In 2007, public health laboratories in all US state health departments submitted every twentieth NTS isolated from humans to CDC for susceptibility testing by NARMS. MICs were determined by using broth microdilution (Sensititer, Trek Diagnostics, Westlake, OH, USA) and interpreted according to Clinical Laboratory Standards Institute criteria, where available.
Of the 2,165 human NTS isolates submitted to NARMS in 2007, a total of 100 (4.6%) displayed elevated MICs (>
2 mg/L) of ceftriaxone or ceftiofur, extended-spectrum cephalosporins used in human and veterinary medicine, respectively. Genomic DNA prepared from the 100 isolates and a PCR screen obtained by using degenerate primers capable of detecting all CTX-M enzymes identified 3 positive isolates, including serotypes Typhimurium; I 4,5,12:i:–; and Concord (4
). Most (66%) of the remaining 97 isolates harbored a blaCMY
The 3 CTX-M–producing Salmonella spp. infections occurred in 2 female patients (8 months of age and 72 years of age) and 1 male patient (1 year of age). Interviews were available for 2 patients, the 8-month-old (her parents) and the 72-year-old; in both instances, gastrointestinal symptoms with diarrhea were reported, and medical care was sought. Both patients received antimicrobial drug treatment (azithromycin and levofloxacin, respectively). The 72-year-old patient had traveled internationally before illness onset; the 8-month-old patient, who was infected with S. enterica serovar Concord, was an adoptee from Ethiopia.
All 3 isolates displayed resistance to β-lactams and extended-spectrum cephalosporins (). The S. enterica serovar Typhimurium and Concord isolates displayed additional multiresistance phenotypes. In addition, the serovar Typhimurium isolate displayed resistance to the quinolone nalidixic acid, a resistance phenotype associated with decreased susceptibility to fluoroquinolones. The serovar Typhimurium and Concord isolates showed decreased susceptibility to ciprofloxacin (MIC 0.25 mg/L and 0.125 mg/L, respectively). PCR for the plasmid-mediated mechanisms qnrA,B,S and aac(6′)Ib-cr showed a qnrA gene in the serovar Concord isolate. Sequencing confirmed qnrA1.
Characteristics of non-Typhi Salmonella isolates harboring blaCTX-M genes reported to the National Antimicrobial Resistance Monitoring System, United States, 2007*
Group-specific PCR primers were used to characterize the presumed blaCTX-M
). S. enterica
serovar Concord and I 4,5,12:i:– harbored group I enzymes, whereas the S. enterica
serovar Typhimurium isolate harbored a group II enzyme. Sequencing showed blaCTX-M-15
in the serovar Concord isolate, blaCTX-M-5
in the serovar Typhimurium isolate, and blaCTX-M-55/57
in the serovar I 4,5,12:i:– isolate. Presence of other β-lactamase–encoding genes (blaTEM
, and blaOXA
) was investigated by using PCR (6–9
). Amplification and sequencing confirmed a blaOXA-1
and a blaSHV-12
gene in the serovar Typhimurium and Concord isolates, respectively (7,8
The genetic environment of each blaCTX-M
gene was investigated by using PCR aimed at identifying insertion elements ISEcpI
, and CR1 (formerly known as orf513) (10
). Amplification and sequencing of the PCR products confirmed the ISEcp1
element upstream of each blaCTX-M
). In addition to ISEcp1
, an IS26
element was detected upstream of the blaCTX-M-5
To determine whether the CTX-M enzymes were plasmid borne, we extracted and transformed plasmids into electrocompetent E. coli DH10B. The blaCTX-M-55/57 gene transferred to E. coli; repeated attempts to transfer the blaCTX-M-5 and blaCTX-M-15 genes were unsuccessful. PCR amplification and plasmid pulsed-field gel electrophoresis confirmed the presence of the blaCTX-M-55/57 gene on a 70-kb plasmid in the transformant. The plasmid was not typeable by PCR-based incompatibility/replicon typing.