On the Low Abundance of Antibiotic Resistance Genes in Bacteriophage Genomes and Their Random Acquisition via Specialized Transduction.

The role of bacteriophages in spreading antimicrobial resistance genes (ARGs) has been debated for over a decade. Several questions regarding the ARG dissemination potential of bacteriophages remain. For example, do phages frequently carry ARGs? Besides generalized transduction (GT), could specialized transduction (ST) play an essential role in the spread of ARGs? To address these questions, we thoroughly analyzed the available phage genomes, viromes, temperate phages, and prophage sequences for the presence of all known ARGs and their genomic context. Out of the 38,861 phage genome sequences we analyzed, 82 phages were found to possess 141 ARGs in their genomes. Interestingly, a few of the Streptococcus phages were found to carry an entire ARG cluster with four or more genes. An uncharacterized Caudoviricetes phage was found to possess the complete vancomycin operon. In literature, the role of ST in phage-based ARG dissemination is often overlooked. Based on the presence of lysogenic markers, the terminal location of ARGs on phage genomes, and ARG clusters transferred to phages, we suggest that ARGs are predominantly acquired from pathogenic hosts by temperate phages via ST. These findings indicate that, in addition to GT, ST can also play a crucial role in phage-based ARG dissemination. Our study also suggests that the acquisition of ARGs by phages is sporadic. Overall, we propose that phage-mediated gene transfer is governed by a complex interplay of gene transfer bottlenecks and microenvironmental parameters, such as microbial density, diversity, and external stress, in addition to phage properties.