Antimicrobial resistance among Enterobacteriaceae is growing, largely due to β-lactamase production. NDM, carried on the blaNDM gene, is the latest addition to this bacterial armamentarium and is a worrisome resistance mechanism (9). Increasing resistance to antimicrobials has been reported for Salmonella spp., due in part to extended-spectrum β-lactamase (ESBL) and AmpC production, often combined with other resistance mechanisms (4, 7). Carbapenemase production in Salmonella spp. has rarely been reported (5), and neither the blaNDM gene nor any other metallo-beta-lactamase (MBL) in Salmonella spp. has been described yet. We report the first case to our knowledge of NDM-producing Salmonella spp. likely acquired in India but detected in the United States.
On 25 January 2011, a sixty-year-old American was transferred from India by air ambulance to a hospital in Maryland. The patient was originally hospitalized in late December 2010 with a catastrophic intracranial bleed in India. Less than 24 h after his arrival at the U.S. hospital, he sustained a fever of 38.5°C; urine, blood, and endotracheal secretions were cultured. His sputum grew a carbapenem-resistant Klebsiella pneumoniae, positive for carbapenemase production by the modified Hodge test using meropenem (3), and the Etest MBL (AB bioMérieux, Durham, NC) revealed MBL production as per the package insert (1). The organism was susceptible to colistin only (MIC of 0.12 μg/liter by broth macrodilution) (Table 1) (3) and was sent to both the Maryland Department of Health and Mental Hygiene (MDHMH) and Centers for Disease Control and Prevention (CDC) for confirmation. The blaNDM gene was identified in the K. pneumoniae isolate by real-time PCR at the CDC (http://www.cdc.gov/HAI/settings/lab/kpc-ndm1-lab-protocol.html).
On February 12, a perirectal surveillance culture for carbapenem-resistant gastrointestinal isolates grew non-typhoid Salmonella spp. positive by the modified Hodge test, with an imipenem/(imipenem plus EDTA) ratio of 4/<1 by the Etest MBL. Serotyping at the MDHMH Laboratories Administration identified the isolate as Salmonella enterica subsp. enterica serovar Senftenberg (2), described as monophasic, with the antigenic formula 3,19:g,s,t:−. The identification was confirmed at the CDC. The CDC also concluded that the strain was blaNDM positive and only susceptible to tetracycline, tigecycline, and trimethoprim-sulfamethoxazole (Table 1). Others have raised the possibility that plasmids carrying blaNDM can easily transfer from one species of Enterobacteriaceae to another via genetic conjugation (6, 9). That does not appear to have happened in this case, as the blaNDM-1 plasmids carried by K. pneumoniae and Salmonella Senftenberg had different restriction profiles, as determined at the CDC.
Considering the potential for food-borne spread of Salmonella carrying NDM, this finding is worrisome and emphasizes the need for epidemiological studies and scrutiny of antimicrobial susceptibility reports from salmonellosis cases identified in or imported from countries where Salmonella is endemic and where NDM is spreading. A recently published study identified numerous NDM-1-positive bacteria, including Shigella boydii and Vibrio cholerae but not Salmonella spp., in water and seepage samples in New Delhi, India (8). Prompt recognition of carbapenem-resistant Enterobacteriaceae and initiation of appropriate infection control measures is essential to avoid spread of these organisms. Thus, clinicians should obtain travel history from patients and initiate infection control measures when carbapenem-resistant organisms are identified.