JAC Advance Access originally published online on March 8, 2006
Journal of Antimicrobial Chemotherapy 2006 57(5):966-969; doi:10.1093/jac/dkl061
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Antimicrobial resistance of coagulase-negative staphylococci from bovine subclinical mastitis with particular reference to macrolide–lincosamide resistance phenotypes and genotypes
Institut für Tierzucht, Bundesforschungsanstalt für Landwirtschaft (FAL), Höltystrasse 10, 31535 Neustadt-Mariensee, Germany
* Corresponding author. Tel: +49-5034-871-241; Fax: +49-5034-871-246; E-mail: stefan.schwarz{at}fal.de
Received 28 November 2005; returned 28 December 2005; revised 24 January 2006; accepted 14 February 2006
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Abstract |
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Objectives: The aim of this study was to analyse coagulase-negative staphylococci (CoNS) for their resistance to antimicrobial agents approved for the control of pathogens involved in bovine mastitis, with particular reference to macrolide and/or lincosamide (ML) resistance and the resistance genes involved.
Methods: A total of 298 CoNS collected between 2003 and 2005 in Germany from cases of subclinical mastitis in dairy cows were identified to the species level and investigated for their MICs by broth microdilution. ML-resistant isolates were subjected to plasmid profiling and electrotransformation experiments. The ML resistance genes were detected using PCR and hybridization. Selected PCR products were cloned and sequenced.
Results: The CoNS isolates used in this study showed a low level of resistance to all antimicrobial agents tested (0–7.4%) except ampicillin (18.1%). In the erythromycin-resistant and/or pirlimycin-resistant isolates, the ML resistance genes erm(B), erm(C), msr(A), mph(C) and lnu(A) were present, either alone or in different combinations. Isolates carrying erm methylase genes or the exporter gene msr(A) showed higher MICs than those harbouring only the genes mph(C) or lnu(A) coding for inactivating enzymes. Most of the ML resistance genes were found on plasmids.
Conclusions: This is the first report of pirlimycin MICs for CoNS collected from cases of bovine subclinical mastitis in Germany. After 3–5 years of veterinary therapeutic use, pirlimycin resistance was rarely detected among CoNS. The finding that five different resistance genes—present in various combinations—were responsible for ML resistance underlines the heterogeneous character of this resistance trait.
Keywords: erythromycin , pirlimycin , Staphylococcus , erm gene , lnu(A) gene , mph(C) gene , msr(A) gene
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Bovine mastitis is a major economic problem in the dairy industry worldwide, with a wide variety of microorganisms involved.1,2 Among the Gram-positive pathogens, Staphylococcus aureus, Streptococcus agalactiae, Streptococcus dysgalactiae and Streptococcus uberis are most frequently seen in cases of clinical mastitis. In contrast, coagulase-negative staphylococci (CoNS) are more often associated with subclinical infections of the udder, characterized by an elevated somatic cell count in milk samples and by decreased milk production.1,2
Antimicrobial agents are commonly applied to dairy cattle either to control or to prevent bacterial infections in lactating and dry cows. Thus, the results of in vitro susceptibility testing are an important tool to guide the veterinarian in selecting the most efficacious antimicrobial agent(s) for therapeutic and prophylactic interventions. In 2001, a new lincosamide antibiotic, pirlimycin, was approved in Germany for the control of staphylococci and streptococci associated with bovine subclinical mastitis. So far, little is known about the susceptibility to pirlimycin of CoNS collected in the post-approval phase. A large number of genes mediating resistance to macrolides, lincosamides and streptogramins (MLS antibiotics) by different resistance mechanisms have been identified,3 and a continuously updated list of MLS resistance genes is available at http://faculty.washington.edu/marilynr/.
In the present study, we investigated 298 CoNS isolates from confirmed cases of subclinical mastitis for their susceptibility to pirlimycin and other antimicrobials commonly used for mastitis therapy. Moreover, all macrolide and/or lincosamide (ML) resistant isolates were investigated for the genetic basis of resistance and the location of the corresponding resistance genes.
