JAC Advance Access originally published online on November 12, 2005
Journal of Antimicrobial Chemotherapy 2006 57(1):46-51; doi:10.1093/jac/dki407
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Deletion of pT181-like sequence in an smr-encoding mosaic plasmid harboured by a persistent bovine Staphylococcus warneri strain
1 Norwegian School of Veterinary Science, PO Box 8146 Dep, N-0033 Oslo, Norway; 2 National Veterinary Institute, PO Box 8156 Dep, N-0033 Oslo, Norway; 3 TINE Research and Development, N-0950 Oslo, Norway; 4 Matforsk AS, Norwegian Food Research Institute, N-1430 Ås, Norway
* Corresponding author. Tel: +47-22-96-48-73; Fax: +47-22-59-70-83; E-mail: jostein.bjorland{at}veths.no
Received 29 August 2005; returned 28 September 2005; revised 10 October 2005; accepted 14 October 2005
 |
Abstract |
|---|
 Top Abstract IntroductionMaterials and methodsResultsDiscussionTransparency declarationsReferences |
|---|
Objectives: The aim was to study the persistence and characteristics of Staphylococcus warneri strains resistant to quaternary ammonium compounds (QACs), including sequencing and analysis of two plasmids proved to carry the smr gene.
Methods: During a 3.5 year period quarter milk samples were collected on three occasions from all lactating cows in a dairy herd. The samples were screened with regard to QAC-resistant bacteria using a selective medium. Thirty randomly selected QAC-resistant S. warneri were typed by PFGE and subjected to plasmid isolation and analysis followed by gene detection using PCR. Two smr-containing plasmids in S. warneri isolates were sequenced.
Results: All isolates from the initial collection of quarter milk contained smr residing on a 5.8 kb plasmid (pSW174), which contained regions with high similarities to various plasmids, including pT181, pSK108 and pPI-2. The pT181-like sequence was flanked by 148 bp direct repeats, denoted ISLE49, with high similarity to previously reported sequences of 
148 bp, including ISLE39 flanking the insertion sequence IS257 in methicillin-resistant Staphylococcus aureus. All isolates from subsequent collections of quarter milk harboured a smaller smr-containing plasmid (pSW49). Sequence analyses revealed pSW49 (3552 bp) to be an in-part deleted version of pSW174 (5767 bp).
Conclusions: The IS-associated elements found in this study may have a wider role in the integration and excision of DNA sequences in staphylococci than previously reported. The mosaic plasmid structure based on genetic elements of various origins contributes to further knowledge on the flexibility of smr-encoding plasmids.
Keywords: Staphylococcus , resistance , quaternary ammonium compounds , tetracycline
|
Introduction |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsDiscussionTransparency declarationsReferences |
|---|
Antiseptic and disinfectant preparations based on quaternary ammonium compounds (QACs) have a wide range of applications in veterinary medicine and play an important role in the control of infectious diseases in animals.1 QAC-containing teat preparations are commonly used in dairy herds to prevent mastitis.2
QAC resistance in various staphylococcal species is normally associated with the presence of plasmid-borne genes encoding efflux-based multidrug resistance proteins. Several QAC resistance genes have been described in staphylococci recovered from different sources.3–8 The gene smr (formerly qacC) was the first identified multidrug resistance determinant in staphylococci encoding an efflux protein of the small multidrug resistance (SMR) family. This gene has been detected on either large conjugative plasmids (pSK41, pTZ22)4,9 or small non-conjugative plasmids <3 kb (pSK89, pSK108, pNVH99, pST827, pPI-2).4,5,10–12 Increased attention is being paid to plasmid-encoded resistance to biocides in antibiotic-resistant staphylococci. For example, qacA/B and the ß-lactamase gene blaZ have proved to co-reside on large plasmids in various staphylococcal species.13–16 In addition, it was recently reported that the SMR pump is involved in low-level resistance to ß-lactam antibiotics.17
Antibiotic treatment of bovine mastitis contributes substantially to the overall antibacterial drug use in veterinary medicine in many countries. It is generally accepted that selection pressure from the use of antibiotics is a main factor in the development of antibiotic resistance. Norwegian guidelines emphasize the prudent use of antimicrobial drugs in mastitis therapy and recommend that broad-spectrum preparations be avoided. Before 1990, tetracycline was used quite frequently for the treatment of bovine clinical mastitis. However, the use of tetracycline for systemic and intramammary treatment decreased gradually during the period 1990–97.18 Plasmid-borne tetracycline resistance in staphylococci is commonly mediated by the tet(K) gene (1380 bp) harboured by the plasmid pT181 (4440 bp) from Staphylococcus aureus.19
Bacterial genomes are plastic, dynamic structures with a range of survival mechanisms to draw on in order to respond to environmental challenge.20 Sequencing of bacterial genomes during the last decade has proved horizontal gene transfer to be more common among bacteria than previously realized. Bacteria have obtained a significant proportion of their genetic diversity through the acquisition of sequences from distantly related organisms.21 Horizontal gene transfer leads to extremely dynamic genomes in which substantial amounts of DNA are introduced into and deleted from the chromosome. These lateral transfers have extensively influenced the ecological and pathogenic character of bacterial species.21
A QAC-resistant Staphylococcus warneri strain was identified in bulk tank milk from a dairy cattle herd in Norway.16 Preliminary plasmid analysis revealed the presence of a medium-sized plasmid (5.8 kb), on which the smr gene was detected by Southern blotting and hybridization analysis. Based on these findings, our intention was to study the distribution and persistence of this S. warneri strain in the herd as well as characterize the medium-sized smr-containing plasmid.
