Macrolides and lincomycin susceptibility of Mycoplasma hyorhinis and variable mutation of domain II and V in 23S ribosomal RNA

Evaluation of a Method for Standardized Antimicrobial Susceptibility Testing with Mycoplasma hyorhinis Field Isolates

Organizations like the Clinical and Laboratory Standards Institute (CLSI) or the European Committee of Antimicrobial Susceptibility Testing (EUCAST) provide standardized methodologies for antimicrobial susceptibility testing of a wide range of nonfastidious and fastidious bacteria, but so far not for Mycoplasma spp. of animal origin. Recently, a proposed method for the standardized broth microdilution testing of Mycoplasma hyorhinis using commercial Sensititre microtiter plates was presented. In this study, we evaluated this broth microdilution method with 37 field isolates and tested their susceptibility toward the following antimicrobial agents: doxycycline, enrofloxacin, erythromycin, florfenicol, gentamicin, marbofloxacin, tetracycline, tiamulin, tilmicosin, tulathromycin, and tylosin. The isolates originated from different countries, isolation sites, and years. The broth microdilution method was carried out using a modified Friis broth as the culture and test medium. For macrolides and lincosamides, a bimodal distribution with elevated MIC values could be observed for almost half of the tested field isolates, deducing reduced susceptibility toward these substances. With a recently published protocol, we were able to test a variety of field isolates, and consistent data could be obtained. Using this method, monitoring studies of Mycoplasma hyorhinis isolates can be carried out in a comparable manner, and the observed susceptibility profiles can be screened for possible changes in MIC values in the future.

Towards a Standardized Antimicrobial Susceptibility Testing Method for Mycoplasma hyorhinis

Conducting antimicrobial susceptibility testing (AST) in a comparable manner requires the availability of a standardized method. Organizations, such as the Clinical and Laboratory Standards Institute (CLSI) or the European Committee on Antimicrobial Susceptibility Testing (EUCAST), provide standardized protocols for a range of fastidious bacteria but not for Mycoplasma hyorhinis. We developed a broth microdilution method for testing M. hyorhinis in a standardized and harmonized way using a modified Friis broth devoid of antimicrobial or otherwise bacterial growth-inhibiting agents. The type strain M. hyorhinis DSM 25591 was chosen to establish the methodology. The antimicrobial agents of interest were doxycycline, enrofloxacin, erythromycin, florfenicol, gentamicin, marbofloxacin, tetracycline, tiamulin, tilmicosin, tulathromycin, and tylosin, tested by using commercial Sensititre^TM microtiter plates. In addition, the suitability of the methodology was evaluated via variation of the individual ingredients of the modified Friis broth by either using different batches or choosing other distributors. Despite these alterations, the method provided reliable results. We obtained repeatable minimal inhibitory concentrations for all six tested field isolates and the M. hyorhinis type strain. With this newly proposed method, we aim to provide an improved AST method for diagnostic laboratories and monitoring purposes with better comparability between times and countries. In addition, this new method will allow for an improvement of targeted treatments using antimicrobial agents and thereby reduce the options for resistance development.

Antimicrobial susceptibility profiles of Mycoplasma hyorhinis strains isolated from five European countries between 2019 and 2021

Mycoplasma hyorhinis is an emerging swine pathogen bacterium causing polyserositis and polyarthritis in weaners and finishers. The pathogen is distributed world-wide, generating significant economic losses. No commercially available vaccine is available in Europe. Therefore, besides improving the housing conditions for prevention, antimicrobial therapy of the diseased animals is the only option to control the infection. Our aim was to determine the minimal inhibitory concentrations (MIC) of ten antimicrobials potentially used against M. hyorhinis infection. The antibiotic susceptibility of 76 M. hyorhinis isolates from Belgium, Germany, Hungary, Italy and Poland collected between 2019 and 2021 was determined by broth micro-dilution method and mismatch amplification mutation assay (MAMA). Low concentrations of tiamulin (MIC₉₀ 0.312 μg/ml), doxycycline (MIC₉₀ 0.078 μg/ml), oxytetracycline (MIC₉₀ 0.25 μg/ml), florfenicol (MIC₉₀ 2 μg/ml) and moderate concentrations of enrofloxacin (MIC₉₀ 1.25 μg/ml) inhibited the growth of the isolates. For the tested macrolides and lincomycin, a bimodal MIC pattern was observed (MIC₉₀ >64 μg/ml for lincomycin, tulathromycin, tylosin and tilmicosin and 5 μg/ml for tylvalosin). The results of the MAMA assay were in line with the conventional method with three exceptions. Based on our statistical analyses, significant differences in MIC values of tiamulin and doxycycline were observed between certain countries. Our results show various levels of antimicrobial susceptibility among M. hyorhinis isolates to the tested antibiotics. The data underline the importance of susceptibility monitoring on pan-European level and provides essential information for proper antibiotic choice in therapy.

