A novel chromosomal aminoglycoside 6′-N-acetyltransferase, AAC(6′)-Io, confers resistance to multiple aminoglycosides identified from Bacillus cereus

Background: The emergence of various resistance determinants in microbes is a growing concern for the clinical application of antimicrobial agents to treat bacterial infections. Research on the aminoglycoside resistance mechanism may help us to determine the complexity of bacterial resistance mechanisms and effective treatment of infectious diseases. Methods: Bacteria were isolated from environmental samples via the plate streak method. The minimum inhibitory concentration (MIC) of the antibiotics was determined using the agar dilution method. Gene cloning and antibiotic susceptibility testing were conducted to confirm the function of the new resistance gene. The kinetic parameters of the enzyme were determined after the protein AAC(6’)-Io was expressed in E. coli. Whole-genome sequencing and bioinformatic analysis were subsequently conducted to analyze the structure and evolution of the resistance gene-related sequences. Results: A novel aminoglycoside resistance gene, aac(6')-Io, which was identified in the chromosome of B. cereus DW444, confers resistance to tobramycin, kanamycin, amikacin, netilmicin, sisomicin and ribostamycin. Of the aminoglycoside substrates tested, AAC(6')-Io demonstrated the highest catalytic efficiency for netilmicin (k_cat/K_m, 2.11 × 10² M⁻¹·s⁻¹). Among the functionally characterized antimicrobial resistance proteins, AAC(6')-Io demonstrated the highest amino acid (aa) sequence similarity (47.51%) to AAC(6')-34, and it had the functional essential residues or domains of the AAC(6’)-I proteins, including F¹⁰⁰-G¹⁰² and G¹¹²T¹¹³, which are Ac-CoA binding sites, and L¹¹⁵, which is the key site for the acetylation of amikacin. Conclusion: The new aminoglycoside resistance gene aac(6')-Io was described in this study, along with its molecular characteristics. Elucidating the antibiotic resistance mechanism of this pathogen will benefit the clinical application of aminoglycosides to treat infections caused by bacteria carrying its homogous genes.