|
Materials and methods |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsDiscussionTransparency declarationsReferences |
|---|
Herd, animals and sampling procedures
Following the detection of QAC-resistant S. warneri in bulk milk from a dairy herd in 2001, quarter milk samples (QMS) were collected on three occasions from all lactating cows in the herd during the subsequent 3.5 years. Times of sampling, number of sampled cows and the total number of samples are shown in Table 1. All 219 QMS were screened with respect to QAC-resistant staphylococci.
|
To prevent mastitis, a QAC-containing teat preparation had been used twice a day for more than 15 years in the herd, and was still being applied at the times of sample collection. Penicillin was the most commonly used antibiotic for the treatment of clinical cases of mastitis. Up to the mid-1990s tetracycline had been used occasionally for mastitis treatment.
Screening procedure
Milk (100 µL) was spread on blood agar plates containing benzalkonium chloride (BC) (11 mg/L) followed by incubation at 37°C for 24 h. From plates where growth was detected colonies were selected, as described previously,16 for further examination with regard to efflux-mediated resistance, using the following procedure. Isolates were grown for 24 h at 37°C on Mueller–Hinton agar containing ethidium bromide (0.5 mg/L), followed by inspection for fluorescence under UV light as described previously.16 Cells accumulating ethidium bromide had a red fluorescence and were considered QAC-sensitive; cells that did not accumulate ethidium bromide were white and were thus defined as QAC-resistant.
Plasmid isolation
Plasmid DNA was isolated using the QIAprep Spin Miniprep Kit (QIAGEN, Hilden, Germany) modified by adding lysostaphin (Sigma–Aldrich, St Louis, MO, USA) as described previously.5 Plasmid DNA was separated by gel electrophoresis in 0.8% SeaKem LE agarose (FMC BioProducts, Rockland, ME, USA) with Supercoiled DNA Ladder (Life Technologies, Paisley, UK) as molecular weight marker. The size of plasmids was estimated using the GeneGenius gel documentation and analysis system (SynGene Laboratories, Cambridge, UK).
PCR amplification
PCR was carried out to identify QAC resistance genes. The primers used for PCR amplification (Invitrogen, Paisley, UK) of qacA/B and smr were the same as used previously.5,15 Primers specific for the genes qacG, qacH and qacJ were designed from previously known sequences (EMBL accession numbers Y16944 [GenBank] , Y16945 [GenBank] and AJ512814 [GenBank] , respectively): qacG-For (5'-CAACAGAAATAATCGGAACT-3')/Rev (5'-TACATTTAAGAGCACTACA-3'); qacH-For (5'-ATAGTCAGTGAAGTAATAG-3')/Rev (5'-AGTGTGATGATCCGAATGT-3'); and qacJ-For (5'-CTTATATTTAGTAATAGCG-3')/Rev (5'-GATCCAAAAACGTTAAGA-3'). The following strains were used as positive controls: Staphylococcus haemolyticus NVH97A (qacA/B),15 S. aureus NVH99 (smr),5 S. warneri ST94 (qacG),6 Staphylococcus saprophyticus ST2H6 (qacH)7 and S. aureus NVH01 (qacJ).8 The composition of each 50 µL PCR mixture was as recommended by the manufacturer, using DyNAzymeTM II DNA Polymerase with supplementary reagents (Finnzymes Oy, Espoo, Finland). The annealing temperature was variable: 40°C (qacA/B) or 48°C (smr, qacG, qacH, qacJ). In order to confirm the presence of pSW49 in the S. warneri isolates from the last sampling, PCR was carried out for two randomly selected isolates using the following primer pair: pSW49-For (5'-ACACTGCCCATTTACATGC-3')/pSW49-Rev (5'-TTGCCTTATGGGTTTGGAAC-3'), using plasmid DNA from pSW49 as positive control. The design of this primer pair was based on the initial sequencing results of one strand of pSW174. Thus, the primer pSW49-Rev differs from the final correct plasmid sequence (5'-TTGCCTTATGGTTTGGAAC-3') that was based on sequencing of both strands. The PCR product was visualized using gel electrophoresis as described for plasmid isolation.
