Identification, characterization, and distribution of novel amidase gene aphA in sphingomonads conferring resistance to amphenicol antibiotics

Amphenicol antibiotics, such as chloramphenicol (CHL), thiamphenicol (TAP), and florfenicol (Ff), are high-risk emerging pollutants. Their extensive usage in aquaculture, livestock, and poultry farming has led to an increase in bacterial antibiotic resistance and facilitated the spread of resistance genes. Yet, limited research has been conducted on the co-resistance of CHL, TAP, and Ff. Herein, a novel amidase AphA was identified from a pure cultured strain that can concurrently mediate the hydrolytic inactivation of CHL, TAP, and Ff, yielding products p-nitrophenylserinol, thiamphenicol amine (TAP-amine), and florfenicol amine (Ff-amine), respectively. The antibacterial activity of these antibiotic hydrolysates exhibited a significant reduction or complete loss in comparison to the parent compounds. Notably, AphA shared less than 26% amino acid sequence identity with previously reported enzymes and exhibited high conservation within the sphingomonad species. Through enzymatic kinetic analysis, the AphA exhibited markedly superior affinity and catalytic activity toward Ff in comparison to CHL and TAP. Site-directed mutagenesis analysis revealed the indispensability of catalytic triad residues, particularly serine 153 and histidine 277, in forming crucial hydrogen bonds essential for AphA’s hydrolytic activity. Comparative genomic analysis showed that aphA genes in some species are closely adjacent to various transposable elements, indicating that there is a high potential risk of horizontal gene transfer (HGT). This study established a hydrolysis resistance mechanism of amphenicol antibiotics in sphingomonads, which offers theoretical guidance and a novel marker gene for assessing the prevalent risk of amphenicol antibiotics in the environment.