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Human Salmonella infection yielding CTX-M [beta]-lactamase, United States.

To the Editor: In the United States most third-generation cephalosporin resistance among salmonellae is due to AmpC plasmid-mediated [beta]-lactamases. Extended-spectrum [beta]-lactamases (ESBLs) have rarely been reported (1). The CTX-M [beta]-lactamases constitute a group of ESBL enzymes that are increasing in prevalence worldwide. Currently, the CTX-M enzymes are classified into 5 different subgroups on the basis of DNA sequence similarities (2). We report on a domestically acquired CTX-M--producing Salmonella isolate in the United States.

In 2003, public health laboratories in all US state health departments submitted every 20th non-Typhi Salmonella (NTS) isolate from humans to the Centers for Disease Control and Prevention (CDC) for susceptibility testing by the National Antimicrobial Resistance Monitoring System (NARMS). MICs were determined by broth microdilution and interpreted according to Clinical and Laboratory Standards Institute standards (www., when available. Resistance to cefquinome was defined as [greater than or equal to] 32 mg/L.

Among the 1,864 human NTS isolates submitted to NARMS in 2003, 105 (5.6%) displayed elevated MICs ([greater than or equal to] 2 mg/L) to ceftriaxone or ceftiofur, third-generation cephalosporins used in human and veterinary medicine, respectively. Genomic DNA was prepared from the 105 isolates, and a PCR with degenerate primers capable of detecting all CTX-M enzymes identified a single positive S. enterica ser. Typhimurium (3). The isolate came from a stool sample collected in September 2003 from a white, non-Hispanic, US-born, 3-month-old boy who lived in the state of Georgia. The patient had diarrhea and fever for [approximately equal to] 1 week. Because neither the patient nor his family had traveled internationally in the 3 months before specimen collection, the infection appears to have been domestically acquired. The patient did not receive any antimicrobial agents before illness but was treated for 14 days with cefpodoxime. The infant recovered from the illness without complications.

The isolate displayed resistance to [beta]-lactams, aminoglycosides, phenicols, tetracyclines, and folate pathway inhibitors (Table). Two [beta]-lactamases (isoelectric pH [pI] 7.5 and 8.8) were resolved by isoelectric focusing.

Group-specific PCR primers were used to characterize the presumed [bla.sub.CTX-M] gene (4). Primers TOHO1--2F and TOHO1--1R yielded a 351-bp product, confirming a group II [bla.sub.CTX-M] gene. To perform sequencing of the entire gene, a ClustalW alignment with representatives from group II was performed to identify primers (DNASTAR, Madison, WI, USA). The sequence of the gene was identical to the sequence of the [bla.sub.CTX-M-5] gene detected in other isolates of S. enterica ser. Typhimurium (GenBank accession nos. U95364 and AF286192) as well as to the kluA-2 gene of Kluyvera ascorbata (GenBank accession no. AJ251722).

The genetic environment of the [bla.sub.CTX-M-5] gene was investigated by PCR specific for upstream insertion elements (ISEcp1, IS26, and ORF513) and the downstream sequence sul1 (5). Amplification with primer ISEcp1 and an internal [bla.sub.CTX-M-5] primer yielded a PCR product of [approximately equal to] 350 bp. Sequencing confirmed presence of the 3' end inverted repeat region of the ISEcp1.

Presence of other [beta]-lactamase--encoding genes ([bla.sub.TEM], [bla.sub.SHV], and [bla.sub.OXA]) was investigated by PCR (6-8). Amplification with primers OXA-1F and OXA-1R yielded a 595-bp product with a sequence consistent with that of [bla.sub.OXA-1] (8).

To determine whether the CTX-M enzyme was plasmid-borne, plasmids were extracted and transformed into electrocompetent Escherichia coli DH10B. The transformant exhibited resistance to cefotaxime but not to ceftazidime (Table). In addition, the transformant exhibited resistance to cefquinome and cefepime. The presence of a [bla.sub.CTX-M] gene was confirmed by PCR (3,4). The [bla.sub.OXA] gene could not be amplified from the E. coli transformant (8).