Staphylococcal species determination
Identification of staphylococci was based on standard laboratory criteria (colony morphology, haemolytic zones, production of catalase and coagulase) and the use of Staph-Zym biochemical test kit (Rosco, Tåstrup, Denmark). In addition, 83% of the sodA gene, the internal fragment denoted sodAint, was sequenced using a degenerate primer pair d1/d2 (Invitrogen) as described previously.22
Nucleotide sequencing
The sequencing was carried out with the capillary sequencer 3100-Avant Genetic Analyzer (Applied Biosystems, Foster City, CA, USA) and BigDye Terminator v3.1 Cycle Sequencing (Applied Biosystems). Sequences were edited and aligned using the BioEdit program (http://www.mbio.ncsu.edu/BioEdit/bioedit.html), and homology searches were performed with the NCBI GeneBlast via the Internet. Two plasmids, pSW174 and pSW49, identified in two randomly selected isolates from the first and the second sampling, respectively, were subjected to sequencing by primer walking, starting with the primer pair smr-For/Rev.5 All sequencing, editing, alignment and homology searches were performed as described for staphylococcal species determination. The nucleotide sequences of plasmids pSW174 and pSW49 have been deposited in the EMBL database under accession numbers AM040729 [GenBank] and AM040730 [GenBank] .
Antimicrobial susceptibility
MICs of BC and cetyltrimethylammonium bromide (CTAB) were determined in a microtitre assay at 0.5 mg/L concentration intervals from 0 to 18 mg/L in Mueller–Hinton broth as described previously.5 The MICs of the following antibiotics were determined by Etest (AB Biodisk, Solna, Sweden): benzylpenicillin, streptomycin, tetracycline, trimethoprim, sulfadiazine, fusidic acid and oxacillin. The breakpoints used were those defined by the NCCLS (CLSI) M31-A. S. aureus ATCC 29213 was included as a control strain.
PFGE
Molecular fingerprinting of staphylococci was performed for 32 staphylococcal isolates (including two cured isolates) by PFGE with a CHEF-DR®III apparatus (Bio-Rad Laboratories, Hercules, CA, USA) as described previously.16,23
Curing of QAC resistance plasmids
Plasmid curing of the two isolates used for plasmid sequencing was performed by growth in increasing sublethal concentrations of novobiocin in Mueller–Hinton broth as described by Heir et al.7
Growth studies
Overnight cultures grown in Luria–Bertani (LB) medium of the S. warneri containing pSW174 or pSW49, and the two isolates cured for the same plasmids, were diluted to an OD600 of 0.05 in 100 mL of LB medium containing 4 mg/L CTAB and in pure LB medium, followed by incubation at 37°C for 24 h. The OD600 was determined at the beginning of the experiment and subsequently at 40 min intervals for 7 h.
|
Results |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsDiscussionTransparency declarationsReferences |
|---|
Characterization of QAC-resistant staphylococci
The numbers of cows and udder quarters from which QAC-resistant staphylococci were obtained are shown in Table 1. Subsets of 10 QAC-resistant staphylococcal isolates for species determination, genetic characterization and susceptibility testing were randomly selected among the isolates collected on each of the three sampling occasions. Species determination by biochemical methods and a molecular genetic method were in agreement, and all isolates were identified as S. warneri. MICs of BC and CTAB were 2.5–3.0 mg/L and 8.0–8.5 mg/L, respectively, compared with 
1.0 and 2.5 mg/L for strains cured for plasmids. MICs for isolates collected on different occasions did not differ. All isolates were susceptible to benzylpenicillin, streptomycin, tetracycline, trimethoprim, sulfadiazine, fusidic acid and oxacillin. PFGE of the 30 S. warneri isolates and 2 plasmid-cured isolates showed indistinguishable banding patterns.
