Journal of Antimicrobial Chemotherapy

JAC Advance Access originally published online on January 5, 2006
Journal of Antimicrobial Chemotherapy 2006 57(3):424-429; doi:10.1093/jac/dki478

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Distribution of a transposon-like element carrying bla_CMY-2 among Salmonella and other Enterobacteriaceae

Lin-Hui Su^1,2, Hsiu-Ling Chen⁴, Ju-Hsin Chia^1,2, Shu-Ying Liu⁵, Chishih Chu⁶, Tsu-Lan Wu^1,2 and Cheng-Hsun Chiu^3,4,*

¹ Department of Clinical Pathology, Chang Gung Memorial Hospital, 5 Fu-Hsin Street, Kweishan, Taoyuan 333, Taiwan; ² Department of Medical Biotechnology and Laboratory Science, Chang Gung University, 259 Wenhua 1st Road, Kweishan, Taoyuan 333, Taiwan; ³ Department of Medicine, Chang Gung University, 259 Wenhua 1st Road, Kweishan, Taoyuan 333, Taiwan; ⁴ Department of Pediatrics, Chang Gung Children's Hospital, 5 Fu-Hsin Street, Kweishan, Taoyuan 333, Taiwan; ⁵ Department of Molecular Biotechnology, Da-Yeh University, 112 Shan-Jiau Road, Da-Tsuen, Changhua 515, Taiwan; ⁶ Department of Applied Microbiology, National Chiayi University, 300 University Road, Chiayi 600, Taiwan

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^* Corresponding author. Tel: +886-3-3281200 ext. 8896; Fax: +886-3-3288957; E-mail: chchiu{at}adm.cgmh.org.tw

Received 29 August 2005; returned 25 September 2005; revised 16 November 2005; accepted 12 December 2005

	Abstract

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Objectives: The dissemination of cephamycin resistance in Enterobacteriaceae and its correlation with a transposon-like DNA element consisting of a specific tnpA-bla_CMY-2-blc-sugE structure were investigated.

Methods: A total of 140 enterobacterial isolates belonging to 17 species (10 genera) of Enterobacteriaceae phenotypically characterized as putative AmpC-producers were evaluated. The isolates were examined by PCR analysis, DNA–DNA hybridization and nucleotide sequencing.

Results: The bla_CMY-2-carrying element was detected in 34 isolates from 10 species (9 genera), including all 14 Salmonella and 4 Shigella isolates as well as 7 of the 10 Escherichia coli isolates tested. The remaining 9 isolates were from 112 isolates of the other 14 species tested. The genetic structure of the bla_CMY-2-carrying element was identical in 29 isolates, while in 3 E. coli and 2 Citrobacter isolates an additional insertion sequence IS1 was found inserted at various nucleotide positions close to the 3' end, either within or downstream, of tnpA. In 12 of the 14 representative isolates examined, the bla_CMY-2-carrying element was found inserted in the finQ gene of various-sized plasmids with highly conserved 8 bp direct repeats flanking the junction regions. Among the other 106 non-CMY-2-producing isolates, plasmid-mediated ampC genes were found only in one isolate of Enterobacter aerogenes which carried a bla_DHA-1-like gene.

Conclusions: bla_CMY-2 is the most prevalent plasmid-mediated ampC gene among Enterobacteriaceae. All the bla_CMY-2 genes identified in the present study were associated with a specific transposon-like element that may be responsible for the spread of bla_CMY-2 among Enterobacteriaceae.