Provisional Use of CLSI-Approved Quality Control Strains for Antimicrobial Susceptibility Testing of Mycoplasma ('Mesomycoplasma') hyorhinis

Antimicrobial susceptibility testing (AST) should be conducted in a standardized manner prior to the start of an antimicrobial treatment. For fastidious bacteria, such as porcine Mycoplasma ('Mesomycoplasma') spp., specifically M. hyorhinis, neither guidelines or standards for the performance of AST, nor quality control strains for the validation of AST results are approved by organizations like the Clinical and Laboratory Standards Institute (CLSI) or the European Committee on Antimicrobial Susceptibility Testing (EUCAST). The CLSI- and EUCAST-approved quality control strains Enterococcus faecalis ATCC 29212 and Staphylococcus aureus ATCC 29213 were chosen to validate AST by broth microdilution using modified Friis broth, developed as growth medium for porcine Mycoplasma ('Mesomycoplasma') spp. The antimicrobial agents doxycycline, enrofloxacin, erythromycin, florfenicol, gentamicin, marbofloxacin, tetracycline, tiamulin, tilmicosin, tulathromycin, and tylosin were examined using customized Sensititre^TM microtiter plates. Minimal inhibitory concentrations, determined after 24, 48, and 72 h, were mostly within the CLSI-approved quality control ranges for defined antimicrobial agents. We propose the use of the combination of E. faecalis ATCC 29212 and S. aureus ATCC 29213 as surrogate quality control strains for the validation of future AST results obtained for M. hyorhinis by broth microdilution using modified Friis broth.

Development of a molecular biological assay for the detection of markers related to decreased susceptibility to macrolides and lincomycin in Mycoplasma hyorhinis

The control of Mycoplasma hyorhinis infection relies mainly on antimicrobial therapy. However, the antibiotic susceptibility testing of the bacteria is usually not performed before applying the treatment, and thus therapeutic failures are not uncommon. In the case of M. hyorhinis, several antibiotic-resistance-related single nucleotide polymorphisms (SNPs) are known but assays for their detection have not been described yet. The aims of the present study were to investigate macrolide- and lincomycin-resistance-related SNPs in Hungarian M. hyorhinis isolates and to develop mismatch amplification mutation assays (MAMA) to detect the identified resistance markers. Minimal inhibitory concentrations (MIC) of different drugs and whole genome sequences of 37 M. hyorhinis isolates were used to find the resistance-related mutations. One MAMA assay was designed to detect the mutation of the 23S rRNA gene at nucleotide position 2058 (Escherichia coli numbering). For further evaluation, the assay was challenged with 17 additional isolates with available MIC data and 15 DNA samples from clinical specimens. The genotypes of the samples were in line with the MIC test results. The developed assay supports the practice of targeted antibiotic usage; hence it may indirectly reduce some bacterial resistance-related public health concerns.

Whole-Genome Sequence of the Mycoplasma (Mesomycoplasma) hyorhinis DSM 25591 Type Strain

The whole-genome sequence of the type strain Mycoplasma (“Mesomycoplasma”) hyorhinis DSM 25591 is reported and compared to the available sequences of the corresponding type strains from other strain collections to ascertain conformity. Knowledge of the identity of type strains is of importance for their application in standardized test systems.