Resistance plasmid identification and characterization
Plasmid isolation and subsequent PCR revealed the presence of smr in all isolates. No other QAC resistance genes were detected. In the 10 isolates collected in 2002, smr resided on a plasmid of 5.8 kb (pSW174). In the 20 isolates collected in 2004 (Sample 2) and 2005 (Sample 3), smr resided on a plasmid of 3.6 kb (pSW49). The presence of the same plasmid (pSW49) in isolates from the third sampling was confirmed by PCR. Sequencing of the two plasmids showed that the complete pSW49 sequence (3552 bp) was present in pSW174 (5767 bp) with 100% identity, comprising the smr gene and a putative replication gene rep49 (Figure 1). In addition, the pSW174 sequence contained a partial pT181 sequence19 of 2067 bp, comprising a non-functional gene 
tet(K) of 1322 bp, truncated by 58 bp in the 3' end compared with the native tet(K) gene (Figure 1).
|
Relatedness to other known staphylococcal plasmids
The pSW174 plasmid has a mosaic structure containing genetic elements of various origins (Figure 1). The conserved sequence of pSW174 (bp 1–3552), synonymous to pSW49, contains a 1762 bp sequence (bp 1791–3552) with nearly 100% identity to the corresponding sequence in pSK108 reported previously in a coagulase-negative Staphylococcus sp., including an overlapping 59 codon ORF (bp 1958–2137), designated qacC' by Leelaporn et al., 10 which appears to result from a 142 bp partial duplication (bp 1958–2099) of the smr gene, thus containing the 5'-end of the smr gene. This 59 codon ORF sequence, denoted smr' in this paper, is unlikely to confer any significant effect on the resistance of harbouring bacteria since the product of this ORF would contain only the N-terminal 32 amino acids of the protein Smr.10 The conserved sequence (pSW49) also contains a putative replication gene (rep49; bp 674–1594) with 88% identity to a putative replication gene in the S. warneri plasmid pPI-2.12 The rep49 sequence is flanked downstream by a 160 bp sequence (bp 486–645), a putative single-strand origin, ssoA, showing 87% identity to a corresponding region present in various rolling-circle replicating plasmids, for example, plasmids pT181 and pC223 (NC_005243
[GenBank]
). This conserved region has been proposed to contain a specific recombination site, RSB.24 The other recombination site, RSA, is known as the specific site necessary for mobilization and recombination mediated by a Mob/Pre protein, present in staphylococcal plasmids such as pT181, pE194 and pSTE2.25,26
Direct repeat sequence ISLE49
The pT181-like sequence was flanked by 148 bp direct repeats (DRs), denoted ISLE49 (Figures 1 and 2). One copy of ISLE49 was present in pSW49 (Figure 1). Alignment searches identified four additional sequences closely related to ISLE49 presented in Figure 2. Sequence 1 (one copy) is present in close association with IS257 (S. P. Yazdankhah, Norwegian Institute of Public Health, Oslo, personal communication) in the S. haemolyticus plasmid pNVH96 (AJ302698 [GenBank] ). Two copies of Sequence 2 are present in the genome sequence of Staphylococcus epidermidis RP62A, a methicillin-resistant biofilm-producing strain (AJ249487 [GenBank] ). Two copies of Sequence 3 are located near the mobile S. aureus pathogenicity island SaPIbov2 encoding the biofilm-associated protein Bap.27 One copy of the ISLE39 (Sequence 5) was previously found to be involved in a novel type of IS257-related genomic organization of the downstream region of qacA/B in methicillin-resistant S. aureus.28 These five sequences, including ISLE49 in this study (Sequence 4), have inverted repeat sequences (IRs) flanking the segments. The 14 bp of the left terminal IRs (IR-L) and the 17 bp in the right end (IR-R) are identical with the terminal IRs of IS257 as well as other bacterial insertion sequences of the IS6 family.29 The sequences shown in Figure 2 exhibited identities of 66–75%, except those from pNVH96 and RP62A which showed 95% identity.