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Material and methods |
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Bacterial isolates and MIC determination
A total of 298 CoNS isolated from cases of bovine subclinical mastitis between 2003 and 2005 were provided by different diagnostic laboratories all over Germany on the basis of one isolate per herd. All isolates were further identified to the species level using the ID32 Staph system (bioMérieux, Nürtingen, Germany). The MICs of seven antimicrobial agents or combinations, including penicillin/novobiocin (1:2), erythromycin, pirlimycin, ampicillin, oxacillin, cefalotin and ceftiofur, were determined using the broth microdilution method. Additional MIC values of clindamycin were determined using broth macrodilution (Table 1). Both tests were performed and the results evaluated according to CLSI documents M31-A2 and M31-S1. S. aureus ATCC 29213 served as the reference strain for quality control purposes.
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DNA isolation and PCRs
Plasmid and whole cell DNA were prepared using standard protocols. PCR assays for the genes erm(A), erm(B), erm(C), msr(A), lnu(A) and mef(A) and the regulatory region of constitutively expressed erm(C) genes were performed as described previously.4,5 Another two PCR assays were used for the detection of the mph(C) gene (forward primer: 5'-GAGACTACCAAGAAGACCTGACG-3'; reverse primer: 5'-CATACGCCGATTCTCCTGAT-3'; annealing temperature 59°C) and the linkage of the genes msr(A) and mph(C) [forward primer from msr(A): 5'-GCAAATGGTGTAGGTAAGACAACT-3'; reverse primer from mph(C): 5'-AATTCATCTGATAC(AG)CCATAAG-3'; annealing temperature 55°C]. The mecA gene was detected as described previously.6 For each gene, at least one amplicon was cloned and sequenced. Sequence comparisons were performed using the BLAST program available at http://www.ncbi.nlm.nih.gov/BLAST/.
Southern-blot hybridization and transformation experiments
Uncut plasmid DNA or HindIII-digested whole cell DNA was transferred from agarose gels to nylon membranes (Roth, Karlsruhe, Germany) using the capillary blot procedure. The cloned amplicons specific for erm(B), erm(C), msr(A), mph(C) and lnu(A) were labelled using the DIG-High Prime DNA labelling and detection system and used as gene probes. Hybridization and signal detection were performed according to the recommendations given by the manufacturer (Roche, Diagnostics GmbH, Mannheim, Germany). Electrotransformation into the recipient strain S. aureus RN4220 was performed as described previously.7 Transformants were selected on blood agar plates containing erythromycin (4–15 mg/L) or pirlimycin (0.25 mg/L).
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Results and discussion |
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Species distribution and MIC distribution
The comparison of studies on antimicrobial resistance of CoNS is often biased not only because of the use of different systems for species identification but also because of the use of different methodologies for susceptibility testing. Using a standardized identification kit available commercially, we identified Staphylococcus chromogenes (99 isolates, 33.2%), Staphylococcus simulans (69 isolates, 23.2%), Staphylococcus epidermidis (35 isolates, 11.7%), Staphylococcus xylosus and Staphylococcus haemolyticus (28 isolates of each, 9.4%) as the most prevalent CoNS species in our test collection. These species were also the most frequently identified CoNS in a previous study.1 In addition, Staphylococcus warneri (13 isolates), Staphylococcus sciuri (8 isolates), Staphylococcus equorum (6 isolates), Staphylococcus saprophyticus (3 isolates), Staphylococcus capitis, Staphylococcus cohnii, Staphylococcus hominis (2 isolates of each) and single isolates of Staphylococcus caprae, Staphylococcus arlettae and Staphylococcus gallinarum were identified. The distribution of the MICs of the antimicrobial agents and combinations tested is summarized in Table 1. For penicillin/novobiocin and cefalotin no resistant isolates were detected. Two mecA-positive isolates were resistant to oxacillin, one of which also represented the single ceftiofur-resistant isolate. In contrast, 54 isolates proved to be resistant to ampicillin. For erythromycin and pirlimycin, 22 and 19 isolates proved to be resistant, respectively. Most of the MICs of clindamycin were equal to or not more than two dilution steps lower than those of pirlimycin. In general, the resistance rates of CoNS obtained in this study corresponded well to those reported in other studies.1,8
Distribution of ML resistance phenotypes and resistance genes
Based on their MICs of erythromycin and pirlimycin, which are approved for the control of mastitis pathogens, the 31 resistant isolates could be subdivided into five different groups (Table 2). Group 1 comprised isolates highly resistant to both antibiotics (MICs 
