rarity of mcr-1-positive bacteria in patients with blood infection is not a reason to ignore its significance in clinical practice

Since the discovery of the plasmid-borne mcr-1 gene, which confers colistin resistance, the dissemination of strains harboring this gene has been widely reported. Colistin resistance is a significant challenge for clinical application of last-line antibiotics, polymyxins, against severe Gram-negative bacterial infections, especially those caused by multidrug-resistant bacteria. Many reports have focused on investigation of mcr-1-encoding plasmids in bacterial isolates from animals. Its presence in bacterial isolates from clinical patients has not been systematically surveyed, although some investigations have revealed that bacteria containing the plasmid-borne mcr-1 gene exist among human gut microbiota. Recently, Dr. Yunsong Yu organized a multicenter longitudinal study to investigate the prevalence of mcr-1-positive Escherichia coli and Klebsiella pneumonia isolated from patients with blood infection. Among E. coli isolates from patients treated at 28 hospitals in China, only 1.3% (20/1,495) were found to be mcr-1-positive; among K. pneumoniae isolates, <0.2% (1/571) were positive. The authors also found coexistence of MCR-1 and NDM-5 in one E. coli strain. The good news is that mcr-1-positive isolates remain sensitive to one or several antibiotics other than colistin. Additionally, the presence of mcr-1-positive isolates remains rare in clinical patients with blood infection and mcr-1 transmission to multidrug-resistant strains has not yet been reported. Nevertheless, these results caution us to strengthen surveillance of mcr-1-positive strains in clinical patients, especially when we select polymyxins for patients with severe Gram-negative bacterial infections. Rare prevalence does not indicate a decreased possibility of outbreak in a hospital after uncontrolled polymyxin use, which can select for mcr-1-positive clones. Soil is recognized as a reservoir of antibiotic-resistant genes because soil-dwelling bacteria can produce various antibiotics and therefore bacteria in the same ecosystem can naturally develop antibiotic resistance. Naturally-evolved resistant genes could then be horizontally transferred into the microbial community, including pathogenic bacteria in hospitals. Therefore, discovery of mcr-1-positive isolates alerts us to rapid emergence and spread of this resistance, rendering effective treatment of severe infections difficult. Clinicians should take care not to administer polymyxin to patients before obtaining antibiotic susceptibility results.