|
Growth rates
No significant difference in growth rates was observed between S. warneri strains containing pSW174 or pSW49. Likewise, there was no significant difference in growth rates between the cured S. warneri isolates.
|
Discussion |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsDiscussionTransparency declarationsReferences |
|---|
One particular PFGE type of QAC-resistant S. warneri, originally found in bulk milk from a dairy herd, was repeatedly recovered from udder quarters of a large proportion of the cows in this herd during the subsequent 3.5 years. In isolates collected at the initial screening of the herd, the smr gene resided on a 5767 bp plasmid (designated pSW174). However, 2 years later, the smr gene was harboured by a plasmid of 3552 bp (designated pSW49), present in all isolates subjected to molecular characterization. The latter plasmid was also found in the isolates from the third screening of the herd. Thus, bacteria of the S. warneri carrying pSW174 seemed to have been ousted entirely by those that carried the smaller plasmid.
The reason for the superior viability of the latter S. warneri sub-population is unclear. Functionally, there seemed to be no differences between the two plasmids. Sequencing showed that the entire pSW49 was contained in pSW174, while the remaining pSW174 sequence included a pT181-like sequence containing a non-functional tet(K) gene. Sequence analysis shows that truncation of a native tet(K) gene by 58 bp in the 3' end will result in a change of the reading frame. The insertion of ISLE49 into the tet(K) reading frame renders an alternative stop codon at position 447, located within the 14th transmembrane segment of the proposed Tet protein. This structural change might explain why tet(K) does not appear to confer any tetracycline resistance. For repC it is obvious that the start of the repC gene is missing and hence translation of this gene cannot occur.
MICs of QACs were found to be similar to those for S. warneri isolates harbouring pSW49 and those containing pSW174. However, the ability to survive under QAC exposure in the herd might have been influenced by very small differences in MICs that were not detected by the MIC determination procedure in our study.
The metabolic burden associated with plasmid replication increases with increasing plasmid size. Therefore, one would expect bacteria carrying the smaller of the two plasmids to be the more competitive, provided that they occur with similar copy numbers and the products encoded by the plasmids do not favour replication or viability of one type relative to the other. Culture in LB broth did not reveal any difference in growth rate between S. warneri isolates harbouring the two plasmids. However, since our culture conditions ensured abundant supply of essential substrates, it is not likely that this procedure was able to mimic the varying environment surrounding the bacteria in the mammary gland and potential extramammary reservoirs.
Little is known about the mechanisms behind persistence of staphylococci associated with bovine mastitis. The pathogenicity island SaPIbov2 has proved to facilitate the persistence of S. aureus in an intramammary gland infection model.27 The similarity between the DRs presented in Figure 2 indicates the mobility potential of genetic elements in staphylococci from different sources. The linkage of mobile genetic elements to pathogenicity islands, involved in biofilm production and antimicrobial resistance, is of interest, since biofilm environments are considered as optimal niches for microbial gene transfer. There is a need for further studies in this field.
Where analysed, members of the IS6 family give rise exclusively to replicon fusions (cointegrates) in which the donor and target replicons are separated by two directly repeated IS copies.29 The observed pT181- and pSK108-like sequences in pSW174 could be considered as remnants from a previous cointegration between a functional tetracycline and QAC resistance plasmid. Such a cointegration might have been a selective advantage for bacterial survival in environments where both QAC-based disinfectants and tetracycline have been in common use over extended time periods. Decrease in tetracycline use may have reduced the selective tetracycline pressure maintaining a native and functional tetracycline determinant. A partial deletion of a functional tet(K) gene might have been an evolutionary event resulting in improved fitness of the host bacteria in a tetracycline-free environment. Moreover, the presence of a plasmid containing a non-functional tet(K) could have been a burden more than an advantage, leading to excision of this segment together with one copy of ISLE49. This recombination event has generated the plasmid pSW49 retaining the same functions (i.e. conferring QAC resistance) and a single copy of ISLE49 within a smaller, more competitive genetic construct. This could explain the presence of pSW49 in all isolates of the second and third sampling and the pSW174 presence of in all isolates from the first sampling.
