Detection of the sul2–strA–strB gene cluster in an ice core from Dome Fuji Station, East Antarctica

•Antimicrobial resistance genes (ARGs) were detected from 1200–1400-year-old Antarctic ice.•Sequences of the ARGs were identical to those harboured by present-day bacteria.•Antarctic-derived ARGs can be expressed in Escherichia coli. Bacteria harbouring antimicrobial resistance genes (ARGs) have been isolated from various locations, including ancient microbiomes, indicating that these genes pre-date the discovery of antibiotics. To gain further information regarding ARGs in the pre-antibiotic era, ice samples derived from Dome Fuji Station, Eastern Antarctica, were examined. DNA was extracted from firn or ice core samples (n=3; 1200–1400ybp, 1700–2100ybp and 2200–2800ybp, respectively) under sterile conditions. Whole-genome amplification and PCR analyses were utilised to detect ARGs. A 2764-bp gene cluster containing the type II dihydropteroate synthase gene sul2 and the aminoglycoside phosphotransferase genes strA and strB was detected in the 1200–1400-year-old Antarctic ice core (DF-63.5). The sul2–strA–strB gene cluster is frequently associated with plasmid RSF1010 and transposon Tn5393; however, these elements were not detected in sample DF-63.5. The gene cluster exhibited a high level of sequence identity to sequences harboured in present-day bacteria, although there were sequence polymorphisms in the strA gene. Furthermore, expression of this gene cluster in Escherichia coli resulted in reduced susceptibility to dihydrostreptomycin and sulfamethoxazole. The results of this study provide further evidence that certain ARGs existed in the pre-antibiotic era. Because the sul2 gene confers resistance to the synthetic compound sulfamethoxazole, these findings suggest that ARGs against synthetic antimicrobials emerged in bacteria during the pre-antibiotic era.