The whole-genome sequence of the type strain Mycoplasma (Mesomycoplasma) hyorhinis DSM 25591 is reported and compared to the available sequences of the corresponding type strains from other strain collections to ascertain conformity. Knowledge of the identity of type strains is of importance for their application in standardized test systems.

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.

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.

Mutations associated with change of susceptibility to lincosamides and/or macrolides in field and laboratory-derived Mycoplasma californicum strains in Japan, and development of a rapid detection method for these mutations

Five mutations involved in changing of susceptibility to lincosamides and/or macrolides were investigated in field isolates of Mycoplasma californicum in Japan, and reconfirmed in laboratory-derived mutants. In addition, a quick and easy detection method for these mutations was established. Guanine at position 748 (Escherichia coli numbering) of the 23S rRNA gene (rrl) was shown to be involved with decreased susceptibility to 16-membered macrolides, and adenines at positions 2059 and 2062 of rrl were involved with decreased susceptibility to both lincosamides and macrolides. Both guanine at position 2576, and change from cytosine to thymine at position 2611 of rrl were found to be involved with decreased susceptibility to lincosamides, and the latter mutation also increased the susceptibility to erythromycin. These mutations were easily induced by several to approximately 30 passages in a medium containing the respective antimicrobial, but they did not return after their initial appearance. The melting curve analysis using hybridization probes revealed the existence of these mutations by the change in the melting curve shape and/or decrease in the melting peak temperature. The detection limit in milk samples with a somatic cell count up to 716 × 10³ cell/mL was 133 cfu/mL, but an excessive increase in the cell count in milk or storage of the milk sample at chilling or freezing temperature decreased the sensitivity. This method requires only a few hours, so field veterinarians can make a same-day determination of susceptibility to macrolides and lincosamides, which are first-line antibiotics for bovine mycoplasmal mastitis.

Antibiotic susceptibility profiles of Mycoplasma hyorhinis strains isolated from swine in Hungary

Mycoplasma hyorhinis is a common pathogen of swine causing mainly polyserositis and arthritis, but it has also been implicated as a cause of pneumonia. The economic losses due to M. hyorhinis infection could be reduced by antibiotic treatment. The aim of this study was to determine minimal inhibitory concentrations (MIC) of antibiotics potentially used to combat M. hyorhinis in swine production. Thirty-eight Hungarian M. hyorhinis strains isolated between 2014 and 2017 were examined by microbroth dilution tests for fifteen antimicrobial agents. Low MIC values of tetracyclines (MIC₅₀ 0.078 μg/ml for doxycycline, ≤0.25 μg/ml for oxytetracycline) and pleuromutilins (MIC₅₀ 0.156 μg/ml for tiamulin, ≤0.039 μg/ml for valnemulin) were detected against all strains. Fluoroquinolones (MIC₅₀ 0.625 μg/ml), gentamicin (MIC₅₀ 1 μg/ml) and florfenicol (MIC₅₀ 2 μg/ml) inhibited the growth of Hungarian isolates at moderate MIC values. Most of the strains were inhibited by spectinomycin with low or moderate MIC values (MIC₅₀ 4 μg/ml) except one strain (>64 μg/ml). Numerous isolates showed decreased susceptibility to macrolides and lincomycin (MIC₉₀ >64 for tylosin, tilmicosin, tulathromycin, gamithromycin, lincomycin, 8 μg/ml for tylvalosin). This study serves as evidence for the increasing resistance to macrolides and lincomycin in mycoplasmas, and also reports the occurrence of strains with extremely high MIC values to spectinomycin thus emphasizes the importance of the prudent use of antibiotics. Based on our results, tetracyclines and pleuromutilins are the most active compounds in vitro against the Hungarian M. hyorhinis strains.