Keywords: plasmid-mediated AmpC ß-lactamases , cephamycin resistance , DHA-1-like ß-lactamases

	Introduction

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Among Gram-negative bacteria, the emergence of resistance to extended-spectrum cephalosporins has constituted a global problem. Production of AmpC ß-lactamases is one of the major mechanisms leading to such resistance. These ß-lactamases are of great concern since they not only confer resistance to 7--methoxy-cephalosporins but also are not inhibited by commercially available ß-lactamase inhibitors.¹ Genes encoding AmpC ß-lactamases were originally located on the chromosome of many Enterobacteriaceae and other Gram-negative bacteria.² In 1989, Bauernfeind et al.³ first described that the resistance of a Klebsiella pneumoniae isolate to cefoxitin, cefotetan, oxyimino-cephalosporins, and monobactams could be transferred to Escherichia coli. The ß-lactamase detected was termed CMY-1 for its cephamycinase activity.³ Subsequently, such plasmid-mediated AmpC ß-lactamases were discovered widely in many bacterial species not naturally producing AmpC.¹ These plasmid-mediated enzymes have recently been classified into six genetic clusters,⁴ in which CMY-2 is the most prevalent and has been found in many areas of the world.¹

In the United States, plasmid-mediated CMY-2 has been found in clinical isolates of Salmonella,^5,6 thereby posing a serious threat to public health. Several Salmonella serotypes have been involved, such as Salmonella Typhimurium, Salmonella Choleraesuis, Salmonella Cremieu, Salmonella Enteritidis, Salmonella Gloucester, Salmonella Hadar, Salmonella Kimuenza, Salmonella Mons, Salmonella Newport, Salmonella Redba, Salmonella Schleissheim and Salmonella Senftenberg.^5–11 We recently isolated a strain of Salmonella Choleraesuis, SC-B67, which was resistant to ceftriaxone and ciprofloxacin from a patient with sepsis.¹² The resistance to ceftriaxone was attributed to the presence of a plasmid-mediated bla_CMY-2 gene.^12,13 Analysis of the genetic structure of the bla_CMY-2 gene in SC-B67 showed that it was located on a transposon-like DNA element consisting of a specific tnpA-bla_CMY-2-blc-sugE structure.¹² To explore the extent to which the bla_CMY-2-carrying element may have contributed to the cephamycin resistance among Enterobacteriaceae, putative AmpC-producers were collected from a wide spectrum of enterobacterial species with phenotypic methods and subjected to molecular investigations.

	Materials and methods

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Bacterial isolates

Bacterial isolates belonging to the family Enterobacteriaceae were collected during 2002–2003 from clinical specimens of patients treated at Chang Gung Memorial Hospital and Children's Hospital, Taoyuan, Taiwan, using standard methods.¹⁴ Isolates showing resistance to cefoxitin in a disc test were considered as putative AmpC-producers and subjected to further molecular studies. After being genotyped by a previously described method, infrequent-restriction-site PCR,¹⁵ only isolates of different genotypes were included in the study. A maximum of 10 isolates of each species were selected except that all Salmonella isolates were included in the study due to their rarity.

Detection of the transposon-like bla_CMY-2-carrying DNA element

Extraction of bacterial DNA was performed using a DNeasy Tissue Kit (QIAGEN GmbH, Hilden, Germany), according to the instructions of the manufacturer. For the analysis of genetic arrangement of the resistance genes, overlapping PCR amplification of internal regions of the transposon-like element that carried bla_CMY-2 was performed.¹² The associated primers used are listed in Table 1. As shown in Figure 1, Tn-F/AmpC-R, AmpC-F/SugE-R, AmpC-F/AmpC-R and Tn-F/SugE-R were used as paired primers to amplify various regions of the DNA element and the expected molecular sizes of the PCR products were 2930, 2583, 1646 and 3867 bp, respectively. Salmonella Choleraesuis SC-B67 that carried the specific DNA element containing the bla_CMY-2 gene was used as a positive control in each experiment.¹²

View this table: [in this window] [in a new window]   Table 1.. Primers used for PCR and sequence analysis

 

View larger version (12K): [in this window] [in a new window]   Figure 1.. Genetic structure of genes surrounding blaCMY-2 identified previously in the AmpC-producing Salmonella enterica serotype Choleraesuis SC-B67 (see reference 12) and various enterobacteria examined in this study. The nucleotide positions of the four primers used are indicated by arrows. The insertion sites of an insertion sequence, IS1, and sequences of the flanking direct repeats are listed for the five isolates that possessed the insertion sequence.

