Isolation of carbapenem-resistant NDM-1-positive Providencia rettgeri in Mexico

Sir, Bacteria of the genus Providencia are Gram-negative opportunistic pathogens that have been isolated from a wide variety of environments, including human stool samples. They comprise part of the natural human gut flora but may also cause infections, including travellers' diarrhoea.1 They are also responsible for urinary tract and other nosocomial infections in humans.2 The New Delhi metallo-β-lactamase (NDM-1) is the most recently discovered transferable molecular class B metallo-β-lactamase. The gene encoding this enzyme was located on a 178 kb plasmid belonging to incompatibility group A/C in a Providencia stuartii clinical isolate.3 However, it has been described in different plasmid types (IncA/C, IncF, IncL/M, IncN or untypeable) and is also chromosomally integrated.4 This work describes four Providencia rettgeri clinical isolates obtained from patients with urinary tract infection in the intensive care unit (ICU) of the University Hospital of Monterrey, Mexico, between January and June 2012 (Table 1). The P. rettgeri isolates were identified using the API 20E galleries (bioMérieux, Durham, NC, USA) and confirmed by Sensititre (TEK Diagnostic Systems Inc., Cleveland, OH, USA). In addition, the 16S rRNA gene was characterized by using primers previously described.5 A possible epidemiological link between the patients was a surgical resident involved in the care of the four patients while in the ICU. None of the patients had evidence of infection prior to being transferred to the ICU; the average time to positive urine cultures for P. rettgeri was 29 days (range 12–68 days). Characteristics of metallo-β-lactamase NDM-1-producing P. rettgeri isolates IPM, imipenem; MEM, meropenem; CIP, ciprofloxacin; GEN, gentamicin; PIP, piperacillin; CAZ, ceftazidime; CTX, cefotaxime; TGC, tigecycline; CST, colistin. aThe plasmids appearing in bold or underlined correspond to the plasmids obtained in the mating and transformation experiments, respectively. bThe patient died of non-infectious complications. cThe patient died of neurological complications. dThe patient was transferred to a community hospital 10 days after isolation with no signs of infection. eThe patient was discharged without clinical signs of infection and remained asymptomatic 6 weeks after discharge. Characteristics of metallo-β-lactamase NDM-1-producing P. rettgeri isolates IPM, imipenem; MEM, meropenem; CIP, ciprofloxacin; GEN, gentamicin; PIP, piperacillin; CAZ, ceftazidime; CTX, cefotaxime; TGC, tigecycline; CST, colistin. aThe plasmids appearing in bold or underlined correspond to the plasmids obtained in the mating and transformation experiments, respectively. bThe patient died of non-infectious complications. cThe patient died of neurological complications. dThe patient was transferred to a community hospital 10 days after isolation with no signs of infection. eThe patient was discharged without clinical signs of infection and remained asymptomatic 6 weeks after discharge. MICs were determined by broth microdilution following CLSI recommendations6 and the phenotypic screening to determine the production of a carbapenemase enzyme was carried out using a double-disc synergy test (meropenem, imipenem and EDTA).6 The isolates were screened using PCR for genes encoding a range of carbapenemase enzymes, including KPC, GES, IMP, SIM, GIM, SPM, VIM and NDM.7 Genotyping was performed using PFGE and the results were analysed following the guidelines of Tenover et al.8 using GelCompar II (Applied Math, Kortrijk, Belgium). The plasmid profile was analysed using ion-interchange columns (Qiagen, Valencia, CA, USA); subsequently, a Southern hybridization was carried out with a non-radioactive probe for the NDM-1 gene. Mating and transformation experiments and identification of plasmid incompatibility groups by PCR replicon typing were undertaken as described previously.9,10 The genotyping analysis of the four P. rettgeri isolates showed one clonal group (data not shown). All isolates were susceptible to tigecycline (1 mg/L), but resistant to imipenem, meropenem, piperacillin, ceftazidime, cefotaxime, ciprofloxacin and colistin (Table 1). The isolates were positive for carbapenemase activity and the PCR assays and sequencing demonstrated the presence of the gene encoding NDM-1. According to the Southern hybridization (data not shown) and PCR replicon typing results obtained with the transconjugants and recombinants, the NDM-1 gene was identified on a 310 kb IncK plasmid (Table 1). However, the mating and the transformation experiments showed the respective transconjugant (310 kb and 160 kb) and transformant (310 kb and 50 kb) with two different plasmids harboured in the clinical isolates (Table 1). Similar results have been recently described suggesting that a helper plasmid is necessary for the mobilization of the plasmid-borne NDM-1.11 In this work, the 160 kb plasmid could be playing the role of helper. This is the first known report of an NDM-1-producing P. rettgeri in Mexico. This finding points to the need to enforce the molecular epidemiological surveillance of these pathogens and enzymes in order to prevent their dissemination among hospitals as well as to other bacterial genera. This work was supported by grants CB-2009-01-136339 from CONACyT (Mexican Council for Science and Technology). Rio Arronte provided the sequencing equipment. None to declare.