Implications of different waterfowl farming on cephalosporin resistance: Investigating the role of bla(CTX-M-55)

We investigated the cephalosporin resistance of Escherichia coli from waterfowl among different breeding mode farms. In 2021, we isolated 200 strains of E. coli from waterfowl feces samples collected from Sichuan, Heilongjiang, and Anhui provinces. The key findings are: Out of the 200 strains, 80, 80, and 40 strains were isolated from waterfowl feces samples in intensive, courtyard, and outdoor breeding mode farms, respectively. The overall positive rate of the ESBL phenotype, detecting by the double disk diffusion method, was 68.00% (136/200). In particular, the rates for intensive, courtyard, and outdoor breeding modes were 98.75%, 36.25%, and 70.00%, respectively. Results of MIC test showed drug resistance rates in the intensive breeding mode: 100.00% for cephalothin, 38.75% for cefoxitin, 100.00% for cefotaxime, and 100.00% for cefepime. In courtyard breeding mode, the corresponding rates were 100.00%, 40.00%, 63.75%, and 45.00%, respectively. In outdoor breeding mode, the corresponding rates were 100.00%, 52.50%, 82.50%, and 77.50%, respectively. The PCR results for bla(CTX-M), bla(TEM), bla(OXA), and bla(SHV) showed the detection rate of bla(CTX-M) was highest at 75.50%, with bla(CTX-M-55) is the main subtype gene, followed by bla(TEM) at 73.50%. We screened 58 donor strains carrying bla(CTX-M-55), including 52 strains from the intensive breeding mode. These donor bacteria can transfer different plasmids to recipient E. coli J53, resulting in recipient bacteria acquiring cephalosporin resistance, and the conjugational transfer frequency ranged from 1.01 × 10(-)(5) to 6.56 × 10(-)(2). The transferred plasmids remained stable in recipient bacteria for up to several days without significant adaptation costs observed. During molecular typing of E. coli with conjugational transfer ability, the bla(CTX-M-55) was found to be widely present in different ST strains with several phylogenetic groups. In summary, cephalosporin resistance of E. coli carried by waterfowl birds in intensive breeding mode farm was significantly higher than in courtyard and outdoor mode farms. The bla(CTX-M-55) subtype gene was the prevalent ARGs and can be horizontally transferred through plasmids, which plays a key role in the spread of cephalosporin drug resistance.