Mycoplasma bovis isolates from dairy calves in Japan have less susceptibility than a reference strain to all approved macrolides associated with a point mutation (G748A) combined with multiple species-specific nucleotide alterations in 23S rRNA

Erythromycin, tylosin and tilmicosin are approved for use in cattle in Japan, the latter two being used to treat Mycoplasma bovis infection. In this study, 58 M. bovis isolates obtained from Japanese dairy calves all exhibited reduced susceptibility to these macrolides, this widespread reduced susceptibility being attributable to a few dominant lineages. All 58 isolates contained the G748A variant in both the rrl3 and rrl4 alleles of 23S rRNA, whereas a reference strain (PG45) did not. G748 localizes in the central loop of domain II (from C744 to A753) of 23S rRNA, which participates in binding to mycinose, a sugar residue present in both tylosin and tilmicosin. A number of in vitro-selected mutants derived from M. bovis PG45 showed reduced susceptibility to tylosin and tilmicosin and contained a nucleotide insertion within the central loop of domain II of rrl3 (U747-G748Ins_CU/GU or A743-U744Ins_UA), suggesting that mutations around G748 confer this reduced susceptibility phenotype. However, other Mycoplasma species containing G748A were susceptible to tylosin and tilmicosin. Sequence comparison with Escherichia coli revealed that M. bovis PG45 and isolates harbored five nucleotide alterations (U744C, G745A, U746C, A752C and A753G) in the central loop of domain II of 23S rRNA, whereas other Mycoplasma species lacked at least two of these five nucleotide alterations. It was therefore concluded that G748 mutations in combination with species-specific nucleotide alterations in the central loop of domain II of 23S rRNA are likely sufficient to reduce susceptibility of M. bovis to tylosin and tilmicosin.

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.

Bovine mastitis outbreak in Japan caused by methicillin-resistant Staphylococcus aureus New York/Japan clone

Many methicillin-resistant Staphylococcus aureus (MRSA) strains are multidrug-resistant; consequently, infectious diseases involving MRSA are recognized as troublesome diseases not only in human health care but also in animal health care. A bovine mastitis case caused by MRSA isolates of the New York/Japan clone (NJC), which occurred in Japan in 2005, was monitored in the current study. Isolates of the NJC are typical of hospital-acquired MRSA in Japan. The genetic backgrounds of these strains differ from those of bovine-associated S. aureus, which are typically of clonal complex (CC)97, CC705, and CC133. Moreover, the NJC isolates in this bovine outbreak possessed a β-hemolysin-converting bacteriophage and an immune evasion cluster, as found in the NJC isolates from humans, so it is possible that this clone was introduced into the dairy herd by a human carrier. Most bovine intramammary infections (IMIs) caused by the NJC isolates in our study were asymptomatic, and obvious clinical signs were recognized in only the first 3 infected cows. Of a total of 78 cows, 31 cows were MRSA carriers, and these carrier cows were detected by testing the milk of all lactating cows at 1-month intervals. These S. aureus carrier cows were culled or the infected quarter was dried off and no longer milked. Both IMI and mastitis caused by MRSA were completely eradicated after 5 months. Genotyping data suggested that exchanging of the staphylococcal cassette chromosome mec (the determining factor in methicillin resistance) occurred easily between MRSA and methicillin-sensitive S. aureus in the udders of carrier cows. This case study demonstrates an effective procedure against the spread of MRSA in a dairy herd, and highlights the risk of emergence of new MRSA strains in a dairy herd.

Decontamination of mycoplasma-contaminated Orientia tsutsugamushi strains by repeating passages through cell cultures with antibiotics

Background

Mycoplasmas-contamination of Orientia tsutsugamushi, one of the obligated intracellular bacteria, is a very serious problem in in vitro studies using cell cultures because mycoplasmas have significant influence on the results of scientific studies. Only a recommended decontamination method is to passage the contaminated O. tsutsugamushi strains through mice to eliminate only mycoplasmas under influence of their immunity. However, this method sometimes does not work especially for low virulent strains of O. tsutsugamushi which are difficult to propagate in mice. In this study, we tried to eliminate mycoplasmas contaminants from both high virulent and low virulent strains of the contaminated O. tsutsugamushi by repeating passage through cell cultures with antibiotics in vitro.

