addition, each derivative displays a unique set of non-specific interactions with the 16S rRNA.

Mycoplasma bovis: Mechanisms of Resistance and Trends in Antimicrobial Susceptibility

Mycoplasma bovis is a cell-wall-less bacterium and belongs to the class Mollicutes. It is the most important etiological agent of bovine mycoplasmoses in North America and Europe, causing respiratory disease, mastitis, otitis media, arthritis, and reproductive disease. Clinical disease associated with M. bovis is often chronic, debilitating, and poorly responsive to antimicrobial therapy, resulting in significant economic loss, the full extent of which is difficult to estimate. Until M. bovis vaccines are universally available, sanitary control measures and antimicrobial treatment are the only approaches that can be used in attempts to control M. bovis infections. However, in vitro studies show that many of the current M. bovis isolates circulating in Europe have high minimum inhibitory concentrations (MIC) for many of the commercially available antimicrobials. In this review we summarize the current MIC trends indicating the development of antimicrobial resistance in M. bovis as well as the known molecular mechanisms by which resistance is acquired.

Tetracycline Antibiotics and Resistance

Tetracyclines possess many properties considered ideal for antibiotic drugs, including activity against Gram-positive and -negative pathogens, proven clinical safety, acceptable tolerability, and the availability of intravenous (IV) and oral formulations for most members of the class. As with all antibiotic classes, the antimicrobial activities of tetracyclines are subject to both class-specific and intrinsic antibiotic-resistance mechanisms. Since the discovery of the first tetracyclines more than 60 years ago, ongoing optimization of the core scaffold has produced tetracyclines in clinical use and development that are capable of thwarting many of these resistance mechanisms. New chemistry approaches have enabled the creation of synthetic derivatives with improved in vitro potency and in vivo efficacy, ensuring that the full potential of the class can be explored for use against current and emerging multidrug-resistant (MDR) pathogens, including carbapenem-resistant Enterobacteriaceae, MDR Acinetobacter species, and Pseudomonas aeruginosa.

In vitro antimicrobial susceptibility of Mycoplasma bovis clinical isolates recovered from bison (Bison bison)

Mycoplasma bovis is a pathogen globally affecting cattle and bison herds, causing pneumonia, arthritis, mastitis, abortions, and other symptoms, leading to huge economic losses. Many studies have been done regarding the antimicrobial susceptibility of M. bovis isolated from cattle, but no such study is available for isolates recovered from bison. For the first time, in vitro susceptibilities of 40 M. bovis clinical isolates collected from bison herds in Canada are reported here. Minimal inhibitory concentration (MIC) values were determined using Sensititre® plates. The most effective MIC50 and MIC90 were for spectinomycin (1 and >64 μg/mL), tiamulin (1 and >32 μg/mL), and tulathromycin (16 and 64 μg/mL), whereas tetracyclines, fluoroquinolones, and florfenicol failed to inhibit growth of M. bovis bison isolates. Isolates were nonsusceptible to tetracyclines (100%), fluoroquinolones (97.5%), and tilmicosin (100%), whereas the highest susceptibility of bison clinical isolates was seen with spectinomycin (95%) and tulathromycin (67.5%). Two lung isolates (Mb283 and 348) were found resistant to both spectinomycin and tulathromycin. These results show a marked difference in antimicrobial susceptibility of bison isolates as compared with previously reported and laboratory reference cattle isolates, emphasizing the necessity of testing antimicrobial susceptibility of M. bovis bison isolates and to generate better therapeutic regime for improved recovery chances for infected bison herds across North America.