ACC-1 β-Lactamase–producing Salmonella enterica Serovar Typhi, India

To the Editor: Typhoid fever, caused by Salmonella enterica serovar Typhi, is a serious form of enteric fever. In 2000, the worldwide number of typhoid cases was estimated to be >21,000,000, and there were >200,000 deaths from this disease (1).

ported. To date, there are no reports of AmpC β-lactamases in typhoidal salmonellae. AmpC β-lactamases confer resistance to a broad spectrum of β-lactams, which greatly limits therapeutic options. We investigated an isolate of S. Typhi by using serotyping, antimicrobial drug susceptibility testing, PCR screening for β-lactamase genes, and sequence analysis to confi rm the identity of the isolate and the β-lactamase gene involved in conferring resistance to this isolate.
The isolate was obtained in Bangalore, India, in August 2009, from the blood of a female patient (14 years of age) who was hospitalized because of signs and symptoms of enteric fever. She had no history of having received antimicrobial drugs. After a blood sample was cultured, the patient was empirically treated with ceftriaxone but did not clinically improve.
Culture yielded gram-negative bacteria after 48 hours. The isolate was identifi ed by standard biochemi-cal methods as S. Typhi. Identifi cation was confi rmed by using Salmonella spp. polyvalent O, O9, and H:d antisera (Murex Biotech, Dartford, UK). Susceptibility to antimicrobial drugs was assessed by using the Kirby-Bauer disk diffusion method according to Clinical and Laboratory Standards Institute guidelines (www.clsi.org). The isolate was resistant to ampicillin, piperacillin, cefoxitin, cefotaxime, ceftazidime, ceftriaxone, aztreonam, amoxicillin/clavulanate, and cefepime. It was susceptible to chloramphenicol, trimethoprim/sulfamethoxazole, nalidixic acid, ciprofl oxacin, and meropenem.
Treatment was changed to ciprofl oxacin (500 mg every 12 h for 7 d).
PCR screening and sequencing was performed to identify β-lactamase resistance genes bla TEM , bla SHV , bla OXA-1 group, bla CTX-M , and AmpC as described (6,7). Sequencing of β-lactamase gene amplicons was conducted at the Vector Control Research Centre in Pondicherry, India. The BLASTN program (www.ncbi.nlm.nih.gov/BLAST) was used for database searching. We also used a nested PCR specifi c for the fl agellin gene of S. Typhi to confi rm identity of the isolate (8). The nested PCR amplicon was sequenced to confi rm identity of the fl agellin (fl iC) gene of S. Typhi. Sequencing of the fl agellin gene product was conducted by Cistron Bioscience (Chennai, India).
The isolate was negative for ESBL production. PCR amplifi cation and sequencing showed that the isolate harbored bla TEM-1 and bla ACC-1. The isolate was negative by PCR for other β-lactamases tested. TEM-1 is one of the most commonly encountered β-lactamases in the family Enterobacteriaceae and can hydrolyze narrow-spectrum penicillins and cephalosporins.
We report ACC-1 AmpC β-lactamase in typhoidal salmonellae. S. Typhi could have acquired the AmpC β-lactamase from drug-resistant bowel fl ora. After the isolate was found to be highly resistant to ceftriaxone, the change in therapy to ciprofl oxacin helped in recovery of the patient without any sequelae.
ACC-1 AmpC β-lactamases originated in Hafnia alvei and are now found in various members of the family Enterobacteriaceae (9). The ACC-1 AmpC β-lactamases are exceptional in that they do not confer resistance to cephamycins (10). Our isolate contained bla TEM-1 and bla ACC-1 and was resistant to cefoxitin and cefepime but susceptible to meropenem. Bidet et al. (9) reported isolating Klebsiella pneumoniae resistant to cefoxitin and cefepime and intermediate resistance to imipenem. Atypical resistance was attributed to ACC-1 β-lactamase production and loss of a 36-kDa major outer membrane protein (9). We did not analyze changes in the outer membrane proteins responsible for alteration of permeability. Continual monitoring of drug resistance patterns is imperative. Antimicrobial drug susceptibility testing should be conducted for clinical isolates, and empirical antimicrobial drug therapy should be changed accordingly. AmpC β-lactamase genes will eventually be transferred to typhoidal salmonellae, which may pose a threat to public health. Spread of broad-spectrum β-lactamases would greatly limit therapeutic options and leave only carbapenems and tigecycline as secondary antimicrobial drugs.