128 mg/L) which carried the constitutively expressed gene erm(C) and/or the gene erm(B). Constitutive expression of erm(C) was due to deletions of 111 bp (five isolates) and 74 bp (one isolate) within the erm(C) regulatory region. Both deletions were either indistinguishable from or closely related to deletions described previously.5,9 Two erm(C)-positive isolates also carried the genes msr(A)–mph(C), and three erm(B)-carrying isolates carried the gene lnu(A). Group 2 consisted of isolates highly resistant to erythromycin only (MICs 64 to 128 mg/L) and harboured an inducibly expressed erm(C) gene or the gene msr(A), alone or in combination with mph(C). One isolate harboured all three genes. Whenever they were detected in the same isolate, the genes msr(A) and mph(C) were found to be physically linked. Sequence analysis revealed that they were separated by non-coding spacer sequences of 348 or 98 bp, which proved to be closely related to those described previously for S. aureus (accession no. AB179623
[GenBank]
) and S. haemolyticus (accession no. AP006716
[GenBank]
), respectively. The two isolates of Group 3 exhibited low-level resistance to erythromycin (MICs 8 and 16 mg/L). The mph(C) gene was the only resistance gene detected in these isolates. A single isolate represented Group 4. It harboured the genes mph(C) and lnu(A) and showed low-level resistance to both of the antibiotics (erythromycin MIC 16 mg/L; pirlimycin MIC 8 mg/L). The nine isolates of Group 5 carried the lnu(A) gene as the sole resistance gene and were borderline pirlimycin resistant (MICs 4 mg/L).
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These data are in good accordance with observations on ML resistance genes in CoNS from human and animal sources published previously,10–12 although none of these studies investigated the isolates for the presence of the gene mph(C). This gene was originally detected on the staphylococcal plasmid pMS97, which carried another two ML resistance genes;13 so far it has been described to be present only in connection with the gene msr(A) in staphylococci.
In addition, we detected the mph(C) gene in two isolates with intermediate resistance to erythromycin (MICs 1–4 mg/L) and the lnu(A) gene in nine susceptible isolates with pirlimycin MICs of 2 mg/L (eight isolates) and 1 mg/L (one isolate). However, it should be noted that the currently valid CLSI breakpoints for pirlimycin do not include an intermediate category. Hence, isolates with MICs 4 mg/L are classified as resistant, while those with MICs 
2 mg/L are considered to be susceptible. An intermediate category including MICs of 1 and 2 mg/L might help to avoid classifying isolates that carry lnu(A) genes as susceptible. Since the gene lnu(A) is expressed constitutively, mutations within the promoter region may lead to increased expression of lnu(A). The same is true for the up-regulation of the copy numbers of plasmids that carry lnu(A). In both cases, increased numbers of lnu(A) transcripts may result in increased amounts of lincosamide-inactivating enzymes and consequently elevate the MICs from 1 or 2 mg/L to 4 mg/L, which will then classify the corresponding isolates as borderline resistant.
Location of detected resistance genes
A plasmid location could be shown for all but one of the erm(C) genes. They were located on small plasmids of 
2.3–4 kb. The erm(B) genes instead were all located on larger plasmids with sizes of 25–30 kb. Three of these latter plasmids also carried the gene lnu(A). The msr(A)–mph(C) genes were located on plasmids of 20–25 kb in three isolates and the mph(C) gene in one case on a plasmid of 30 kb. In eight pirlimycin-resistant isolates of Groups 4 and 5 (Table 1), the lnu(A) genes were located on small plasmids of <3 kb.
While the erm(C) gene is commonly found on small plasmids of 2.5 kb,5,7 there have also been reports of plasmid-borne genes erm(B) and lnu(A) in animal staphylococci.14,15 The plasmid location of a resistance gene may favour its distribution across species and sometimes even genus borders. The observation in this study that erm(C) as well as lnu(A) genes were found on similar sized and structurally closely related plasmids (data not shown) among different CoNS species suggested an interspecies exchange of such plasmids. Moreover, the detection of the same mobile ML resistance genes in S. aureus and CoNS from humans and animals5,7,10–15 underlines their potential for spreading and the need for more detailed knowledge of the prevalence of ML resistance genes in veterinary pathogens.
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Nothing to declare.
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Acknowledgements |
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We thank Gabriele Luhofer, Peter Krabisch, Sylvia Kleta and Michael Zschöck for providing the CoNS isolates. P. L. is supported by a Georg-Christoph-Lichtenberg scholarship of the county Lower Saxony. This study was supported financially by Pfizer Animal Health, Sandwich, UK.
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