Comparative Metagenomics and Network Analyses Provide Novel Insights Into the Scope and Distribution of β-Lactamase Homologs in the Environment.

The β-lactams are the largest group of clinically applied antibiotics, and resistance to these is primarily associated with β-lactamases. There is increasing understanding that these enzymes are ubiquitous in natural environments and henceforth, elucidating the global diversity, distribution, and mobility of β-lactamase-encoding genes is crucial for holistically understanding resistance to these antibiotics. In this study, we screened 232 shotgun metagenomes from ten different environments against a custom-designed β-lactamase database, and subsequently analyzed β-lactamase homologs with a suite of bioinformatic platforms including cluster and network analyses. Three interrelated β-lactamase clusters encompassed all of the human and bovine feces metagenomes, while β-lactamases from soil, freshwater, glacier, marine, and wastewater grouped within a separate "environmental" cluster that displayed high levels of inter-network connectivity. Interestingly, almost no connectivity occurred between the "feces" and "environmental" clusters. We attributed this in part to the divergence in microbial community composition (dominance of and vs. , respectively). The β-lactamase diversity in the "environmental" cluster was significantly higher than in human and bovine feces microbiomes. Several class A, B, C, and D β-lactamase homologs ( , , ) were ubiquitous in the "environmental" cluster, whereas bovine and human feces metagenomes were dominated by class A (primarily ) β-lactamases. Collectively, this study highlights the ubiquitous presence and broad diversity of β-lactamase gene precursors in non-clinical environments. Furthermore, it suggests that horizontal transfer of β-lactamases to human-associated bacteria may be more plausible from animals than from terrestrial and aquatic microbes, seemingly due to phylogenetic similarities.