 
Isolates with PCR products of the same sizes as those expected were considered as having the same specific DNA element as SC-B67 and no further sequence analysis were performed, except that all amplicons produced by the primer pair AmpC-F/AmpC-R were sequenced to specify the associated ampC genes. Such PCR products and those with sizes different from the expected were purified using the Microcon® PCR Centrifugal Filter Devices (Millipore Corporation, Bedford, MA, USA) and sequenced by using the ABI 3100-Avant Genetic Analyzer (Perkin-Elmer, Applied Biosystems, Foster City, CA, USA). The nucleotide sequences obtained were compiled and analysed using the Lasergene software (DNASTAR, Inc., Madison, WI, USA). The search for homologous sequences was conducted in the GenBank database using the Basic Local Alignment Search Tool (BLAST) through the Internet (http://www.ncbi.nlm.nih.gov/BLAST/).

Detection of other plasmid-mediated ampC genes by using a multiplex PCR and DNA sequence analysis

Because the primers used in the above screening were purposely designed to identify the presence of the specific structure containing the bla_CMY-2-carrying DNA element, CMY-2-producing isolates that do not harbour the specific structure may not be amplified. Thus isolates negative in the above screening were further examined by using a multiplex PCR⁴ previously developed for the detection of family-specific plasmid-mediated ampC genes. The multiplex PCR was shown to amplify the majority of the six genetic groups of currently known plasmid-mediated ampC genes, including bla_CMY-2.⁴ PCR-sequencing was further performed for any isolates positive in the multiplex PCR using respective primers described previously¹⁶ or those listed in Table 1.

Plasmid analysis and DNA–DNA hybridization

The presence of plasmids was checked using a modified alkaline lysis method as described previously.¹⁷ DNA–DNA hybridization was performed by using the method of Southern,¹⁸ except that the probe for the target CMY-2 ß-lactamase gene was prepared and labelled with digoxigenin-11-deoxyuridine triphosphate (Roche, Molecular Biochemicals, Mannheim, Germany).

DNA sequence analysis of the flanking regions of bla_CMY-2

To reveal the insertion site of the specific bla_CMY-2-containing DNA element among different organisms, the plasmid DNA of representative strains was extracted using a QIAGEN Plasmid Kit (QIAGEN GmbH, Hilden, Germany) and sequenced using the procedure described above. Two primers, Tn-R and SugE-F (Table 1), were used for the sequence analysis of junction regions between the plasmid sequence and tnpA on the 5' end and sugE and the plasmid sequence on the 3' end, respectively.

	Results

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A total of 140 Gram-negative bacterial isolates belonging to 17 species (10 genera) of the family Enterobacteriaceae were evaluated in this study (Table 2). The bla_CMY-2-carrying transposon-like element was detected in 34 isolates from 10 species (9 genera), including all 14 Salmonella and 4 Shigella isolates as well as 7 of the 10 E. coli isolates tested. In contrast, only 9 of the remaining 112 isolates from the other 14 species tested had the specific element (Table 2).

View this table: [in this window] [in a new window]   Table 2.. Distribution of the transposon-like blaCMY-2-carrying DNA element and other plasmid-mediated AmpC in different species of Enterobacteriaceae

