Molecular and Phenotypic Characteristics of Escherichia coli Isolates from Farmed Minks in Zhucheng, China

Abstract

In this study, the prevalence, phenotypes, and clonal relationships of Escherichia coli (E. coli) strains isolated from minks were investigated. In July 2017, a total of 62 fresh faecal swab samples were randomly collected from one large-scale mink farm in Zhucheng, Shandong Province, China. In all the samples, 50 E. coli strains were isolated and then assigned to serotyping, antimicrobial susceptibility test, detection of antimicrobial resistance genes and the Class 1 integrons, and multilocus sequence typing (MLST). Four pathogenic serotypes were identified among all the isolates, while the most common serotype was enterohemorrhagic E. coli O104:H4 (6.0 %). Antimicrobial sensitivity testing revealed that most isolates were susceptible to cefoxitin (96.0 %) and amikacin (82.0 %), while most isolates were resistant to ampicillin (92.0 %) and tetracycline (90.0 %). An analysis of the nucleotide sequences revealed that 7 isolates (14.0%) carried 4 types of Class 1 integron cassette, including dfrA27+aadA2+qnrA (57.1%), dfrA17+aadA5 (14.3%), dfrA12+aadA2 (14.3%), and dfrA1+aadA1 (14.3%). PCR screening showed that 14 antibiotic resistance genes were presented in 50 isolates, while the most prevalent resistance gene was qnrS, which was detected in 60.0 % of isolates, followed by sul2 (40.0%) and oqxA (38.0%). MLST analysis showed that 32 sequence types (STs) were identified, while ST46 was the predominant genotype among all isolates. Clonal complex 3 (CC3) was dominant. Compared with 340 human E. coli STs reported in China, the ST10 clonal complex, known as the largest human clonal complex, was also found in the 50 mink E. coli isolates. Meanwhile, mink-derived strain ST206 formed a new clonal complex, CC206, which was different from human ST strains. Our results showed that farmed minks could be reservoirs of antimicrobial-resistant E. coli with Class 1 integron cassettes and resistance genes, which were likely to pose a threat to public health. Therefore, continuous inspections and monitoring of E. coli in minks are essential for detecting and controlling emerging E. coli with different serovars as well as antibiotic resistance.

Figure 1

Antimicrobial resistance phenotypes of 50 E. coli isolates.

Figure 2

Minimum spanning tree analysis of E. coli isolated from free-range mink farms. Note. Each circle represents one ST, and the area of the circle corresponds to the number of isolates. The colour of the circle indicates the clonal complex to which the isolate belongs. The grey region indicates that strains of isolates belong to a clonal complex. ST11 is included as a reference.

Figure 3

eBURST diagram generated by comparing the isolated strains dataset with the E. coli MLST database from human strains in China. Note. The E. coli MLST database from human strains in China was the reference dataset. STs in the profiles window are coloured differentially dependent on their membership of the two datasets. In pink, STs found in both datasets; in green, STs found in the QUERY dataset only; in blue, the primary ST founder of the clonal complexes; in yellow, the subgroup founder of the clonal complexes; and in black, all other STs.

Figure 4

Diversity profiles of the phylogenetic tree, drug resistance genes, antimicrobial resistance, and Class 1 integron structure. Note. Dendrogram of seven allele sequence types from 50 E. coli isolates from minks. ST11 is included as the reference. The three shaded parts in the figure represent three clonal complexes, CC10, CC46, and CC176 from top to bottom. The unit information is the number of isolated strains followed by the ST type and Class 1 integron structure IntA, dfrA27+aadA2+qnrA; IntB, dfrA17+aadA5; IntC, dfrA12+aadA2; or IntD, dfrA1+aadA1. (1) The 14 detected resistance genes were shown in the left matrix: a red square indicates that the strain carries the resistance gene, and a light blue square indicates that the resistance gene is not detected. (2) Antibiotics used in this experiment were shown in the right matrix: a blue square indicates that the strain is resistant to the antibiotic, and a light blue square indicates that the strain is not resistant.