A CTX-M--producing Salmonella isolate has been reported only once previously in the United States (9). This was in 1994, when an isolate of Salmonella ser. Typhimurium var. Copenhagen with a CTX-M-5 was recovered from a 4-month-old girl adopted from Russia; that infection was not domestically acquired (9). We compared the 1994 isolate and the isolate in this study by pulsed-field gel electrophoresis; the isolates showed distinct patterns.

The ISEcp1 insertion sequence has been described as a flanking region of several [bla.sub.CTX-M] genes and has been implicated in the expression and mobilization of the genes (5). A recent study by Lartigue et al. showed that a CTX-M-2 progenitor in K. ascorbata could be mobilized and transferred to a conjugative E. coli plasmid by the ISEcp1B element; enhanced mobilization was observed in the presence of ceftazidime, cefotaxime, and piperacillin (10).

This Salmonella isolate's resistance to cefepime and cefquinome, fourth-generation cephalosporins, is troubling. Cefquinome is not approved for use in the United States but has been used in Europe for treating food animals since 1994. ESBLs, including CTX-M enzymes, are more common in Europe than in the United States (1). Further studies are warranted to clarify the extent to which the use of cefquinome has contributed to high CTX-M prevalence in Europe.

In conclusion, we report a domestically acquired CTX-M--producing Salmonella isolate in the United States. Because third-generation cephalosporins are important for treating invasive Salmonella infections, continued monitoring of ESBL-producing bacteria is important.


We thank the NARMS-participating public health laboratories for submitting isolates, Matt Mikoleit for confirming the Salmonella serotype, Anne Whitney for DNA sequencing, and the Georgia Division of Public Health for providing patient interview information.

This work was supported by an interagency agreement between CDC and the Food and Drug Administration Center for Veterinary Medicine.


(1.) Bush K. Extended-spectrum beta-lactamases in North America, 1987-2006. Clin Microbiol Infect. 2008;14(Suppl 1):134-43. DOI: 10.1111/j.1469-0691 .2007.01848.x

(2.) Bonnet R. Growing group of extended-spectrum beta-lactamases: the CTX-M enzymes. Antimicrob Agents Chemother. 2004;48:1-14. DOI: 10.1128/AAC.48.1.1-14.2004

(3.) Bonnet R, Recule C, Baraduc R, Chanal C, Sirot D, De Champs C, et al. Effect of D240G substitution in a novel ESBL CTX-M-27. J Antimicrob Chemother. 2003;52:29-35. DOI: 10.1093/jac/dkg256

(4.) Pitout JD, Hossain A, Hanson ND. Phenotypic and molecular detection of CTX-M-beta-lactamases produced by Escherichia coli and Klebsiella spp. J Clin Microbiol. 2004;42:5715-21. DOI: 10.1128/ JCM.42.12.5715-5721.2004

(5.) Eckert C, Gautier V, Saladin-Allard M, Hidri N, Verdet C, Ould-Hocine Z, et al. Dissemination of CTX-M-type beta-lactamases among clinical isolates of Enterobacteriaceae in Paris, France. Antimicrob Agents Chemother. 2004;48:1249-55. DOI: 10.1128/AAC.48.4.1249-1255.2004

(6.) Brinas L, Zarazaga M, Saenz Y, Ruiz-Larrea F, Torres C. Beta-lactamases in ampicillin-resistant Escherichia coli isolates from foods, humans, and healthy animals. Antimicrob Agents Chemother. 2002;46:3156-63. DOI: 10.1128/ AAC.46.10.3156-3163.2002

(7.) Rasheed JK, Jay C, Metchock B, Berkowitz F, Weigel L, Crellin J, et al. Evolution of extended-spectrum beta-lactam resistance (SHV-8) in a strain of Escherichia coli during multiple episodes of bacteremia. Antimicrob Agents Chemother. 1997;41:647-53.