 
The genetic structure of the bla_CMY-2-carrying element was identical to that of Salmonella Choleraesuis SC-B67¹² in 29 of the 34 isolates. However, a minor genetic diversity of the DNA element was found in the remaining 5 isolates (Citrobacter diversus #204; Citrobacter amalonaticus #638; and E. coli #71, #910 and #929). The tnpA sequences were exactly the same among the 5 isolates as well as the other 12 isolates described below (Table 3). However, an additional insertion sequence IS1, 768 bp in length, was found at the 3' end of tnpA in the C. diversus and the 3 E. coli isolates, while a 754 bp IS1 was found inserted between tnpA and bla_CMY-2 in the C. amalonaticus isolate. The exact nucleotide positions where the IS1 inserted in the bla_CMY-2-containing element and the sequences of the respective flanking direct repeats are listed in Figure 1. In two of the E. coli isolates (#910 and #929) the IS1 was found inserted at the same nucleotide positions and thus had identical flanking direct repeats. In the remaining E. coli isolate (#71), the IS1 was found inserted at a different nucleotide position but was the same as that found in the C. diversus isolate (#204). Plasmid analysis and DNA–DNA hybridization indicated that the bla_CMY-2-carrying transposon-like element was located on large plasmids of sizes ranging between 90 and 140 kb (data not shown).

View this table: [in this window] [in a new window]   Table 3.. DNA sequence analysis of the flanking regions upstream of tnpA and downstream of sugE and the 8 bp direct repeats at the respective junction regions for representative CMY-2 ß-lactamase-producing strains from Table 2

 
Furthermore, sequences of the plasmid DNA flanking the bla_CMY-2-carrying element were analysed in a total of 14 representative isolates (Table 3). Similar to what has been reported in the AmpC-producing Salmonella Choleraesuis SC-B67,¹² the transposon-like element that carries bla_CMY-2 was found inserted in the finQ gene of the transfer region of IncI1 plasmid ColIb-P9¹⁹ in 12 of the 14 isolates tested, including E. coli #71 and C. amalonaticus #638, which had the additional IS1 at the end of tnpA. Only the Enterobacter cloacae and Salmonella Infantis isolates had different gene sequences flanking the bla_CMY-2-containing transposon-like element (Table 3). The 8 bp direct repeats at junction regions surrounding the element were highly conserved in these isolates; only the two isolates with different flanking sequences had no such specific repeats (Table 3).

On the other hand, the bla_CMY-2-carrying element was not identified in a total of 106 isolates, in which only one isolate of Enterobacter aerogenes was found to carry a bla_DHA-1-like gene, by using the multiplex PCR for plasmid-mediated ampC genes. This DHA-1-like ß-lactamase had an identical amino acid sequence to that of DHA-1 except that one mutation occurred at amino acid position 29, where an alanine (GCG) was substituted by a valine (GTG).

	Discussion

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CMY-2 ß-lactamase is the most prevalent type of the plasmid-mediated AmpC enzymes and is distributed in a wide geographical area, having been reported in France, Greece, Turkey, the United Kingdom and the United States.³ In Taiwan, spread of bla_CMY-2-carrying plasmids among Salmonella and E. coli isolates in both hospital and community settings have been frequently reported.^10,16,20,21 The first report of plasmid-mediated CMY-2-type AmpC ß-lactamase identified among Shigella sonnei was also reported recently in Taiwan.²² In the present study, we confirmed that CMY-2 ß-lactamase is the most prevalent AmpC enzyme in the Enterobacteriaceae family and that all the bla_CMY-2 genes were also bracketed by a specific transposon-like DNA element. As shown in Table 2, the bla_CMY-2-carrying transposon-like element is widely distributed among 10 of the 17 species of the Enterobacteriaceae tested, notably in AmpC-producing Salmonella (14/14), Shigella (4/4) and E. coli (7/10). Clinical isolates of other species, such as E. cloacae (1/10), C. diversus (2/10), Morganella morganii (1/10), Proteus rettgeri (1/4) and Providencia stuartii (1/10), although known as natural inducible AmpC-producers,¹ also carried this bla_CMY-2-containing DNA element at a lower rate. To our knowledge, this is the first report of the presence of bla_CMY-2 among these bacterial species. These results raised a speculation that the spread of bla_CMY-2 occurred through lateral gene transfer among these bacterial species.

