Genomic Characterization and Antimicrobial Resistance of Four Mcr-1 Escherichia coli Strains Isolated from Human and Environment Sources, Hainan Province, Tropical China

Background: The rapid spread of multidrug-resistant Gram-negative bacteria, particularly the increase in carbapenem-resistant strains, has led to a critical level of resistance in clinical settings. Colistin is considered to be the last line of defense against these Gram-negative bacteria. Objectives: This study aims to investigate the genetic characteristics and antimicrobial resistance (AMR) patterns of four mcr-1 (mobile colistin resistance) Escherichia coli isolates from various sources in Hainan Province, tropical China, and to determine their genetic relationships with both domestic and global strains. Methods: Samples were collected from swimming pools, marine beaches, and beach personnel in Haikou and Sanya, Hainan Province, China. Escherichia coli isolates were obtained, multilocus sequence typing was performed, and the mcr family resistance genes were amplified by polymerase chain reaction. The mobile sequences, AMR genes, phenotypes, and virulence genotypes of the mcr-1 strains were analyzed. Phylogenetic analyses were conducted to investigate the relationships between the mcr-1E. coli strains and those in the NCBI database. Results: In the current study, the mcr-1 resistance gene was detected in 4 strains, accounting for 2.94% (4/136) of the total. The 4 strains of E. coli were isolated from a freshwater swimming pool, a pool wall, seawater, and a human body. Among the four strains, the minimum inhibitory concentration of polymyxin was 8 mg/L, except for one strain, which showed low-level resistance at 4 mg/L. All four strains showed complete resistance to ampicillin, three were resistant to compound sulfamethoxazole and chloramphenicol, and all strains were sensitive to other common antibiotics. The virulence genes cfaA, cfaB, cfaC, and cfaD/cfaE were detected in all E. coli strains. The insertable sequence IS3 was the most widely distributed type. Three isolates were identified as ST987, whereas the isolate from marine sources was identified as ST24. Comparison of the core genes of the strains indicated that the four strains were closely related to the Chrysomya sp. from Northern Thailand. Conclusions: The four mcr-1 genes can mediate low to medium levels of colistin resistance, facilitating their dissemination within the population and the environment. It is crucial to trace the source of multidrug-resistant bacteria in recreational water, which is in direct contact with humans, and to reduce the risks to human health through effective supervision.