(8.) Chen S, Zhao S, White DG, Schroeder CM, Lu R, Yang H, et al. Characterization of multiple-antimicrobial-resistant salmonella serovars isolated from retail meats. Appl Environ Microbiol. 2004;70:1-7. DOI: 10.1128/AEM.70.1.1-7.2004

(9.) Zirnstein G, Swaminathan B, Angulo F, Tenover F, Rasheed JK. Plasmid-mediated CTX-M-5 [beta]-lactamase conferring resistance to ceftriaxone and cefotaxime in a Salmonella serotype Typhimurium var. Copenhagen isolate from an infant adopted from Russia. In: Proceedings of the 2nd International Conference on Emerging Infectious Diseases, Atlanta, Georgia, USA. July 16-19, 2000. Washington: American Society for Microbiology; 2000.

(10.) Lartigue MF, Poirel L, Aubert D, Nordmann P. In vitro analysis of ISEcp1B-mediated mobilization of naturally occurring beta-lactamase gene [bla.sub.CTX-M] of Kluyvera ascorbata. Antimicrob Agents Chemother. 2006;50:1282-6. DOI: 10.1128/ AAC.50.4.1282-1286.2006

Author affiliations: Centers for Disease Control and Prevention, Atlanta, Georgia, USA (M. Sjolund, J. Yam, J. Schwenk, K. Joyce, F. Medalla, E. Barzilay, J.M. Whichard); and Atlanta Research and Education Foundation, Atlanta (M. Sjolund, J. Yam, K. Joyce)

DOI: 10.3201/eid1412.080494

Address for correspondence: Maria Sjolund, Centers for Disease Control and Prevention, 1600 Clifton Rd NE, Mailstop G29, Atlanta GA 30333, USA; email:
Table. MIC values of antimicrobial drugs for the Salmonella ser.
Typhimurium isolate and its Escherichia coli DH10B transformant

 MIC, mg/L

 S. ser. E. coli DH10B
Antimicrobial agent Typhimurium transformant
Amikacin 1 1
Amoxicillin-clavulanic acid 32 16
Ampicillin >32 >32
Aztreonam 32 32
Cefepime 32 32
Cefotaxime >64 >64
Cefotaxime-clavulanic acid 0.25 0.12
Cefoxitin 2 8
Cefquinome >32 >32
Ceftazidime 8 8
Ceftazidime-clavulanic acid 0.5 0.25
Ceftiofur >8 >8
Ceftriaxone >64 >64
Chloramphenicol >32 [less than or
 equal to] 2
Ciprofloxacin [less than or [less than or
 equal to] 0.016 equal to] 0.016
Gentamicin 4 [less than or
 equal to] 0.25
Imipenem 0.5 0.25
Kanamycin >64 [less than or
 equal to] 8
Nalidixic acid 4 1
Piperacillin-tazobactam >64 2
Streptomycin [less than or >64
 equal to] 32
Sulfisoxazole >256 [less than or
 equal to] 16
Tetracycline >32 [less than or
 equal to] 4
Trimethoprim-sulfamethoxazole >4 [less than or
 equal to] 0.12

 MIC, mg/L

 E. coli
Antimicrobial agent DH10B
Amikacin 1
Amoxicillin-clavulanic acid 4
Ampicillin 4
Aztreonam 0.12
Cefepime [less than or
 equal to] 0.06
Cefotaxime [less than or
 equal to] 0.06
Cefotaxime-clavulanic acid [less than or
 equal to] 0.06
Cefoxitin 8
Cefquinome [less than or
 equal to] 0.06
Ceftazidime 0.5
Ceftazidime-clavulanic acid 0.12
Ceftiofur 0.5
Ceftriaxone [less than or
 equal to] 0.25
Chloramphenicol [less than or
 equal to] 2
Ciprofloxacin [less than or
 equal to] 0.016
Gentamicin [less than or
 equal to] 0.25
Imipenem 0.25
Kanamycin [less than or
 equal to] 8
Nalidixic acid 1
Piperacillin-tazobactam 2
Streptomycin >64
Sulfisoxazole [less than or
 equal to] 16
Tetracycline [less than or
 equal to] 4
Trimethoprim-sulfamethoxazole [less than or
 equal to] 0.12
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Title Annotation:LETTERS
Author:Sjolund, Maria; Yam, Jennifer; Schwenk, Jillian; Joyce, Kevin; Medalla, Felicita; Barzilay, Ezra; Wh
Publication:Emerging Infectious Diseases
Article Type:Letter to the editor
Date:Dec 1, 2008
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