Experimental assessment of granular activated carbon's effect on horizontal gene transfer

With the total number of annual deaths related to antimicrobial resistance estimated to reach ten million globally by 2050, it is essential to evaluate the impact of every process such as adsorption on horizontal gene transfer. Activated carbon is the most frequently applied adsorbent for elimination of a broad range of chemicals like antibiotics from water. The current study assessed the influence of three types of granular activated carbons (GACs) on the transfer frequency of vancomycin resistance genes from donor to recipient Enterococcus faecalis strains. The bacterial attachment on the GAC surfaces was evaluated using scanning electron microscope (SEM) imagery, and a method was developed to correlate bacterial spatial proximity and cell-to-cell contacts on the GAC surface with gene transfer frequency in SEM micrographs. The results indicated that regardless of different surface properties, all studied GACs supported bacterial attachment and facilitated horizontal gene transfer. For example, loading 0.5 g of Filtrasorb-400 into 5 mL growth media increased the gene transfer frequency by more than 28 700 times, from 3.33 × 10−9 (in the bulk) to 9.55 × 10−5. The response patterns of gene transfer frequency between bacteria and GAC dose were similar across GAC types but varied in amplitude. The normalized values of cell-to-cell contacts on the number of bacteria on a unit surface area of activated carbon showed a high correlation with its observed gene transfer frequency. Findings of this research emphasized the importance of considering adsorbents' (e.g. activated carbon) contribution in stimulation of horizontal gene transfer, along with operational parameters in designing water treatment processes.