Mobile colistin resistance (mcr-1) gene-positive Escherichia coli from chickens in Nigeria is potentially pathogenic and transfers colistin resistance to other organisms

Colonization of chicken gut by pathogenic Escherichia coli harboring mobile colistin resistance (mcr-1) gene is a huge public and animal health risk. This study was undertaken to evaluate colistin (COL) resistance transfer potentials and pathogenicity of mcr-1-positive E. coli isolates from chickens in Southeastern Nigeria (SEN). In vitro mcr-1/COL resistance transfer of 22 mcr-1-positive E. coli isolates was assayed by conjugation test. A 1:1 v/v broth culture containing 1 × 108 colony-forming unit/mL of each test isolate (donor) and sodium azide–resistant E. coli J53 (recipient) was mated overnight at 37 °C in nutrient broth (NB), and transconjugants were selected on MacConkey agar containing COL (4 µg/mL) and sodium azide (150 µg/mL). Four groups of 10 healthy 1-week-old chickens devoid of COL-resistant organism were orally inoculated with 0.5 mL of 1 × 108 cfu/mL broth culture of three strains of mcr-1-positive E. coli and E. coli J53, and one unchallenged group was used as a control. Clinical signs were monitored regularly and recorded. Periodically collected cloacal swab samples and organs obtained from animals sacrificed 21 days post-experimental challenge were cultured on selective media and isolates were characterized. Pathogenicity of the donors in the birds was assessed grossly and histomorphologically. Fifty-nine percent of the 22 mcr-1-positive isolates transferred COL resistance to E. coli J53 at a frequency of 5.0 × 10−7 to 4.5 × 10−6. Histopathologically, no lesion was observed in tissue sections of birds in the control group. But in the challenged birds, the liver had mild hyperaemia, hepatocyte degeneration/necrosis, and mononuclear cell aggregation. Their spleen had moderate to severe hyperaemia with reactive white pulp while their heart revealed mild to moderate hyperaemia, oedema of myocardial interstices, multifocal areas of myocardial fiber necrosis, and mononuclear cell infiltration. Potentially pathogenic mcr-1-positive E. coli is spreading COL resistance among Enterobacterales colonizing the gut of chickens in SEN and this has clinical and public health significance. © 2022, The Author(s), under exclusive licence to Springer-Verlag London Ltd., part of Springer Nature.