The IS257 element has previously been shown to mediate integration of small tet(K)-encoding plasmids into large plasmids.30 The present study also indicates that shorter sequence repeats, with high similarity to repeats flanking IS257 and repeats flanking the pathogenicity island SaPIbov2, may be involved in the genetic flexibility of small plasmids. It is possible that the 148 bp ISLE49 flanked by the 14 bp IRs could be a relic of an IS element. It is also possible that a transposase could mobilize the pT181-like sequence flanked by the 14 bp IRs. The present data indicate that these elements may have a potentially wider role in integration and excision of genetic elements in staphylococci than previously reported. The presence of these elements within a new genetic context described in this study suggests that extensive genetic transfer and recombination within the staphylococcal population have been involved in the construction of plasmids pSW174 and pSW49. This is also supported by the mosaic-like structure of these plasmids demonstrated by sequence similarities to a variety of plasmids of diverse origin. The presence of repeated genetic elements like those found in this study may contribute to the staphylococcal genetic flexibility, enhancing the effect of beneficial genetic rearrangements as shown for IS25731 and affecting the ability of the organism to adapt to and survive in diverse environments.
|
Transparency declarations |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsDiscussionTransparency declarationsReferences |
|---|
No declarations were made by the authors of this paper.
|
Acknowledgements |
|---|
This study was supported by a grant from the Research Council of Norway (grant no. 140723/110).
|
References |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsDiscussionTransparency declarationsReferences |
|---|
1. Russell AD, Hugo WB. Chemical disinfectants. In: Linton AH, Hugo WB, Russell AD, eds Disinfection in Veterinary and Farm Animal Practice. Oxford: Blackwell Scientific Publication, 1987; 20–3.
2. Bramley AJ, Dodd FH. Reviews of the progress of dairy science: mastitis control—progress and prospects. J Dairy Res 1984; 51: 481–512.[Web of Science][Medline]
3.
Paulsen IT, Brown MH, Littlejohn TG et al. Multidrug resistance proteins QacA and QacB from Staphylococcus aureus: membrane topology and identification of residues involved in substrate specificity. Proc Natl Acad Sci USA 1996; 93: 3630–5.
4. Littlejohn TG, DiBerardino D, Messerotti JL et al. Structure and evolution of a family of genes encoding antiseptic and disinfectant resistance in Staphylococcus aureus. Gene 1991; 101:59–66.[CrossRef][Web of Science][Medline]
5.
Bjorland J, Sunde M, Waage S. Plasmid-borne smr gene causes resistance to quaternary ammonium compounds in bovine Staphylococcus aureus. J Clin Microbiol 2001; 39: 3999–4004.
6. Heir E, Sundheim G, Holck AL. The qacG gene on plasmid pST94 confers resistance to quaternary ammonium compounds in staphylococci isolated from the food industry. J Appl Microbiol 1999; 86: 378–88.[CrossRef][Medline]
7. Heir E, Sundheim G, Holck AL. The Staphylococcus qacH gene product: a new member of the SMR family encoding multidrug resistance. FEMS Microbiol Lett 1998; 163: 49–56.[CrossRef][Medline]
8.
Bjorland J, Steinum T, Sunde M et al. Novel plasmid-borne gene qacJ mediates resistance to quaternary ammonium compounds in equine Staphylococcus aureus, Staphylococcus simulans, and Staphylococcus intermedius. Antimicrob Agents Chemother 2003; 47: 3046–52.
9. Sasatsu M, Shibata Y, Noguchi N et al. High-level resistance to ethidium bromide and antiseptics in Staphylococcus aureus. FEMS Microbiol Lett 1992; 72: 109–13.[CrossRef][Medline]
10. Leelaporn A, Firth N, Paulsen IT et al. Multidrug resistance plasmid pSK108 from coagulase-negative staphylococci; relationship to Staphylococcus aureus qacC plasmids. Plasmid 1995; 34: 62–7.[CrossRef][Web of Science][Medline]
11. Heir E, Sundheim G, Holck AL. Resistance to quaternary ammonium compounds in Staphylococcus spp. isolated from the food industry and nucleotide sequence of the resistance plasmid pST827. J Appl Bacteriol 1995; 79: 149–56.[Medline]
12. Aso Y, Koga H, Sashihara T et al. Description of complete DNA sequence of two plasmids from the nukacin ISK-1 producer, Staphylococcus warneri ISK-1. Plasmid 2005; 53: 164–78.[CrossRef][Web of Science][Medline]
13. Sidhu MS, Heir E, Sørum H et al. Genetic linkage between resistance to quaternary ammonium compounds and ß-lactam antibiotics in food-related Staphylococcus spp. Microb Drug Resist 2001; 7: 363–71.[CrossRef][Web of Science][Medline]
14.