In contrast to the wide spread of CMY-2-producing isolates among Enterobacteriaceae, a much lower prevalence of the other plasmid-mediated ampC genes was noted in the present study. Similar situations were found in previous studies from Taiwan where only CMY-8 and DHA-1 have been sporadically reported in K. pneumoniae.^23,24 E. aerogenes has been shown to carry a bla_LAT-2-containing plasmid.²⁵ The finding in the present study of a bla_DHA-1-like gene, which was probably originated from M. morganii,⁷ in an isolate of E. aerogenes has never been reported before.

Antibiotic resistance can be disseminated throughout populations by epidemic spread of a particular clone or through the exchange of genetic materials. In the present study, the studied isolates were all of different genotypes, but all the isolates that produced AmpC-type ß-lactamases harboured the specific four-gene segment on their plasmids, suggesting that the dissemination of the resistance gene may be through plasmid transfer.

Genes for the AmpC-type ß-lactamases have been reported to be located on plasmids of sizes varying from 7 to 180 kb.¹ A few of the plasmids are not self-transmissible but are transferred by transformation²⁶ or mobilization.^25,27 Winokur et al., in an earlier study, found that all cephalosporin-resistant Salmonella encoded a CMY-2-type ß-lactamase, and furthermore the resistance gene appeared to be located on large non-conjugative plasmids.²⁶ Conjugation experiments were also performed to determine whether cephalosporin resistance could be transferred from our Salmonella isolates to E. coli DH5. Despite multiple attempts, resistance could not be transferred by conjugation to the recipient E. coli strain using ceftriaxone and lactose utilization (MacConkey agar) as selection markers (C. H. Chiu, unpublished data). In this study, the DNA sequences surrounding the bla_CMY-2-carrying element were examined and in the majority of the Enterobacteriaceae tested this element was found inserted in the finQ gene of IncI1 large plasmids, as evidenced by the identification of an 8 bp direct repeat at both junction regions. Although the plasmids that carried this element appeared non-conjugative, our previous study has proved that the plasmid of Salmonella Choleraesuis SC-B67 contains an oriT sequence and a defective tra gene system,¹³ indicating that these plasmids may be mobilizable.^28,29

A Citrobacter freundii-type bla_CMY-5 gene has been mapped in the Klebsiella oxytoca plasmid pTKH11 to be adjacent to the blc and sugE genes found downstream of ampC on the C. freundii chromosome.³⁰ The ampR gene upstream of ampC on the chromosome is lost and its place has been taken by a putative insertion element, which could have been involved in gene capture. In the present study, DNA structures surrounding the bla_CMY-2 had a similar organization among the clinical isolates of the Enterobacteriaceae checked, suggesting that the resistance determinants were derived directly from the C. freundii chromosome. The finding in five isolates of one additional IS1 inserted close to the end of the tnpA may represent a recent evolution. Furthermore, although plasmids carrying the specific bla_CMY-2-containing element were not identical, the conserved genetic organization of the bla_CMY-2-carrying element and the surrounding DNA sequence among the isolates studied suggests that the whole segment, including the surrounding DNA sequence, might have been transferred altogether on a plasmid to other bacteria of the same or different species.

In conclusion, results of this work suggested that bla_CMY-2 is spread among members of the Enterobacteriaceae family primarily through the transfer, probably by mobilization, of various plasmids carrying the specific transposon-like DNA element. The efficiency of plasmid transfer by mobilization is generally lower than that by conjugation;²⁹ therefore, one could predict that the inter-species spread of the plasmid-mediated resistance would be slow. Nevertheless, the continuous spread of CMY-2-producing organisms appears unavoidable and plasmid DNA can evolve rapidly over time.³¹ Further study on the transfer mechanism of the bla_CMY-2 gene among Enterobacteriaceae is warranted.

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None to declare.

	Acknowledgements

 
This work was supported by grants NSC93-2314-B-182A-071 and NSC94-2321-B-182A-003 from National Science Council, Executive Yen, Taiwan, and CMRPG63011 from Chang Gung Memorial Hospital, Taiwan.

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