While it is well recognized that the environmental resistome is global, diverse, and augmented by human activities, it has been difficult to assess risk because of the inability to culture many environmental organisms, and it is difficult to evaluate risk from current sequence-based environmental methods. The four most important criteria to determine risk are whether the antibiotic-resistance genes (ARGs) are a complete, potentially functional complement; if they are linked with other resistances; whether they are mobile; and the identity of their host. Long-read sequencing fills this important gap between culture and short sequence-based methods. To address these criteria, we collected feces from a ceftiofur-treated cow, enriched the samples in the presence of antibiotics to favor ARG functionality, and sequenced long reads using Nanopore and PacBio technologies. Multidrug-resistance genes comprised 58% of resistome abundance, but only 0.8% of them were plasmid associated; fluroquinolone-, aminoglycoside-, macrolide-lincosamide-streptogramin (MLS)-, and β-lactam-resistance genes accounted for 2.7 to 12.3% of resistome abundance but with 19 to 78% located on plasmids. A variety of plasmid types were assembled, some of which share low similarity to plasmids in current databases. Enterobacteriaceae were dominant hosts of antibiotic-resistant plasmids; physical linkage of extended-spectrum β-lactamase genes (CTX-M, TEM, CMY, and CARB) was largely found with aminoglycoside-, MLS-, tetracycline-, trimethoprim-, phenicol-, sulfonamide-, and mercury-resistance genes. A draft circular chromosome of Vagococcus lutrae was assembled; it carries MLS-, tetracycline- (including tetM and tetL on an integrative conjugative element), and trimethoprim-resistance genes flanked by many transposase genes and insertion sequences, implying that they remain transferrable.
Keywords: ARG host; ARG mobility; Nanopore sequencing; PacBio sequencing; antibiotic-resistance genes.