Identification of Novel Mobilized Colistin Resistance Gene mcr-9 in a Multidrug-Resistant, Colistin-Susceptible Salmonella enterica Serotype Typhimurium Isolate

• Published in: mBio Vol. 10; no. 3
• Main Authors: Carroll, Laura M., Gaballa, Ahmed, Guldimann, Claudia, Sullivan, Genevieve, Henderson, Lory O., Wiedmann, Martin
• Format: Journal Article
• Language: English
• Published: United States American Society for Microbiology 07.05.2019
• Subjects: Anti-Bacterial Agents - pharmacology; Clinical Science and Epidemiology; Colistin - pharmacology; Drug Resistance, Bacterial - genetics; Editor's Pick; Escherichia coli - genetics; Gene Transfer, Horizontal; Genes, Bacterial; Humans; Observation; Salmonella typhimurium - drug effects; Salmonella typhimurium - genetics; Serogroup; Salmonella enterica; multidrug resistance; mobilized colistin resistance; antibiotic resistance; mcr-9; plasmid-mediated resistance; colistin; mcr genes
• ISSN: 2161-2129; 2150-7511; 2150-7511
• DOI: 10.1128/mBio.00853-19

Colistin is a last-resort antibiotic that is used to treat severe infections caused by MDR and extensively drug-resistant (XDR) bacteria. The World Health Organization (WHO) has designated colistin as a “highest priority critically important antimicrobial for human medicine” (WHO, Critically Important Antimicrobials for Human Medicine , 5th revision , 2017, https://www.who.int/foodsafety/publications/antimicrobials-fifth/en/ ), as it is often one of the only therapies available for treating serious bacterial infections in critically ill patients. Plasmid-borne mcr genes that confer resistance to colistin pose a threat to public health at an international scale, as they can be transmitted via horizontal gene transfer and have the potential to spread globally. Therefore, the establishment of a complete reference of mcr genes that can be used to screen for plasmid-mediated colistin resistance is essential for developing effective control strategies. Mobilized colistin resistance ( mcr ) genes are plasmid-borne genes that confer resistance to colistin, an antibiotic used to treat severe bacterial infections. To date, eight known mcr homologues have been described ( mcr-1 to -8 ). Here, we describe mcr-9 , a novel mcr homologue detected during routine in silico screening of sequenced Salmonella genomes for antimicrobial resistance genes. The amino acid sequence of mcr-9 , detected in a multidrug-resistant (MDR) Salmonella enterica serotype Typhimurium ( S . Typhimurium) strain isolated from a human patient in Washington State in 2010, most closely resembled mcr-3 , aligning with 64.5% amino acid identity and 99.5% coverage using Translated Nucleotide BLAST (tblastn). The S. Typhimurium strain was tested for phenotypic resistance to colistin and was found to be sensitive at the 2-mg/liter European Committee on Antimicrobial Susceptibility Testing breakpoint under the tested conditions. mcr-9 was cloned in colistin-susceptible Escherichia coli NEB5α under an IPTG (isopropyl-β- d -thiogalactopyranoside)-induced promoter to determine whether it was capable of conferring resistance to colistin when expressed in a heterologous host. Expression of mcr-9 conferred resistance to colistin in E. coli NEB5α at 1, 2, and 2.5 mg/liter colistin, albeit at a lower level than mcr-3 . Pairwise comparisons of the predicted protein structures associated with all nine mcr homologues (Mcr-1 to -9) revealed that Mcr-9, Mcr-3, Mcr-4, and Mcr-7 share a high degree of similarity at the structural level. Our results indicate that mcr-9 is capable of conferring phenotypic resistance to colistin in Enterobacteriaceae and should be immediately considered when monitoring plasmid-mediated colistin resistance. IMPORTANCE Colistin is a last-resort antibiotic that is used to treat severe infections caused by MDR and extensively drug-resistant (XDR) bacteria. The World Health Organization (WHO) has designated colistin as a “highest priority critically important antimicrobial for human medicine” (WHO, Critically Important Antimicrobials for Human Medicine , 5th revision , 2017, https://www.who.int/foodsafety/publications/antimicrobials-fifth/en/ ), as it is often one of the only therapies available for treating serious bacterial infections in critically ill patients. Plasmid-borne mcr genes that confer resistance to colistin pose a threat to public health at an international scale, as they can be transmitted via horizontal gene transfer and have the potential to spread globally. Therefore, the establishment of a complete reference of mcr genes that can be used to screen for plasmid-mediated colistin resistance is essential for developing effective control strategies.