Functional Cloning and Characterization of Antibiotic Resistance Genes from the Chicken Gut Microflora

A recent study using human fecal samples in conjunction with a culture-independent approach revealed immense diversity of antibiotic resistance (AR) genes in the human gut microflora. We hypothesize that food animal gut microflora also contain diverse and novel AR genes which could contribute to the emergence and transmission of AR in pathogens important in animal and human health. To test this, we examined AR reservoir in chicken gut microflora using a metagenomic, functional cloning method. Total genomic DNA was extracted from individual cecal contents of two free range chickens and two conventionally raised chickens. The DNAs were physically sheered into 1 to 3 kb fragments, cloned into expression vector pZE21-MCS, and transformed into E. coli TOP10 host strain, resulting in four metagenomic libraries of a total size of 10⁸ base pairs per library. The AR transformants from the libraries were selected on plates containing the specific antibiotic of interest; six antibiotics including ampicillin, tetracycline, chloramphenicol, spectinomycin, ciprofloxacin and norfloxacin were used for screening. Plasmids from selected transformants were extracted and subjected to sequence analysis of inserted fragments. Identified AR genes were annotated and aligned with homologs that have been deposited in GenBank. A total of 12 AR genes and 3 AR genes were identified from the microbiome in conventionally raised chickens and free-range chickens, respectively. Of the identified 15 AR genes, 8 genes that confer resistance to ampicillin, spectinomycin or chloramphenicol shared low sequence similarity (58% - 76% at amino acid level) with the corresponding AR genes previously identified using culture-dependent approaches. Notably, among the 8 novel AR genes identified in this study, 4 genes also shared low sequence similarities (59%-76% at amino acid level) with recently identified AR genes in human gut. An E. coli-Campylobacter shuttle vector bearing the flaA sigma 28 promoter was constructed. Two novel genes conferring resistance to ampicillin (FRAmp1.1) and spectinomycin (FRSpe1.1) were cloned into this new expression vector, respectively. The derived vectors have conferred increased AR in C. jejuni, a leading zoonotic bacterial pathogen causing human gastroenteritidis in many industrialized countries. Together, findings from this study showed the effectiveness of the metagenomic approach for examination of AR reservoir in food animals, revealed novel AR resistance genes in chicken gut microflora, and demonstrated the functionality of such AR genes in foodborne human pathogens.