Differences in Antibiotic Resistance of MDR Enterobacteriaceae between Broiler and Laying Farms in Which Two mcr-1 Positive Plasmids Revealed their Risk of Resistance

Background: Existing studies on poultry (broilers and layers) suggest that strong screening of antibiotics leads to the wide spread of antibiotic resistance genes (ARGs). However, due to the different breeding cycle, antibiotic use and breeding environment characteristics of layers and broilers, they have different antibiotic resistance risks. Until now, there are few studies on the difference in risk of antibiotic resistance between broilers and layers, which is still a gap in our knowledge. Results: In the current study, high-throughput sequencing-based metagenomic revealed that firmicutes and bacteroidetes were the dominant bacteria in broiler and layer farms. Aph(3’’’)-III and ermF were the ARGs with the highest absolute abundance in broiler farms, which was higher than that of laying farms. TetW was the highest absolute abundance of ARG in laying hens, but it was still lower than broiler chickens. Three hundred fecal samples were used to isolate multidrug-resistant Enterobacteriaceae (MDRE). It was found that the detection rates of mcr-1, aph(3')-Ia, aadA2, blaOXA-1, blaCTX-M, fosB, qnrD, sul1, tetA and catA1 in broiler source MDRE were significantly higher than that in laying farms (P<0.05). Further studies showed that a chimeric plasmid p20432-mcr which carried the novel integron In1866 was isolated from broiler source MDRE. High frequence of conjugation (10-1 to 10-3) and wide range of hosts made p20432-mcr likely play an important role for the high detection rate of mcr-1, aph(3')-Ia and aadA2 in broiler farms. Conclusions: We comprehensively compared the prevalence of pathogen MDRE and their ARGs in the two environments and the mechanism of transmission. It is found that the mcr-1 positive plasmid p20432-mcr has a wide host range which may play an important part in the high detection rate of ARGs in broiler source MDRE. These findings will help to optimize disinfection and antibiotic resistant bacteria surveillance programs in poultry farms, especially in broilers.