Sidhu MS, Heir E, Leegaard T et al. Frequency of disinfectant resistance genes and genetic linkage with ß-lactamase transposon Tn552 among clinical staphylococci. Antimicrob Agents Chemother 2002; 46: 2797–803.
15.
Anthonisen IL, Sunde M, Steinum TM et al. Organization of the antiseptic resistance gene qacA and Tn552-related ß-lactamase genes in multidrug-resistant Staphylococcus haemolyticus strains of animal and human origins. Antimicrob Agents Chemother 2002; 46: 3606–12.
16.
Bjorland J, Steinum T, Kvitle B et al. Widespread distribution of disinfectant resistance genes among staphylococci of bovine and caprine origin in Norway. J Clin Microbiol 2005; 43: 4363–8.
17. Fuentes DE, Navarro CA, Tantalean JC et al. The product of the qacC gene of Staphylococcus epidermidis CH mediates resistance to ß-lactam antibiotics in gram-positive and gram-negative bacteria. Res Microbiol 2005; 156: 472–7.[Medline]
18. Grave K, Greko C, Nilsson L et al. The usage of veterinary antibacterial drugs for mastitis in cattle in Norway and Sweden during 1990–1997. Prev Vet Med 1999; 42: 45–55.[CrossRef][Web of Science][Medline]
19. Guay GG, Khan SA, Rothstein DM. The tet(K) gene of plasmid pT181 of Staphylococcus aureus encodes an efflux protein that contains 14 transmembrane helices. Plasmid 1993; 30: 163–6.[CrossRef][Web of Science][Medline]
20. Rice LB. Bacterial monopolists: the bundling and dissemination of antimicrobial resistance genes in gram-positive bacteria. Clin Infect Dis 2000; 31: 762–9.[CrossRef][Web of Science][Medline]
21. Ochman H, Lawrence JG, Groisman EA. Lateral gene transfer and the nature of bacterial innovation. Nature 2000; 405: 299–304.[CrossRef][Medline]
22.
Poyart C, Quesne G, Boumaila C et al. Rapid and accurate species-level identification of coagulase-negative staphylococci by using the sodA gene as a target. J Clin Microbiol 2001; 39: 4296–301.
23. Tenover FC, Arbeit RD, Goering RV et al. Interpreting chromosomal DNA restriction patterns produced by pulsed-field gel electrophoresis: criteria for bacterial strain typing. J Clin Microbiol 1995; 33: 2233–9.[Web of Science][Medline]
24.
Kramer MG, Espinosa M, Misra TK et al. Lagging strand replication of rolling-circle plasmids: specific recognition of the ssoA-type origins in different gram-positive bacteria. Proc Natl Acad Sci USA 1998; 95: 10505–10.
25.
Gennaro ML, Kornblum J, Novick RP. A site-specific recombination function in Staphylococcus aureus plasmids. J Bacteriol 1987; 169: 2601–10.
26.
Hauschild T, Lüthje P, Schwarz S. Staphylococcal tetracycline-MLSB resistance plasmid pSTE2 is the product of an RSA-mediated in vivo recombination. J Antimicrob Chemother 2005; 56: 399–402.
27. Ubeda C, Tormo MA, Cucarella C et al. Sip, an integrase protein with excision, circularization and integration activities, defines a new family of mobile Staphylococcus aureus pathogenicity islands. Mol Microbiol 2003; 49: 193–210.[CrossRef][Web of Science][Medline]
28. Alam MA, Kobayashi N, Uehara N et al. Analysis on distribution and genomic diversity of high-level antiseptic resistance genes qacA and qacB in human clinical isolates of Staphylococcus aureus. Microb Drug Resist 2003; 2: 109–21.
29.
Mahillon J, Chandler M. Insertion sequences. Microbiol Mol Biol Rev 1998; 62: 725–74.
30. Werckenthin C, Schwarz S, Roberts MC. Integration of pT181-like tetracycline resistance plasmids into large staphylococcal plasmids involves IS257. Antimicrob Agents Chemother 1996; 40: 2542–4.[Abstract]
31.
Simpson AE, Skurray RA, Firth N. An IS257-derived hybrid promoter directs transcription of a tetA(K) tetracycline resistance gene in Staphylococcus aureus chromosomal mec region. J Bacteriol 2000; 182: 3345–52.
CiteULike 
Connotea 
Del.icio.us What's this?
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||