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

Objectives: 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.

Methods: 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.

Results: 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.

Conclusion: 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.