Results

We cultured a contaminated, high virulent strain of O. tsutsugamushi using a mouse lung fibroblasts cell line, L-929 cell in the culture medium containing lincomycin at various concentrations and repeated passages about every seven days. At the passage 5 only with 10 μg/ml of lincomycin, we did not detect mycoplasmas by two PCR based methods whereas O. tsutsugamushi continued good growth. During following four passages without lincomycin, mycoplasmas did not recover. These results suggested that mycoplasmas were completely eliminated from the high virulent strain of O. tsutsugamushi. Furthermore, by the same procedures with 10 μg/ml of lincomycin, we also eliminated mycoplasmas from a contaminated, low virulent strain of O. tsutsugamushi. Our additional assay showed that 50 μg/ml of lyncomycin did not inhibit the growth of O. tsutsugamushi, although MICs of many mycoplasmas contaminants were less than 6 μg/ml as shown previously.

Conclusion

Our results showed an alternative method to eliminate mycoplasmas from the contaminated O. tsutsugamushi strains in place of in vivo passage through mice. Especially this notable method works for the decontamination not only from the high virulent strain also from the low virulent strain of O. tsutsugamushi. For further elimination, lincomycin at the limit concentration, which does not inhibit the growth of O. tsutsugamushi, can possibly eliminate most mycoplasmas from contaminated O. tsutsugamushi strains.

Characterization of in vivo-acquired resistance to macrolides of Mycoplasma gallisepticum strains isolated from poultry

The macrolide class of antibiotics, including tylosin and tilmicosin, is widely used in the veterinary field for prophylaxis and treatment of mycoplasmosis. In vitro susceptibility testing of 50 strains of M. gallisepticum isolated in Israel during the period 1997-2010 revealed that acquired resistance to tylosin as well as to tilmicosin was present in 50% of them. Moreover, 72% (13/18) of the strains isolated from clinical samples since 2006 showed acquired resistance to enrofloxacin, tylosin and tilmicosin. Molecular typing of the field isolates, performed by gene-target sequencing (GTS), detected 13 molecular types (I-XIII). Type II was the predominant type prior to 2006 whereas type X, first detected in 2008, is currently prevalent. All ten type X strains were resistant to both fluoroquinolones and macrolides, suggesting selective pressure leading to clonal dissemination of resistance. However, this was not a unique event since resistant strains with other GTS molecular types were also found. Concurrently, the molecular basis for macrolide resistance in M. gallisepticum was identified. Our results revealed a clear-cut correlation between single point mutations A2058G or A2059G in domain V of the gene encoding 23S rRNA (rrnA, MGA_01) and acquired macrolide resistance in M. gallisepticum. Indeed, all isolates with MIC ≥ 0.63 μg/mL to tylosin and with MIC ≥ 1.25 μg/mL to tilmicosin possess one of these mutations, suggesting an essential role in decreased susceptibility of M. gallisepticum to 16-membered macrolides.

Transition mutations in 23S rRNA account for acquired resistance to macrolides in Ureaplasma urealyticum

OBJECTIVE

To study the mechanisms of acquired resistance of Ureaplasma urealyticum (Uu) to Macrolide antibiotics.

METHODS

Eighteen strains of Uu clinical isolates with different phenotypes of resistance to Macrolide antibiotics were screened for mutations in 23S rRNA in comparison with the reference strain ATCC 27618 and AF272627-1 in GenBank, which is susceptible to Macrolide antibiotics.

RESULTS

Compared to the sequence of reference strains ATCC 27618 and AF272627-1, three transitions were found in 23S rRNA of Uu clinical isolates with acquired resistance to Macrolide antibiotics. The C2243N (T or C) transition was found in the 23S rRNA sequence of five strains with the phenotype of acquired resistance to Roxithromycin (ROM) and Azithromycin (AZM). The two changes of A2149C and A2181T were found in the 23S rRNA sequence of nine strains with the phenotype of resistance to ROM and midrange resistance to AZM, and they were also found in that of four strains with the phenotype of sensitivity to macrolides (Josamycin, Clarithromycin, ROM, and AZM). No mutation was found in the ATCC 27618.

CONCLUSION

The mechanisms of Uu resistance to ROM and AZM may be associated with the C2243N (T or C) mutation in 23S rRNA. Further studies are necessary to confirm this hypothesis.